Journal of Instrumentation

Fast-neutron imaging spectrometer based onliquid scintillator loaded capillariesTo cite this article I Mor et al 2012 JINST 7 C04021

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This content was downloaded from IP address 46119142172 on 20112021 at 1543

2012 JINST 7 C04021

PUBLISHED BY IOP PUBLISHING FOR SISSA MEDIALAB

RECEIVED February 6 2012ACCEPTED February 27 2012

PUBLISHED April 27 2012

2nd INTERNATIONAL WORKSHOP ON FAST NEUTRON DETECTORS AND APPLICATIONSNOVEMBER 6ndash11 2011EIN GEDI ISRAEL

Fast-neutron imaging spectrometer based on liquidscintillator loaded capillaries

I Morab D Vartskya1 M Brandisa MB Goldbergab D Bara I Mardora

V Dangendorfb and B Brombergerb

aSoreq Nuclear Research Center (SOREQ NRC)Yavne 81800 Israel

bPhysikalisch-Technische Bundesanstalt (PTB)Braunschweig 38116 Germany

E-mail davidsoreqgovil

ABSTRACT A fast-neutron imaging detector based on micrometric glass capillaries loaded withhigh refractive index liquid scintillator has been developed Neutron energy spectrometry is basedon event-by-event detection and reconstruction of neutron energy from the measurement of theknock-on proton track length and the amount of light produced in the track In addition the de-tector can provide fast-neutron imaging with position resolution of tens of microns The detectorprinciple of operation simulations and experimental results obtained with a small detector proto-type are described We have demonstrated by simulation energy spectrum reconstruction for inci-dent neutrons in the range of 4ndash20 MeV The energy resolution in this energy range was 10ndash15Preliminary experimental results of detector spectroscopic capabilities are presented

KEYWORDS Scintillators and scintillating fibres and light guides Neutron detectors (cold ther-mal fast neutrons) Neutron radiography

1Corresponding author

ccopy 2012 IOP Publishing Ltd and Sissa Medialab srl doi1010881748-0221704C04021

2012 JINST 7 C04021

Contents

1 Introduction 1

2 The concept of the capillary based detector 221 Irradiation geometries 3

211 Irradiation parallel to the capillary bundle axis 3212 Irradiation perpendicular to the capillary bundle axis 4

22 Determination of neutron energy 5

3 Detector simulations 531 Detector geometry 532 Reconstruction of neutron energy 6

4 Experimental setup 841 Capillary array 842 Optics and track imaging 943 Irradiation configuration 10

5 Experimental results 10

6 Conclusions 12

1 Introduction

High resolution imaging of fast neutrons combined with energy spectroscopy is required in a varietyof applications ranging from fast neutron radiography and tomography nuclear material monitor-ing to solar and atmospheric physics

Over the last decade several groups have been developing fast detectors based on particletracking using scintillating plastic fibers or capillary tubes filled with liquid scintillator mainly inorder to determine the direction of the incident neutron and for high resolution imaging

Ryan et al [1] and Miller et al [2] developed the SONTRAC detector which is based on adensely-packed bundle of 250 microm diameter scintillating plastic fibers stacked in orthogonal layersUsing double neutron-proton scattering and recording images of the ionization tracks of the recoilprotons the detector permits determining the energy and direction of neutrons originating in thesun at energies of 20ndash250 MeV

In addition to SONTRAC the above-mentioned groups have built a Fast Neutron ImagingTelescope mdash FNIT [3] for the energy range of 08ndash20 MeV the purpose being to detect SpecialNuclear Materials (SNM) such as plutonium or Highly-Enriched Uranium (HEU)

Peel et al [4] of Sandia National Lab have developed a directional detector for neutrons of14 MeV With this detector the direction of the neutron is reconstructed by finding the direction

ndash 1 ndash

2012 JINST 7 C04021

and energy of the recoil proton from a single elastic scatter The detector is composed of an arrayof 64 square scintillating plastic fibers (BCF12 of St Gobain [5]) 05times 05times100 mm3 with 23 mmspacings between the fibers

Furthermore Disdier et al [6] described a capillary array detector originally developed atCERN for the CHORUS collaboration [7] It was filled with deuterated liquid scintillator for high-resolution neutron imaging of laser-imploded D-T targets They employed 50 mm long 100x100mm2 coherent arrays of glass capillaries with 85 microm-diameter pores However neutron spec-troscopy with this detector has not been reported

In 2005 the Soreq PTB and Bern University collaboration investigated a capillary array of 20microm in diameter fibres developed by the CHORUS collaboration at CERN The capillaries werefilled with high-refractive-index liquid scintillator developed at Soreq for a Gamma-ray ResonanceAbsorption (GRA) detector [8] The detector was tested with gamma-rays and mixed gamma andneutron events produced by radioactive sources The experiment showed very promising albeitqualitative results Preliminary computer simulation of the detector indicated that it is possible toreconstruct the energy of the incident neutrons provided the proton track projection is determinedwith sufficient accuracy

In this context our principal interest is fast neutron radiography which requires detectors withthe following properties

bull High neutron efficiency (gt 10)

bull Large area or long linear arrays for high resolution radiography of voluminous objects

bull Sub-mm position-resolution capabilities

bull Neutron spectroscopy (for rejection of scattered radiation)

bull Insensitivity to gamma-rays

The imaging neutron detectors we have developed to-date were for Fast Neutron Resonance Ra-diography (FNRR) [10 11] These detectors perform high energy resolution spectroscopy bythe method of measuring neutron time-of flight (TOF) This method requires operating with ananosecond-pulsed neutron source such as a particle accelerator using an intense pulsed deuteronbeam

In this paper we describe the development of a micro-capillary bundle detector filled withliquid scintillator that will permit high spatial resolution imaging and medium-quality energy spec-troscopy of non-pulsed fast neutron sources available from continuous beam particle acceleratorsisotopic neutron sources or reactor beams

2 The concept of the capillary based detector

Figure 1 shows schematically the concept of the capillary bundle detector The detector is basedon a capillary array filled with high-refractive-index liquid scintillator The principal fast neutroninteraction (in the energy range 08ndash14 MeV) within the liquid scintillator is elastic scattering withhydrogen and to a somewhat lesser extent carbon as both elements have comparable atomic den-sity in such substances For the detection process scattering by hydrogen is dominant at such

ndash 2 ndash

2012 JINST 7 C04021

Figure 1 Description of the concept of the capillary bundle detector

energies since the scattered hydrogen nucleus also denoted a recoil-proton is primarily responsi-ble for the excitation of the scintillator molecules leading to emission of scintillation light Thecarbon recoil is of much lower energy and the amount of scintillation light is furthermore stronglyquenched therefore this process does not play a major role in the detection process

The proton on the other hand has a significant energy (up to the original neutron energy) andtherefore a range that can be as large as several millimetres During its motion within the bundle itcreates scintillation light inside the capillaries it traverses A fraction of this light will travel to theend of these capillaries via total internal reflection and is registered in the optical readout systemthereby creating a projection of the proton track

Assuming the incident neutron flight direction relative to the capillary bundle axis is known(this is true for radiographic systems) and there are no multiple neutron interactions in the bundleone can calculate the incident neutron energy using two parameters that characterize the protontrack

1 the amount of light created along the recoil proton track

2 the recoil-proton track-length projection on the capillary array face plane

21 Irradiation geometries

Referring to the configuration of figure 1 we investigated two irradiation geometries Irradiationparallel to the capillary bundle axis and irradiation perpendicular to it

211 Irradiation parallel to the capillary bundle axis

Figure 2-left describes schematically the direction of neutron irradiation and the proton track view-ing direction in geometry of irradiation parallel to the capillary axis x Figure 2-right shows thedependence of proton track projection on polar and azimuthal angles θ and φ respectively andtrack length r Such a configuration possesses axial symmetry and the length of track projection isdependent only on θ and proton energy The energy of the proton is

Ep = En cos2θ

The length of the track r is a function of proton energy Moreover the track projection Pr is relatedto track length r by

Pr = r middot sinθ (21)

ndash 3 ndash

2012 JINST 7 C04021

Figure 2 Geometry of irradiation parallel the capillary axis x

Figure 3 Geometry of irradiation perpendicular to the capillary axis (along z axis)

The projections are usually short since forward-going protons (small θ ) will have high energies(that produce more light) but short track projections As θ increases the proton energy decreasesthus giving rise to shorter tracks and creating less light

The crucial feature for the present application is that this configuration permits neutron imag-ing with very high position resolution (typically tens of microns)

212 Irradiation perpendicular to the capillary bundle axis

Here the projection Pr is related to track length r by

Pr = (r2 sin2θprime cos2

ϕprime+ r2 cos2

θprime)12 (22)

ndash 4 ndash

2012 JINST 7 C04021

There is no axial symmetry along the capillary axis but in contrast to the previous configurationthe projection length always increases with the length of the proton track

The projection angle α relative to the direction of the incident neutron is

tgα = tgθprime cosφ

prime (23)

For φ rsquo=90o α=0 and the projection Pr is reduced to Pr=rmiddotcosθ rsquo Thus for small αrsquos (or forward-going protons) one can use this expression In this configuration neutron imaging is not possibleand the detector can be only used as a spectrometer

22 Determination of neutron energy

The only two measurable quantities available are

bull the total amount of light in the track-L

bull the track projection length-Pr

Ignoring for the time being the energy loss in the capillary walls the amount of light L is propor-tional to the initial proton energy Ep which in turn is related to the incident neutron energy byEp = En cos2 θ In principle knowledge of Ep should yield information on the true track lengthr in the volume of the bundle Knowing the relation between the measured Pr and r provides anestimate of θ Using Ep and the relationship Ep = En cos2 θ one can now determine En

Clearly in a physical system of liquid capillaries which include non-negligible capillary wallsthe situation is more complicated In order to study this issue the capillary system was modelledvia Monte-Carlo simulations

3 Detector simulations

Detailed capillary detector simulations were performed in order to find relations necessary forreconstruction of the incident neutron energy namely recoil-proton energy and recoil-proton track-length vs total created light

31 Detector geometry

An array of 500times 500 round quartz capillaries (refractive index=14632) a section of which isshown in figure 4 was simulated Individual capillary dimensions were 11 microm in diameter (1microm wall thickness) and 3 cm in length The scintillator within the capillaries had the followingproperties density 0964 grcm3 HC ratio 1249 refractive index=157 which correspond to theEJ309 liquid scintillator manufactured by ELJEN company [11]

In the simulations the response of the capillary array in both irradiation geometries parallel tothe capillary array axis and perpendicular to it were investigated The following parameters wererecorded for each recoil proton created

bull Point of interaction

bull Number of capillaries traversed (track-length projection)

ndash 5 ndash

2012 JINST 7 C04021

Figure 4 A section of the capillary array (perpendicular irradiation)

bull Full track length in microm

bull Initial recoil proton energy

bull Energy deposited in each capillary traversed by the proton

bull Number of photons created in each capillary along the track

bull Origin of proton creation (capillary core or capillary wall)

bull For perpendicular irradiation the angle α between the track projection and direction of theincident neutron

The above parameters were determined for incident neutrons with energies of 4 10 13 and 20 MeVFigures 5a and 5b show two recoil proton track-projections (different magnification scale)

generated by 20 MeV neutrons 132 and 6 capillaries long (corresponding to 1716 microm and 78 micromprojected track lengths) respectively As can be seen each track exhibits increased scintillationlight emission at one end indicating the Bragg peak Thus it is possible to determine with highaccuracy (tens of microns) the location of the neutron interaction

Figure 6 shows the relation between proton range (in microns) and energy (in MeV) vs totallight yield per track for 20 MeV incident neutrons

Using these relationships it is possible to determine the proton energy and its track length bymeasuring the total amount of light in the track

32 Reconstruction of neutron energy

The reconstruction of neutron energy from simulated data is performed for each detected neutronusing the following method

bull Count the number of capillaries traversed by the recoil proton

bull Convert this number into the projected proton track length Pr (in microm)

bull Determine the total amount of light in the track

ndash 6 ndash

2012 JINST 7 C04021

a) b)

Figure 5 Images of simulated recoil proton track-projections at different magnification scales a) 132capillaries long (1716 microm) b) 6 capillaries long (78 microm) The colour code is a measure for the amount oflight in a capillary (in number of light photons)

Figure 6 Proton range (left) and energy (right) vs total light in track

bull Use the relations of figure 6 to determine proton energy Ep and its range r from the totalamount of light in the track

bull Determine angle α for neutrons incident perpendicular to capillary axis

Using these parameters and eq (21) or in the case of perpendicular irradiation eq (22) togetherwith eq (23) the energy En of the incident neutron can be determined

Figure 7 shows the reconstructed spectra of neutrons for incident energies 4 10 13 and20 MeV The energy resolution is 17ndash2 MeV (FWHM) Noticeably the peaks are quite broad atthe base of the distribution

The main reason for this broadening is the inaccuracy in converting the number of capillariesin the projection into projection length (in microm) especially for short projections In order to reducethis error it is important that the diameter of the capillaries and the thickness of the walls be assmall as possible

ndash 7 ndash

2012 JINST 7 C04021

Figure 7 Reconstructed neutron spectra (simulated data perpendicular irradiation)

The spectra in figure 7 show only tracks where the projected length is larger than 3 capillariesThe distributions can be improved if we reject the short projections Figure 8 shows a reconstructedmonoenergetic 4 MeV neutron spectrum for tracks with projected length larger than 7 capillariesAs can be observed the spectrum is narrower and without tails However rejecting the short tracksreduces the counting efficiency at this neutron energy by a factor of 2

4 Experimental setup

The capillary detector seen in figure 9 consists of the following components capillary array filledwith liquid scintillator lens assembly time-gated image-intensifier and cooled CCD camera

41 Capillary array

In order to achieve high energy- and spatial-resolution the capillary cross-sectional dimensionsshould be as small as possible Figure 10 shows a small capillary array made from silica glass(14 cm in width 3 cm in length refractive index n=14632) made by the XOS company [12]The dimensions of a single capillary are inner average diameter 11 microm wall thickness 1 microm theusable free area (excluding the triangular patterns in figure 10) is about 72

ndash 8 ndash

2012 JINST 7 C04021

Figure 8 Reconstructed 4 MeV neutron spectrum track projection length gt 7 capillaries

Figure 9 The capillary detector setup

The array was filled by capillary action with EJ309 scintillator manufactured by ELJEN [11]presenting the following characteristics refractive index n=157 light yield = 11500 photonsMeVeedensity 0965 gcm HC ratio = 125 light output 75 of anthracene

42 Optics and track imaging

The rear end of the capillary array was viewed by a magnifying tandem lens configuration whichconsists of f=200 mm and f=50 mm lenses mounted face-to-face resulting in a magnification factorof 4 The image was focused on the photocathode of a 25 mm diameter gateable two multi-channelplates (MCP) image intensifier manufactured by Proxitronic Germany The intensified image atthe phosphor of the intensifier was viewed by a cooled CCD camera ML16083 manufactured byFinger Lakes Instrumentation The CCD size was 4096times 4096 pixels with pixel dimensions of

ndash 9 ndash

2012 JINST 7 C04021

Figure 10 Left Capillary array manufactured by XOS Right enlarged section of the capillary arrayshowing individual capillaries imaged by an electronic microscope

9times9 microm2

43 Irradiation configuration

The detector was tested in a neutron beam using the PTB cyclotron The capillary array wasirradiated from the side ie perpendicular to its central axis by neutrons created in the 9Be(dn)reaction using a 12 MeV deuteron beam that impinges on a thick Be target This reaction yieldsan intense broad-energy neutron spectrum with sufficient neutron intensity up to about 16 MeVThe deuteron beam was pulsed insim1ndash2 ns bursts at 2 MHz repetition rate The average deuteroncurrent was 14 microA Neutron energy was selected using the Time-of-Flight (TOF) technique Thiswas accomplished by time-gating the image-intensifier such that it acts as a very fast shutter (on ans time-scale) collecting light after a pre-determined time relative to the time the deuteron pulseimpinged on the Be target The image intensifier opening time was about 10 ns The detector-targetdistance was 354 m

5 Experimental results

The images were collected by opening the camera for about 1ndash2 seconds thus ensuring that onaverage an image will contain no more than 2 neutron tracks Images were obtained for 2 neutronenergies 86 plusmn12 MeV and 155 plusmn3 MeV

Figure 11a shows an entire image (all CCD pixels) obtained for neutron energy 155 MeV Thisis a net image after the readout and dark noises have been removed In addition a threshold wasapplied such that all values below 16 ADC units (ADU) were set to zero

One can observe a very large number of point-like low-light-intensity tracks and a single pro-ton continuous track (indicated by the arrow) The projection length of this track is 298 micromFigure 11b shows an enlarged image of the proton track Figure 11c shows the same image withthe light intensity threshold increased to 180 ADU

The spots are mainly due to gamma-rays and their number was found to be correlated with theintensity of gamma rays The separation between gamma-ray and neutron events is quite evidentfrom figures 11 Protons generated by neutrons in the scintillating liquid exhibit bright continuoustracks with a Bragg peak at their end Gamma-ray-induced electrons generate small faint blobs of

ndash 10 ndash

2012 JINST 7 C04021

Figure 11 CCD image of the capillary array a) entire image b) image zoomed on the proton track c)thresholded image neutron energy 155 MeV

Figure 12 Examples of 3 proton tracks obtained for 155 MeV neutrons

light that appear in figure 11a as a multitude of specks The separation of electron from protonevents can in principle be performed automatically by using light and track length thresholdingpixel connectivity analysis and the existence of a Bragg peak As is evident from figure 11c asimple thresholding procedure removes most of the gamma-ray induced events

Figure 12 shows 3 additional samples of proton tracks obtained in this run In all of them theBragg peak is clearly discernible

At this stage of the project the identification of the proton tracks has been performed by visualinspection of the CCD images This is a rather labor-consuming task so that a computerized trackrecognition procedure needs to be developed for rapid automatic identification of proton tracks ina large number of CCD images In this experiment about 60 proton tracks with projection lengthabove 7 capillaries were collected and used in the energy reconstruction The energy reconstructionprocedure is identical to that described in section 32

Figure 13 shows the reconstructed energy spectrum Clearly the event statistics is rather poornevertheless there is strong evidence for the existence of two neutron groups at about 86 MeV andat 155 MeV In this reconstruction no correction for variability in light emission over the capillarybundle cross-section was made In addition one has to take into account the non-uniformitiesof light collection and of the image-intensifier photocathode quantum-efficiency over the imagearea The correction for these effects can be made by illuminating the capillary bundle with auniform light source and registering the relative intensity for each CCD pixel This can be used forcorrection of track light intensity

ndash 11 ndash

2012 JINST 7 C04021

Figure 13 Reconstructed spectrum of 86 and 155 MeV neutrons

6 Conclusions

An imaging neutron detector consisting of micro-capillaries filled with a high refractive indexliquid scintillator has been developed The important properties of the detector are high positionresolution (tens of microns) and good rejection of gamma-ray events The detection efficiency isdependent on the size of the capillary array and in principle can be in the range of 10ndash20

The energy reconstruction method presented here based on the determination of light yieldand the projection length of each proton track resulted in 10ndash15 energy resolution in the energyrange of 4ndash20 MeV This energy resolution is inferior by a factor of 2ndash4 to that obtainable withnon-imaging organic spectrometers that use unfolding algorithms [13] and is certainly poorer thanTOF spectrometry However its energy resolution is much better than that of the Bonner multi-sphere method [13] It is planned to investigate the standard unfolding reconstruction procedureswith the capillary detector in order to achieve better energy resolution and yet maintain the highposition resolution

For high resolution neutron imaging and for neutron spectroscopy based on the method de-scribed here the direction of the incident neutron relatively to the capillary axis direction must beknown If the detector is intended for non-spectroscopic purposes but efficient gamma-ray rejec-tion is none-the-less required it is possible to use larger diameter capillaries and the direction ofthe incident neutron is of no importance

References

[1] JM Ryan et al A scintillating plastic fiber tracking detector for neutron and proton imaging andspectroscopy Nucl Instrum Meth A 422 (1999) 49

ndash 12 ndash

2012 JINST 7 C04021

and energy of the recoil proton from a single elastic scatter The detector is composed of an arrayof 64 square scintillating plastic fibers (BCF12 of St Gobain [5]) 05times 05times100 mm3 with 23 mmspacings between the fibers

Furthermore Disdier et al [6] described a capillary array detector originally developed atCERN for the CHORUS collaboration [7] It was filled with deuterated liquid scintillator for high-resolution neutron imaging of laser-imploded D-T targets They employed 50 mm long 100x100mm2 coherent arrays of glass capillaries with 85 microm-diameter pores However neutron spec-troscopy with this detector has not been reported

In 2005 the Soreq PTB and Bern University collaboration investigated a capillary array of 20microm in diameter fibres developed by the CHORUS collaboration at CERN The capillaries werefilled with high-refractive-index liquid scintillator developed at Soreq for a Gamma-ray ResonanceAbsorption (GRA) detector [8] The detector was tested with gamma-rays and mixed gamma andneutron events produced by radioactive sources The experiment showed very promising albeitqualitative results Preliminary computer simulation of the detector indicated that it is possible toreconstruct the energy of the incident neutrons provided the proton track projection is determinedwith sufficient accuracy

In this context our principal interest is fast neutron radiography which requires detectors withthe following properties

bull High neutron efficiency (gt 10)

bull Large area or long linear arrays for high resolution radiography of voluminous objects

bull Sub-mm position-resolution capabilities

bull Neutron spectroscopy (for rejection of scattered radiation)

bull Insensitivity to gamma-rays

The imaging neutron detectors we have developed to-date were for Fast Neutron Resonance Ra-diography (FNRR) [10 11] These detectors perform high energy resolution spectroscopy bythe method of measuring neutron time-of flight (TOF) This method requires operating with ananosecond-pulsed neutron source such as a particle accelerator using an intense pulsed deuteronbeam

In this paper we describe the development of a micro-capillary bundle detector filled withliquid scintillator that will permit high spatial resolution imaging and medium-quality energy spec-troscopy of non-pulsed fast neutron sources available from continuous beam particle acceleratorsisotopic neutron sources or reactor beams

2 The concept of the capillary based detector

Figure 1 shows schematically the concept of the capillary bundle detector The detector is basedon a capillary array filled with high-refractive-index liquid scintillator The principal fast neutroninteraction (in the energy range 08ndash14 MeV) within the liquid scintillator is elastic scattering withhydrogen and to a somewhat lesser extent carbon as both elements have comparable atomic den-sity in such substances For the detection process scattering by hydrogen is dominant at such

ndash 2 ndash

2012 JINST 7 C04021

Figure 1 Description of the concept of the capillary bundle detector

energies since the scattered hydrogen nucleus also denoted a recoil-proton is primarily responsi-ble for the excitation of the scintillator molecules leading to emission of scintillation light Thecarbon recoil is of much lower energy and the amount of scintillation light is furthermore stronglyquenched therefore this process does not play a major role in the detection process

The proton on the other hand has a significant energy (up to the original neutron energy) andtherefore a range that can be as large as several millimetres During its motion within the bundle itcreates scintillation light inside the capillaries it traverses A fraction of this light will travel to theend of these capillaries via total internal reflection and is registered in the optical readout systemthereby creating a projection of the proton track

Assuming the incident neutron flight direction relative to the capillary bundle axis is known(this is true for radiographic systems) and there are no multiple neutron interactions in the bundleone can calculate the incident neutron energy using two parameters that characterize the protontrack

1 the amount of light created along the recoil proton track

2 the recoil-proton track-length projection on the capillary array face plane

21 Irradiation geometries

Referring to the configuration of figure 1 we investigated two irradiation geometries Irradiationparallel to the capillary bundle axis and irradiation perpendicular to it

211 Irradiation parallel to the capillary bundle axis

Figure 2-left describes schematically the direction of neutron irradiation and the proton track view-ing direction in geometry of irradiation parallel to the capillary axis x Figure 2-right shows thedependence of proton track projection on polar and azimuthal angles θ and φ respectively andtrack length r Such a configuration possesses axial symmetry and the length of track projection isdependent only on θ and proton energy The energy of the proton is

Ep = En cos2θ

The length of the track r is a function of proton energy Moreover the track projection Pr is relatedto track length r by

Pr = r middot sinθ (21)

ndash 3 ndash

2012 JINST 7 C04021

Figure 2 Geometry of irradiation parallel the capillary axis x

Figure 3 Geometry of irradiation perpendicular to the capillary axis (along z axis)

The projections are usually short since forward-going protons (small θ ) will have high energies(that produce more light) but short track projections As θ increases the proton energy decreasesthus giving rise to shorter tracks and creating less light

The crucial feature for the present application is that this configuration permits neutron imag-ing with very high position resolution (typically tens of microns)

212 Irradiation perpendicular to the capillary bundle axis

Here the projection Pr is related to track length r by

Pr = (r2 sin2θprime cos2

ϕprime+ r2 cos2

θprime)12 (22)

ndash 4 ndash

2012 JINST 7 C04021

There is no axial symmetry along the capillary axis but in contrast to the previous configurationthe projection length always increases with the length of the proton track

The projection angle α relative to the direction of the incident neutron is

tgα = tgθprime cosφ

prime (23)

For φ rsquo=90o α=0 and the projection Pr is reduced to Pr=rmiddotcosθ rsquo Thus for small αrsquos (or forward-going protons) one can use this expression In this configuration neutron imaging is not possibleand the detector can be only used as a spectrometer

22 Determination of neutron energy

The only two measurable quantities available are

bull the total amount of light in the track-L

bull the track projection length-Pr

Ignoring for the time being the energy loss in the capillary walls the amount of light L is propor-tional to the initial proton energy Ep which in turn is related to the incident neutron energy byEp = En cos2 θ In principle knowledge of Ep should yield information on the true track lengthr in the volume of the bundle Knowing the relation between the measured Pr and r provides anestimate of θ Using Ep and the relationship Ep = En cos2 θ one can now determine En

Clearly in a physical system of liquid capillaries which include non-negligible capillary wallsthe situation is more complicated In order to study this issue the capillary system was modelledvia Monte-Carlo simulations

3 Detector simulations

Detailed capillary detector simulations were performed in order to find relations necessary forreconstruction of the incident neutron energy namely recoil-proton energy and recoil-proton track-length vs total created light

31 Detector geometry

An array of 500times 500 round quartz capillaries (refractive index=14632) a section of which isshown in figure 4 was simulated Individual capillary dimensions were 11 microm in diameter (1microm wall thickness) and 3 cm in length The scintillator within the capillaries had the followingproperties density 0964 grcm3 HC ratio 1249 refractive index=157 which correspond to theEJ309 liquid scintillator manufactured by ELJEN company [11]

In the simulations the response of the capillary array in both irradiation geometries parallel tothe capillary array axis and perpendicular to it were investigated The following parameters wererecorded for each recoil proton created

bull Point of interaction

bull Number of capillaries traversed (track-length projection)

ndash 5 ndash

2012 JINST 7 C04021

Figure 4 A section of the capillary array (perpendicular irradiation)

bull Full track length in microm

bull Initial recoil proton energy

bull Energy deposited in each capillary traversed by the proton

bull Number of photons created in each capillary along the track

bull Origin of proton creation (capillary core or capillary wall)

bull For perpendicular irradiation the angle α between the track projection and direction of theincident neutron

The above parameters were determined for incident neutrons with energies of 4 10 13 and 20 MeVFigures 5a and 5b show two recoil proton track-projections (different magnification scale)

generated by 20 MeV neutrons 132 and 6 capillaries long (corresponding to 1716 microm and 78 micromprojected track lengths) respectively As can be seen each track exhibits increased scintillationlight emission at one end indicating the Bragg peak Thus it is possible to determine with highaccuracy (tens of microns) the location of the neutron interaction

Figure 6 shows the relation between proton range (in microns) and energy (in MeV) vs totallight yield per track for 20 MeV incident neutrons

Using these relationships it is possible to determine the proton energy and its track length bymeasuring the total amount of light in the track

32 Reconstruction of neutron energy

The reconstruction of neutron energy from simulated data is performed for each detected neutronusing the following method

bull Count the number of capillaries traversed by the recoil proton

bull Convert this number into the projected proton track length Pr (in microm)

bull Determine the total amount of light in the track

ndash 6 ndash

2012 JINST 7 C04021

a) b)

Figure 5 Images of simulated recoil proton track-projections at different magnification scales a) 132capillaries long (1716 microm) b) 6 capillaries long (78 microm) The colour code is a measure for the amount oflight in a capillary (in number of light photons)

Figure 6 Proton range (left) and energy (right) vs total light in track

bull Use the relations of figure 6 to determine proton energy Ep and its range r from the totalamount of light in the track

bull Determine angle α for neutrons incident perpendicular to capillary axis

Using these parameters and eq (21) or in the case of perpendicular irradiation eq (22) togetherwith eq (23) the energy En of the incident neutron can be determined

Figure 7 shows the reconstructed spectra of neutrons for incident energies 4 10 13 and20 MeV The energy resolution is 17ndash2 MeV (FWHM) Noticeably the peaks are quite broad atthe base of the distribution

The main reason for this broadening is the inaccuracy in converting the number of capillariesin the projection into projection length (in microm) especially for short projections In order to reducethis error it is important that the diameter of the capillaries and the thickness of the walls be assmall as possible

ndash 7 ndash

2012 JINST 7 C04021

Figure 7 Reconstructed neutron spectra (simulated data perpendicular irradiation)

The spectra in figure 7 show only tracks where the projected length is larger than 3 capillariesThe distributions can be improved if we reject the short projections Figure 8 shows a reconstructedmonoenergetic 4 MeV neutron spectrum for tracks with projected length larger than 7 capillariesAs can be observed the spectrum is narrower and without tails However rejecting the short tracksreduces the counting efficiency at this neutron energy by a factor of 2

4 Experimental setup

The capillary detector seen in figure 9 consists of the following components capillary array filledwith liquid scintillator lens assembly time-gated image-intensifier and cooled CCD camera

41 Capillary array

In order to achieve high energy- and spatial-resolution the capillary cross-sectional dimensionsshould be as small as possible Figure 10 shows a small capillary array made from silica glass(14 cm in width 3 cm in length refractive index n=14632) made by the XOS company [12]The dimensions of a single capillary are inner average diameter 11 microm wall thickness 1 microm theusable free area (excluding the triangular patterns in figure 10) is about 72

ndash 8 ndash

2012 JINST 7 C04021

Figure 8 Reconstructed 4 MeV neutron spectrum track projection length gt 7 capillaries

Figure 9 The capillary detector setup

The array was filled by capillary action with EJ309 scintillator manufactured by ELJEN [11]presenting the following characteristics refractive index n=157 light yield = 11500 photonsMeVeedensity 0965 gcm HC ratio = 125 light output 75 of anthracene

42 Optics and track imaging

The rear end of the capillary array was viewed by a magnifying tandem lens configuration whichconsists of f=200 mm and f=50 mm lenses mounted face-to-face resulting in a magnification factorof 4 The image was focused on the photocathode of a 25 mm diameter gateable two multi-channelplates (MCP) image intensifier manufactured by Proxitronic Germany The intensified image atthe phosphor of the intensifier was viewed by a cooled CCD camera ML16083 manufactured byFinger Lakes Instrumentation The CCD size was 4096times 4096 pixels with pixel dimensions of

ndash 9 ndash

2012 JINST 7 C04021

Figure 10 Left Capillary array manufactured by XOS Right enlarged section of the capillary arrayshowing individual capillaries imaged by an electronic microscope

9times9 microm2

43 Irradiation configuration

The detector was tested in a neutron beam using the PTB cyclotron The capillary array wasirradiated from the side ie perpendicular to its central axis by neutrons created in the 9Be(dn)reaction using a 12 MeV deuteron beam that impinges on a thick Be target This reaction yieldsan intense broad-energy neutron spectrum with sufficient neutron intensity up to about 16 MeVThe deuteron beam was pulsed insim1ndash2 ns bursts at 2 MHz repetition rate The average deuteroncurrent was 14 microA Neutron energy was selected using the Time-of-Flight (TOF) technique Thiswas accomplished by time-gating the image-intensifier such that it acts as a very fast shutter (on ans time-scale) collecting light after a pre-determined time relative to the time the deuteron pulseimpinged on the Be target The image intensifier opening time was about 10 ns The detector-targetdistance was 354 m

5 Experimental results

The images were collected by opening the camera for about 1ndash2 seconds thus ensuring that onaverage an image will contain no more than 2 neutron tracks Images were obtained for 2 neutronenergies 86 plusmn12 MeV and 155 plusmn3 MeV

Figure 11a shows an entire image (all CCD pixels) obtained for neutron energy 155 MeV Thisis a net image after the readout and dark noises have been removed In addition a threshold wasapplied such that all values below 16 ADC units (ADU) were set to zero

One can observe a very large number of point-like low-light-intensity tracks and a single pro-ton continuous track (indicated by the arrow) The projection length of this track is 298 micromFigure 11b shows an enlarged image of the proton track Figure 11c shows the same image withthe light intensity threshold increased to 180 ADU

The spots are mainly due to gamma-rays and their number was found to be correlated with theintensity of gamma rays The separation between gamma-ray and neutron events is quite evidentfrom figures 11 Protons generated by neutrons in the scintillating liquid exhibit bright continuoustracks with a Bragg peak at their end Gamma-ray-induced electrons generate small faint blobs of

ndash 10 ndash

2012 JINST 7 C04021

Figure 11 CCD image of the capillary array a) entire image b) image zoomed on the proton track c)thresholded image neutron energy 155 MeV

Figure 12 Examples of 3 proton tracks obtained for 155 MeV neutrons

light that appear in figure 11a as a multitude of specks The separation of electron from protonevents can in principle be performed automatically by using light and track length thresholdingpixel connectivity analysis and the existence of a Bragg peak As is evident from figure 11c asimple thresholding procedure removes most of the gamma-ray induced events

Figure 12 shows 3 additional samples of proton tracks obtained in this run In all of them theBragg peak is clearly discernible

At this stage of the project the identification of the proton tracks has been performed by visualinspection of the CCD images This is a rather labor-consuming task so that a computerized trackrecognition procedure needs to be developed for rapid automatic identification of proton tracks ina large number of CCD images In this experiment about 60 proton tracks with projection lengthabove 7 capillaries were collected and used in the energy reconstruction The energy reconstructionprocedure is identical to that described in section 32

Figure 13 shows the reconstructed energy spectrum Clearly the event statistics is rather poornevertheless there is strong evidence for the existence of two neutron groups at about 86 MeV andat 155 MeV In this reconstruction no correction for variability in light emission over the capillarybundle cross-section was made In addition one has to take into account the non-uniformitiesof light collection and of the image-intensifier photocathode quantum-efficiency over the imagearea The correction for these effects can be made by illuminating the capillary bundle with auniform light source and registering the relative intensity for each CCD pixel This can be used forcorrection of track light intensity

ndash 11 ndash

2012 JINST 7 C04021

Figure 13 Reconstructed spectrum of 86 and 155 MeV neutrons

6 Conclusions

An imaging neutron detector consisting of micro-capillaries filled with a high refractive indexliquid scintillator has been developed The important properties of the detector are high positionresolution (tens of microns) and good rejection of gamma-ray events The detection efficiency isdependent on the size of the capillary array and in principle can be in the range of 10ndash20

The energy reconstruction method presented here based on the determination of light yieldand the projection length of each proton track resulted in 10ndash15 energy resolution in the energyrange of 4ndash20 MeV This energy resolution is inferior by a factor of 2ndash4 to that obtainable withnon-imaging organic spectrometers that use unfolding algorithms [13] and is certainly poorer thanTOF spectrometry However its energy resolution is much better than that of the Bonner multi-sphere method [13] It is planned to investigate the standard unfolding reconstruction procedureswith the capillary detector in order to achieve better energy resolution and yet maintain the highposition resolution

For high resolution neutron imaging and for neutron spectroscopy based on the method de-scribed here the direction of the incident neutron relatively to the capillary axis direction must beknown If the detector is intended for non-spectroscopic purposes but efficient gamma-ray rejec-tion is none-the-less required it is possible to use larger diameter capillaries and the direction ofthe incident neutron is of no importance

References

[1] JM Ryan et al A scintillating plastic fiber tracking detector for neutron and proton imaging andspectroscopy Nucl Instrum Meth A 422 (1999) 49

ndash 12 ndash

2012 JINST 7 C04021

Figure 1 Description of the concept of the capillary bundle detector

energies since the scattered hydrogen nucleus also denoted a recoil-proton is primarily responsi-ble for the excitation of the scintillator molecules leading to emission of scintillation light Thecarbon recoil is of much lower energy and the amount of scintillation light is furthermore stronglyquenched therefore this process does not play a major role in the detection process

The proton on the other hand has a significant energy (up to the original neutron energy) andtherefore a range that can be as large as several millimetres During its motion within the bundle itcreates scintillation light inside the capillaries it traverses A fraction of this light will travel to theend of these capillaries via total internal reflection and is registered in the optical readout systemthereby creating a projection of the proton track

Assuming the incident neutron flight direction relative to the capillary bundle axis is known(this is true for radiographic systems) and there are no multiple neutron interactions in the bundleone can calculate the incident neutron energy using two parameters that characterize the protontrack

1 the amount of light created along the recoil proton track

2 the recoil-proton track-length projection on the capillary array face plane

21 Irradiation geometries

Referring to the configuration of figure 1 we investigated two irradiation geometries Irradiationparallel to the capillary bundle axis and irradiation perpendicular to it

211 Irradiation parallel to the capillary bundle axis

Figure 2-left describes schematically the direction of neutron irradiation and the proton track view-ing direction in geometry of irradiation parallel to the capillary axis x Figure 2-right shows thedependence of proton track projection on polar and azimuthal angles θ and φ respectively andtrack length r Such a configuration possesses axial symmetry and the length of track projection isdependent only on θ and proton energy The energy of the proton is

Ep = En cos2θ

The length of the track r is a function of proton energy Moreover the track projection Pr is relatedto track length r by

Pr = r middot sinθ (21)

ndash 3 ndash

2012 JINST 7 C04021

Figure 2 Geometry of irradiation parallel the capillary axis x

Figure 3 Geometry of irradiation perpendicular to the capillary axis (along z axis)

The projections are usually short since forward-going protons (small θ ) will have high energies(that produce more light) but short track projections As θ increases the proton energy decreasesthus giving rise to shorter tracks and creating less light

The crucial feature for the present application is that this configuration permits neutron imag-ing with very high position resolution (typically tens of microns)

212 Irradiation perpendicular to the capillary bundle axis

Here the projection Pr is related to track length r by

Pr = (r2 sin2θprime cos2

ϕprime+ r2 cos2

θprime)12 (22)

ndash 4 ndash

2012 JINST 7 C04021

There is no axial symmetry along the capillary axis but in contrast to the previous configurationthe projection length always increases with the length of the proton track

The projection angle α relative to the direction of the incident neutron is

tgα = tgθprime cosφ

prime (23)

For φ rsquo=90o α=0 and the projection Pr is reduced to Pr=rmiddotcosθ rsquo Thus for small αrsquos (or forward-going protons) one can use this expression In this configuration neutron imaging is not possibleand the detector can be only used as a spectrometer

22 Determination of neutron energy

The only two measurable quantities available are

bull the total amount of light in the track-L

bull the track projection length-Pr

Ignoring for the time being the energy loss in the capillary walls the amount of light L is propor-tional to the initial proton energy Ep which in turn is related to the incident neutron energy byEp = En cos2 θ In principle knowledge of Ep should yield information on the true track lengthr in the volume of the bundle Knowing the relation between the measured Pr and r provides anestimate of θ Using Ep and the relationship Ep = En cos2 θ one can now determine En

Clearly in a physical system of liquid capillaries which include non-negligible capillary wallsthe situation is more complicated In order to study this issue the capillary system was modelledvia Monte-Carlo simulations

3 Detector simulations

Detailed capillary detector simulations were performed in order to find relations necessary forreconstruction of the incident neutron energy namely recoil-proton energy and recoil-proton track-length vs total created light

31 Detector geometry

An array of 500times 500 round quartz capillaries (refractive index=14632) a section of which isshown in figure 4 was simulated Individual capillary dimensions were 11 microm in diameter (1microm wall thickness) and 3 cm in length The scintillator within the capillaries had the followingproperties density 0964 grcm3 HC ratio 1249 refractive index=157 which correspond to theEJ309 liquid scintillator manufactured by ELJEN company [11]

In the simulations the response of the capillary array in both irradiation geometries parallel tothe capillary array axis and perpendicular to it were investigated The following parameters wererecorded for each recoil proton created

bull Point of interaction

bull Number of capillaries traversed (track-length projection)

ndash 5 ndash

2012 JINST 7 C04021

Figure 4 A section of the capillary array (perpendicular irradiation)

bull Full track length in microm

bull Initial recoil proton energy

bull Energy deposited in each capillary traversed by the proton

bull Number of photons created in each capillary along the track

bull Origin of proton creation (capillary core or capillary wall)

bull For perpendicular irradiation the angle α between the track projection and direction of theincident neutron

The above parameters were determined for incident neutrons with energies of 4 10 13 and 20 MeVFigures 5a and 5b show two recoil proton track-projections (different magnification scale)

generated by 20 MeV neutrons 132 and 6 capillaries long (corresponding to 1716 microm and 78 micromprojected track lengths) respectively As can be seen each track exhibits increased scintillationlight emission at one end indicating the Bragg peak Thus it is possible to determine with highaccuracy (tens of microns) the location of the neutron interaction

Figure 6 shows the relation between proton range (in microns) and energy (in MeV) vs totallight yield per track for 20 MeV incident neutrons

Using these relationships it is possible to determine the proton energy and its track length bymeasuring the total amount of light in the track

32 Reconstruction of neutron energy

The reconstruction of neutron energy from simulated data is performed for each detected neutronusing the following method

bull Count the number of capillaries traversed by the recoil proton

bull Convert this number into the projected proton track length Pr (in microm)

bull Determine the total amount of light in the track

ndash 6 ndash

2012 JINST 7 C04021

a) b)

Figure 5 Images of simulated recoil proton track-projections at different magnification scales a) 132capillaries long (1716 microm) b) 6 capillaries long (78 microm) The colour code is a measure for the amount oflight in a capillary (in number of light photons)

Figure 6 Proton range (left) and energy (right) vs total light in track

bull Use the relations of figure 6 to determine proton energy Ep and its range r from the totalamount of light in the track

bull Determine angle α for neutrons incident perpendicular to capillary axis

Using these parameters and eq (21) or in the case of perpendicular irradiation eq (22) togetherwith eq (23) the energy En of the incident neutron can be determined

Figure 7 shows the reconstructed spectra of neutrons for incident energies 4 10 13 and20 MeV The energy resolution is 17ndash2 MeV (FWHM) Noticeably the peaks are quite broad atthe base of the distribution

The main reason for this broadening is the inaccuracy in converting the number of capillariesin the projection into projection length (in microm) especially for short projections In order to reducethis error it is important that the diameter of the capillaries and the thickness of the walls be assmall as possible

ndash 7 ndash

2012 JINST 7 C04021

Figure 7 Reconstructed neutron spectra (simulated data perpendicular irradiation)

The spectra in figure 7 show only tracks where the projected length is larger than 3 capillariesThe distributions can be improved if we reject the short projections Figure 8 shows a reconstructedmonoenergetic 4 MeV neutron spectrum for tracks with projected length larger than 7 capillariesAs can be observed the spectrum is narrower and without tails However rejecting the short tracksreduces the counting efficiency at this neutron energy by a factor of 2

4 Experimental setup

The capillary detector seen in figure 9 consists of the following components capillary array filledwith liquid scintillator lens assembly time-gated image-intensifier and cooled CCD camera

41 Capillary array

In order to achieve high energy- and spatial-resolution the capillary cross-sectional dimensionsshould be as small as possible Figure 10 shows a small capillary array made from silica glass(14 cm in width 3 cm in length refractive index n=14632) made by the XOS company [12]The dimensions of a single capillary are inner average diameter 11 microm wall thickness 1 microm theusable free area (excluding the triangular patterns in figure 10) is about 72

ndash 8 ndash

2012 JINST 7 C04021

Figure 8 Reconstructed 4 MeV neutron spectrum track projection length gt 7 capillaries

Figure 9 The capillary detector setup

The array was filled by capillary action with EJ309 scintillator manufactured by ELJEN [11]presenting the following characteristics refractive index n=157 light yield = 11500 photonsMeVeedensity 0965 gcm HC ratio = 125 light output 75 of anthracene

42 Optics and track imaging

The rear end of the capillary array was viewed by a magnifying tandem lens configuration whichconsists of f=200 mm and f=50 mm lenses mounted face-to-face resulting in a magnification factorof 4 The image was focused on the photocathode of a 25 mm diameter gateable two multi-channelplates (MCP) image intensifier manufactured by Proxitronic Germany The intensified image atthe phosphor of the intensifier was viewed by a cooled CCD camera ML16083 manufactured byFinger Lakes Instrumentation The CCD size was 4096times 4096 pixels with pixel dimensions of

ndash 9 ndash

2012 JINST 7 C04021

Figure 10 Left Capillary array manufactured by XOS Right enlarged section of the capillary arrayshowing individual capillaries imaged by an electronic microscope

9times9 microm2

43 Irradiation configuration

The detector was tested in a neutron beam using the PTB cyclotron The capillary array wasirradiated from the side ie perpendicular to its central axis by neutrons created in the 9Be(dn)reaction using a 12 MeV deuteron beam that impinges on a thick Be target This reaction yieldsan intense broad-energy neutron spectrum with sufficient neutron intensity up to about 16 MeVThe deuteron beam was pulsed insim1ndash2 ns bursts at 2 MHz repetition rate The average deuteroncurrent was 14 microA Neutron energy was selected using the Time-of-Flight (TOF) technique Thiswas accomplished by time-gating the image-intensifier such that it acts as a very fast shutter (on ans time-scale) collecting light after a pre-determined time relative to the time the deuteron pulseimpinged on the Be target The image intensifier opening time was about 10 ns The detector-targetdistance was 354 m

5 Experimental results

The images were collected by opening the camera for about 1ndash2 seconds thus ensuring that onaverage an image will contain no more than 2 neutron tracks Images were obtained for 2 neutronenergies 86 plusmn12 MeV and 155 plusmn3 MeV

Figure 11a shows an entire image (all CCD pixels) obtained for neutron energy 155 MeV Thisis a net image after the readout and dark noises have been removed In addition a threshold wasapplied such that all values below 16 ADC units (ADU) were set to zero

One can observe a very large number of point-like low-light-intensity tracks and a single pro-ton continuous track (indicated by the arrow) The projection length of this track is 298 micromFigure 11b shows an enlarged image of the proton track Figure 11c shows the same image withthe light intensity threshold increased to 180 ADU

The spots are mainly due to gamma-rays and their number was found to be correlated with theintensity of gamma rays The separation between gamma-ray and neutron events is quite evidentfrom figures 11 Protons generated by neutrons in the scintillating liquid exhibit bright continuoustracks with a Bragg peak at their end Gamma-ray-induced electrons generate small faint blobs of

ndash 10 ndash

2012 JINST 7 C04021

Figure 11 CCD image of the capillary array a) entire image b) image zoomed on the proton track c)thresholded image neutron energy 155 MeV

Figure 12 Examples of 3 proton tracks obtained for 155 MeV neutrons

light that appear in figure 11a as a multitude of specks The separation of electron from protonevents can in principle be performed automatically by using light and track length thresholdingpixel connectivity analysis and the existence of a Bragg peak As is evident from figure 11c asimple thresholding procedure removes most of the gamma-ray induced events

Figure 12 shows 3 additional samples of proton tracks obtained in this run In all of them theBragg peak is clearly discernible

At this stage of the project the identification of the proton tracks has been performed by visualinspection of the CCD images This is a rather labor-consuming task so that a computerized trackrecognition procedure needs to be developed for rapid automatic identification of proton tracks ina large number of CCD images In this experiment about 60 proton tracks with projection lengthabove 7 capillaries were collected and used in the energy reconstruction The energy reconstructionprocedure is identical to that described in section 32

Figure 13 shows the reconstructed energy spectrum Clearly the event statistics is rather poornevertheless there is strong evidence for the existence of two neutron groups at about 86 MeV andat 155 MeV In this reconstruction no correction for variability in light emission over the capillarybundle cross-section was made In addition one has to take into account the non-uniformitiesof light collection and of the image-intensifier photocathode quantum-efficiency over the imagearea The correction for these effects can be made by illuminating the capillary bundle with auniform light source and registering the relative intensity for each CCD pixel This can be used forcorrection of track light intensity

ndash 11 ndash

2012 JINST 7 C04021

Figure 13 Reconstructed spectrum of 86 and 155 MeV neutrons

6 Conclusions

An imaging neutron detector consisting of micro-capillaries filled with a high refractive indexliquid scintillator has been developed The important properties of the detector are high positionresolution (tens of microns) and good rejection of gamma-ray events The detection efficiency isdependent on the size of the capillary array and in principle can be in the range of 10ndash20

The energy reconstruction method presented here based on the determination of light yieldand the projection length of each proton track resulted in 10ndash15 energy resolution in the energyrange of 4ndash20 MeV This energy resolution is inferior by a factor of 2ndash4 to that obtainable withnon-imaging organic spectrometers that use unfolding algorithms [13] and is certainly poorer thanTOF spectrometry However its energy resolution is much better than that of the Bonner multi-sphere method [13] It is planned to investigate the standard unfolding reconstruction procedureswith the capillary detector in order to achieve better energy resolution and yet maintain the highposition resolution

For high resolution neutron imaging and for neutron spectroscopy based on the method de-scribed here the direction of the incident neutron relatively to the capillary axis direction must beknown If the detector is intended for non-spectroscopic purposes but efficient gamma-ray rejec-tion is none-the-less required it is possible to use larger diameter capillaries and the direction ofthe incident neutron is of no importance

References

[1] JM Ryan et al A scintillating plastic fiber tracking detector for neutron and proton imaging andspectroscopy Nucl Instrum Meth A 422 (1999) 49

ndash 12 ndash

2012 JINST 7 C04021

Figure 2 Geometry of irradiation parallel the capillary axis x

Figure 3 Geometry of irradiation perpendicular to the capillary axis (along z axis)

The projections are usually short since forward-going protons (small θ ) will have high energies(that produce more light) but short track projections As θ increases the proton energy decreasesthus giving rise to shorter tracks and creating less light

The crucial feature for the present application is that this configuration permits neutron imag-ing with very high position resolution (typically tens of microns)

212 Irradiation perpendicular to the capillary bundle axis

Here the projection Pr is related to track length r by

Pr = (r2 sin2θprime cos2

ϕprime+ r2 cos2

θprime)12 (22)

ndash 4 ndash

2012 JINST 7 C04021

There is no axial symmetry along the capillary axis but in contrast to the previous configurationthe projection length always increases with the length of the proton track

The projection angle α relative to the direction of the incident neutron is

tgα = tgθprime cosφ

prime (23)

For φ rsquo=90o α=0 and the projection Pr is reduced to Pr=rmiddotcosθ rsquo Thus for small αrsquos (or forward-going protons) one can use this expression In this configuration neutron imaging is not possibleand the detector can be only used as a spectrometer

22 Determination of neutron energy

The only two measurable quantities available are

bull the total amount of light in the track-L

bull the track projection length-Pr

Ignoring for the time being the energy loss in the capillary walls the amount of light L is propor-tional to the initial proton energy Ep which in turn is related to the incident neutron energy byEp = En cos2 θ In principle knowledge of Ep should yield information on the true track lengthr in the volume of the bundle Knowing the relation between the measured Pr and r provides anestimate of θ Using Ep and the relationship Ep = En cos2 θ one can now determine En

Clearly in a physical system of liquid capillaries which include non-negligible capillary wallsthe situation is more complicated In order to study this issue the capillary system was modelledvia Monte-Carlo simulations

3 Detector simulations

Detailed capillary detector simulations were performed in order to find relations necessary forreconstruction of the incident neutron energy namely recoil-proton energy and recoil-proton track-length vs total created light

31 Detector geometry

An array of 500times 500 round quartz capillaries (refractive index=14632) a section of which isshown in figure 4 was simulated Individual capillary dimensions were 11 microm in diameter (1microm wall thickness) and 3 cm in length The scintillator within the capillaries had the followingproperties density 0964 grcm3 HC ratio 1249 refractive index=157 which correspond to theEJ309 liquid scintillator manufactured by ELJEN company [11]

In the simulations the response of the capillary array in both irradiation geometries parallel tothe capillary array axis and perpendicular to it were investigated The following parameters wererecorded for each recoil proton created

bull Point of interaction

bull Number of capillaries traversed (track-length projection)

ndash 5 ndash

2012 JINST 7 C04021

Figure 4 A section of the capillary array (perpendicular irradiation)

bull Full track length in microm

bull Initial recoil proton energy

bull Energy deposited in each capillary traversed by the proton

bull Number of photons created in each capillary along the track

bull Origin of proton creation (capillary core or capillary wall)

bull For perpendicular irradiation the angle α between the track projection and direction of theincident neutron

The above parameters were determined for incident neutrons with energies of 4 10 13 and 20 MeVFigures 5a and 5b show two recoil proton track-projections (different magnification scale)

generated by 20 MeV neutrons 132 and 6 capillaries long (corresponding to 1716 microm and 78 micromprojected track lengths) respectively As can be seen each track exhibits increased scintillationlight emission at one end indicating the Bragg peak Thus it is possible to determine with highaccuracy (tens of microns) the location of the neutron interaction

Figure 6 shows the relation between proton range (in microns) and energy (in MeV) vs totallight yield per track for 20 MeV incident neutrons

Using these relationships it is possible to determine the proton energy and its track length bymeasuring the total amount of light in the track

32 Reconstruction of neutron energy

The reconstruction of neutron energy from simulated data is performed for each detected neutronusing the following method

bull Count the number of capillaries traversed by the recoil proton

bull Convert this number into the projected proton track length Pr (in microm)

bull Determine the total amount of light in the track

ndash 6 ndash

2012 JINST 7 C04021

a) b)

Figure 5 Images of simulated recoil proton track-projections at different magnification scales a) 132capillaries long (1716 microm) b) 6 capillaries long (78 microm) The colour code is a measure for the amount oflight in a capillary (in number of light photons)

Figure 6 Proton range (left) and energy (right) vs total light in track

bull Use the relations of figure 6 to determine proton energy Ep and its range r from the totalamount of light in the track

bull Determine angle α for neutrons incident perpendicular to capillary axis

Using these parameters and eq (21) or in the case of perpendicular irradiation eq (22) togetherwith eq (23) the energy En of the incident neutron can be determined

Figure 7 shows the reconstructed spectra of neutrons for incident energies 4 10 13 and20 MeV The energy resolution is 17ndash2 MeV (FWHM) Noticeably the peaks are quite broad atthe base of the distribution

The main reason for this broadening is the inaccuracy in converting the number of capillariesin the projection into projection length (in microm) especially for short projections In order to reducethis error it is important that the diameter of the capillaries and the thickness of the walls be assmall as possible

ndash 7 ndash

2012 JINST 7 C04021

Figure 7 Reconstructed neutron spectra (simulated data perpendicular irradiation)

The spectra in figure 7 show only tracks where the projected length is larger than 3 capillariesThe distributions can be improved if we reject the short projections Figure 8 shows a reconstructedmonoenergetic 4 MeV neutron spectrum for tracks with projected length larger than 7 capillariesAs can be observed the spectrum is narrower and without tails However rejecting the short tracksreduces the counting efficiency at this neutron energy by a factor of 2

4 Experimental setup

The capillary detector seen in figure 9 consists of the following components capillary array filledwith liquid scintillator lens assembly time-gated image-intensifier and cooled CCD camera

41 Capillary array

In order to achieve high energy- and spatial-resolution the capillary cross-sectional dimensionsshould be as small as possible Figure 10 shows a small capillary array made from silica glass(14 cm in width 3 cm in length refractive index n=14632) made by the XOS company [12]The dimensions of a single capillary are inner average diameter 11 microm wall thickness 1 microm theusable free area (excluding the triangular patterns in figure 10) is about 72

ndash 8 ndash

2012 JINST 7 C04021

Figure 8 Reconstructed 4 MeV neutron spectrum track projection length gt 7 capillaries

Figure 9 The capillary detector setup

The array was filled by capillary action with EJ309 scintillator manufactured by ELJEN [11]presenting the following characteristics refractive index n=157 light yield = 11500 photonsMeVeedensity 0965 gcm HC ratio = 125 light output 75 of anthracene

42 Optics and track imaging

The rear end of the capillary array was viewed by a magnifying tandem lens configuration whichconsists of f=200 mm and f=50 mm lenses mounted face-to-face resulting in a magnification factorof 4 The image was focused on the photocathode of a 25 mm diameter gateable two multi-channelplates (MCP) image intensifier manufactured by Proxitronic Germany The intensified image atthe phosphor of the intensifier was viewed by a cooled CCD camera ML16083 manufactured byFinger Lakes Instrumentation The CCD size was 4096times 4096 pixels with pixel dimensions of

ndash 9 ndash

2012 JINST 7 C04021

Figure 10 Left Capillary array manufactured by XOS Right enlarged section of the capillary arrayshowing individual capillaries imaged by an electronic microscope

9times9 microm2

43 Irradiation configuration

The detector was tested in a neutron beam using the PTB cyclotron The capillary array wasirradiated from the side ie perpendicular to its central axis by neutrons created in the 9Be(dn)reaction using a 12 MeV deuteron beam that impinges on a thick Be target This reaction yieldsan intense broad-energy neutron spectrum with sufficient neutron intensity up to about 16 MeVThe deuteron beam was pulsed insim1ndash2 ns bursts at 2 MHz repetition rate The average deuteroncurrent was 14 microA Neutron energy was selected using the Time-of-Flight (TOF) technique Thiswas accomplished by time-gating the image-intensifier such that it acts as a very fast shutter (on ans time-scale) collecting light after a pre-determined time relative to the time the deuteron pulseimpinged on the Be target The image intensifier opening time was about 10 ns The detector-targetdistance was 354 m

5 Experimental results

The images were collected by opening the camera for about 1ndash2 seconds thus ensuring that onaverage an image will contain no more than 2 neutron tracks Images were obtained for 2 neutronenergies 86 plusmn12 MeV and 155 plusmn3 MeV

Figure 11a shows an entire image (all CCD pixels) obtained for neutron energy 155 MeV Thisis a net image after the readout and dark noises have been removed In addition a threshold wasapplied such that all values below 16 ADC units (ADU) were set to zero

One can observe a very large number of point-like low-light-intensity tracks and a single pro-ton continuous track (indicated by the arrow) The projection length of this track is 298 micromFigure 11b shows an enlarged image of the proton track Figure 11c shows the same image withthe light intensity threshold increased to 180 ADU

The spots are mainly due to gamma-rays and their number was found to be correlated with theintensity of gamma rays The separation between gamma-ray and neutron events is quite evidentfrom figures 11 Protons generated by neutrons in the scintillating liquid exhibit bright continuoustracks with a Bragg peak at their end Gamma-ray-induced electrons generate small faint blobs of

ndash 10 ndash

2012 JINST 7 C04021

Figure 11 CCD image of the capillary array a) entire image b) image zoomed on the proton track c)thresholded image neutron energy 155 MeV

Figure 12 Examples of 3 proton tracks obtained for 155 MeV neutrons

light that appear in figure 11a as a multitude of specks The separation of electron from protonevents can in principle be performed automatically by using light and track length thresholdingpixel connectivity analysis and the existence of a Bragg peak As is evident from figure 11c asimple thresholding procedure removes most of the gamma-ray induced events

Figure 12 shows 3 additional samples of proton tracks obtained in this run In all of them theBragg peak is clearly discernible

At this stage of the project the identification of the proton tracks has been performed by visualinspection of the CCD images This is a rather labor-consuming task so that a computerized trackrecognition procedure needs to be developed for rapid automatic identification of proton tracks ina large number of CCD images In this experiment about 60 proton tracks with projection lengthabove 7 capillaries were collected and used in the energy reconstruction The energy reconstructionprocedure is identical to that described in section 32

Figure 13 shows the reconstructed energy spectrum Clearly the event statistics is rather poornevertheless there is strong evidence for the existence of two neutron groups at about 86 MeV andat 155 MeV In this reconstruction no correction for variability in light emission over the capillarybundle cross-section was made In addition one has to take into account the non-uniformitiesof light collection and of the image-intensifier photocathode quantum-efficiency over the imagearea The correction for these effects can be made by illuminating the capillary bundle with auniform light source and registering the relative intensity for each CCD pixel This can be used forcorrection of track light intensity

ndash 11 ndash

2012 JINST 7 C04021

Figure 13 Reconstructed spectrum of 86 and 155 MeV neutrons

6 Conclusions

An imaging neutron detector consisting of micro-capillaries filled with a high refractive indexliquid scintillator has been developed The important properties of the detector are high positionresolution (tens of microns) and good rejection of gamma-ray events The detection efficiency isdependent on the size of the capillary array and in principle can be in the range of 10ndash20

The energy reconstruction method presented here based on the determination of light yieldand the projection length of each proton track resulted in 10ndash15 energy resolution in the energyrange of 4ndash20 MeV This energy resolution is inferior by a factor of 2ndash4 to that obtainable withnon-imaging organic spectrometers that use unfolding algorithms [13] and is certainly poorer thanTOF spectrometry However its energy resolution is much better than that of the Bonner multi-sphere method [13] It is planned to investigate the standard unfolding reconstruction procedureswith the capillary detector in order to achieve better energy resolution and yet maintain the highposition resolution

For high resolution neutron imaging and for neutron spectroscopy based on the method de-scribed here the direction of the incident neutron relatively to the capillary axis direction must beknown If the detector is intended for non-spectroscopic purposes but efficient gamma-ray rejec-tion is none-the-less required it is possible to use larger diameter capillaries and the direction ofthe incident neutron is of no importance

References

[1] JM Ryan et al A scintillating plastic fiber tracking detector for neutron and proton imaging andspectroscopy Nucl Instrum Meth A 422 (1999) 49

ndash 12 ndash

2012 JINST 7 C04021

There is no axial symmetry along the capillary axis but in contrast to the previous configurationthe projection length always increases with the length of the proton track

The projection angle α relative to the direction of the incident neutron is

tgα = tgθprime cosφ

prime (23)

For φ rsquo=90o α=0 and the projection Pr is reduced to Pr=rmiddotcosθ rsquo Thus for small αrsquos (or forward-going protons) one can use this expression In this configuration neutron imaging is not possibleand the detector can be only used as a spectrometer

22 Determination of neutron energy

The only two measurable quantities available are

bull the total amount of light in the track-L

bull the track projection length-Pr

Ignoring for the time being the energy loss in the capillary walls the amount of light L is propor-tional to the initial proton energy Ep which in turn is related to the incident neutron energy byEp = En cos2 θ In principle knowledge of Ep should yield information on the true track lengthr in the volume of the bundle Knowing the relation between the measured Pr and r provides anestimate of θ Using Ep and the relationship Ep = En cos2 θ one can now determine En

Clearly in a physical system of liquid capillaries which include non-negligible capillary wallsthe situation is more complicated In order to study this issue the capillary system was modelledvia Monte-Carlo simulations

3 Detector simulations

Detailed capillary detector simulations were performed in order to find relations necessary forreconstruction of the incident neutron energy namely recoil-proton energy and recoil-proton track-length vs total created light

31 Detector geometry

An array of 500times 500 round quartz capillaries (refractive index=14632) a section of which isshown in figure 4 was simulated Individual capillary dimensions were 11 microm in diameter (1microm wall thickness) and 3 cm in length The scintillator within the capillaries had the followingproperties density 0964 grcm3 HC ratio 1249 refractive index=157 which correspond to theEJ309 liquid scintillator manufactured by ELJEN company [11]

In the simulations the response of the capillary array in both irradiation geometries parallel tothe capillary array axis and perpendicular to it were investigated The following parameters wererecorded for each recoil proton created

bull Point of interaction

bull Number of capillaries traversed (track-length projection)

ndash 5 ndash

2012 JINST 7 C04021

Figure 4 A section of the capillary array (perpendicular irradiation)

bull Full track length in microm

bull Initial recoil proton energy

bull Energy deposited in each capillary traversed by the proton

bull Number of photons created in each capillary along the track

bull Origin of proton creation (capillary core or capillary wall)

bull For perpendicular irradiation the angle α between the track projection and direction of theincident neutron

The above parameters were determined for incident neutrons with energies of 4 10 13 and 20 MeVFigures 5a and 5b show two recoil proton track-projections (different magnification scale)

generated by 20 MeV neutrons 132 and 6 capillaries long (corresponding to 1716 microm and 78 micromprojected track lengths) respectively As can be seen each track exhibits increased scintillationlight emission at one end indicating the Bragg peak Thus it is possible to determine with highaccuracy (tens of microns) the location of the neutron interaction

Figure 6 shows the relation between proton range (in microns) and energy (in MeV) vs totallight yield per track for 20 MeV incident neutrons

Using these relationships it is possible to determine the proton energy and its track length bymeasuring the total amount of light in the track

32 Reconstruction of neutron energy

The reconstruction of neutron energy from simulated data is performed for each detected neutronusing the following method

bull Count the number of capillaries traversed by the recoil proton

bull Convert this number into the projected proton track length Pr (in microm)

bull Determine the total amount of light in the track

ndash 6 ndash

2012 JINST 7 C04021

a) b)

Figure 5 Images of simulated recoil proton track-projections at different magnification scales a) 132capillaries long (1716 microm) b) 6 capillaries long (78 microm) The colour code is a measure for the amount oflight in a capillary (in number of light photons)

Figure 6 Proton range (left) and energy (right) vs total light in track

bull Use the relations of figure 6 to determine proton energy Ep and its range r from the totalamount of light in the track

bull Determine angle α for neutrons incident perpendicular to capillary axis

Using these parameters and eq (21) or in the case of perpendicular irradiation eq (22) togetherwith eq (23) the energy En of the incident neutron can be determined

Figure 7 shows the reconstructed spectra of neutrons for incident energies 4 10 13 and20 MeV The energy resolution is 17ndash2 MeV (FWHM) Noticeably the peaks are quite broad atthe base of the distribution

The main reason for this broadening is the inaccuracy in converting the number of capillariesin the projection into projection length (in microm) especially for short projections In order to reducethis error it is important that the diameter of the capillaries and the thickness of the walls be assmall as possible

ndash 7 ndash

2012 JINST 7 C04021

Figure 7 Reconstructed neutron spectra (simulated data perpendicular irradiation)

The spectra in figure 7 show only tracks where the projected length is larger than 3 capillariesThe distributions can be improved if we reject the short projections Figure 8 shows a reconstructedmonoenergetic 4 MeV neutron spectrum for tracks with projected length larger than 7 capillariesAs can be observed the spectrum is narrower and without tails However rejecting the short tracksreduces the counting efficiency at this neutron energy by a factor of 2

4 Experimental setup

The capillary detector seen in figure 9 consists of the following components capillary array filledwith liquid scintillator lens assembly time-gated image-intensifier and cooled CCD camera

41 Capillary array

In order to achieve high energy- and spatial-resolution the capillary cross-sectional dimensionsshould be as small as possible Figure 10 shows a small capillary array made from silica glass(14 cm in width 3 cm in length refractive index n=14632) made by the XOS company [12]The dimensions of a single capillary are inner average diameter 11 microm wall thickness 1 microm theusable free area (excluding the triangular patterns in figure 10) is about 72

ndash 8 ndash

2012 JINST 7 C04021

Figure 8 Reconstructed 4 MeV neutron spectrum track projection length gt 7 capillaries

Figure 9 The capillary detector setup

The array was filled by capillary action with EJ309 scintillator manufactured by ELJEN [11]presenting the following characteristics refractive index n=157 light yield = 11500 photonsMeVeedensity 0965 gcm HC ratio = 125 light output 75 of anthracene

42 Optics and track imaging

The rear end of the capillary array was viewed by a magnifying tandem lens configuration whichconsists of f=200 mm and f=50 mm lenses mounted face-to-face resulting in a magnification factorof 4 The image was focused on the photocathode of a 25 mm diameter gateable two multi-channelplates (MCP) image intensifier manufactured by Proxitronic Germany The intensified image atthe phosphor of the intensifier was viewed by a cooled CCD camera ML16083 manufactured byFinger Lakes Instrumentation The CCD size was 4096times 4096 pixels with pixel dimensions of

ndash 9 ndash

2012 JINST 7 C04021

Figure 10 Left Capillary array manufactured by XOS Right enlarged section of the capillary arrayshowing individual capillaries imaged by an electronic microscope

9times9 microm2

43 Irradiation configuration

The detector was tested in a neutron beam using the PTB cyclotron The capillary array wasirradiated from the side ie perpendicular to its central axis by neutrons created in the 9Be(dn)reaction using a 12 MeV deuteron beam that impinges on a thick Be target This reaction yieldsan intense broad-energy neutron spectrum with sufficient neutron intensity up to about 16 MeVThe deuteron beam was pulsed insim1ndash2 ns bursts at 2 MHz repetition rate The average deuteroncurrent was 14 microA Neutron energy was selected using the Time-of-Flight (TOF) technique Thiswas accomplished by time-gating the image-intensifier such that it acts as a very fast shutter (on ans time-scale) collecting light after a pre-determined time relative to the time the deuteron pulseimpinged on the Be target The image intensifier opening time was about 10 ns The detector-targetdistance was 354 m

5 Experimental results

The images were collected by opening the camera for about 1ndash2 seconds thus ensuring that onaverage an image will contain no more than 2 neutron tracks Images were obtained for 2 neutronenergies 86 plusmn12 MeV and 155 plusmn3 MeV

Figure 11a shows an entire image (all CCD pixels) obtained for neutron energy 155 MeV Thisis a net image after the readout and dark noises have been removed In addition a threshold wasapplied such that all values below 16 ADC units (ADU) were set to zero

One can observe a very large number of point-like low-light-intensity tracks and a single pro-ton continuous track (indicated by the arrow) The projection length of this track is 298 micromFigure 11b shows an enlarged image of the proton track Figure 11c shows the same image withthe light intensity threshold increased to 180 ADU

The spots are mainly due to gamma-rays and their number was found to be correlated with theintensity of gamma rays The separation between gamma-ray and neutron events is quite evidentfrom figures 11 Protons generated by neutrons in the scintillating liquid exhibit bright continuoustracks with a Bragg peak at their end Gamma-ray-induced electrons generate small faint blobs of

ndash 10 ndash

2012 JINST 7 C04021

Figure 11 CCD image of the capillary array a) entire image b) image zoomed on the proton track c)thresholded image neutron energy 155 MeV

Figure 12 Examples of 3 proton tracks obtained for 155 MeV neutrons

light that appear in figure 11a as a multitude of specks The separation of electron from protonevents can in principle be performed automatically by using light and track length thresholdingpixel connectivity analysis and the existence of a Bragg peak As is evident from figure 11c asimple thresholding procedure removes most of the gamma-ray induced events

Figure 12 shows 3 additional samples of proton tracks obtained in this run In all of them theBragg peak is clearly discernible

At this stage of the project the identification of the proton tracks has been performed by visualinspection of the CCD images This is a rather labor-consuming task so that a computerized trackrecognition procedure needs to be developed for rapid automatic identification of proton tracks ina large number of CCD images In this experiment about 60 proton tracks with projection lengthabove 7 capillaries were collected and used in the energy reconstruction The energy reconstructionprocedure is identical to that described in section 32

Figure 13 shows the reconstructed energy spectrum Clearly the event statistics is rather poornevertheless there is strong evidence for the existence of two neutron groups at about 86 MeV andat 155 MeV In this reconstruction no correction for variability in light emission over the capillarybundle cross-section was made In addition one has to take into account the non-uniformitiesof light collection and of the image-intensifier photocathode quantum-efficiency over the imagearea The correction for these effects can be made by illuminating the capillary bundle with auniform light source and registering the relative intensity for each CCD pixel This can be used forcorrection of track light intensity

ndash 11 ndash

2012 JINST 7 C04021

Figure 13 Reconstructed spectrum of 86 and 155 MeV neutrons

6 Conclusions

An imaging neutron detector consisting of micro-capillaries filled with a high refractive indexliquid scintillator has been developed The important properties of the detector are high positionresolution (tens of microns) and good rejection of gamma-ray events The detection efficiency isdependent on the size of the capillary array and in principle can be in the range of 10ndash20

The energy reconstruction method presented here based on the determination of light yieldand the projection length of each proton track resulted in 10ndash15 energy resolution in the energyrange of 4ndash20 MeV This energy resolution is inferior by a factor of 2ndash4 to that obtainable withnon-imaging organic spectrometers that use unfolding algorithms [13] and is certainly poorer thanTOF spectrometry However its energy resolution is much better than that of the Bonner multi-sphere method [13] It is planned to investigate the standard unfolding reconstruction procedureswith the capillary detector in order to achieve better energy resolution and yet maintain the highposition resolution

For high resolution neutron imaging and for neutron spectroscopy based on the method de-scribed here the direction of the incident neutron relatively to the capillary axis direction must beknown If the detector is intended for non-spectroscopic purposes but efficient gamma-ray rejec-tion is none-the-less required it is possible to use larger diameter capillaries and the direction ofthe incident neutron is of no importance

References

[1] JM Ryan et al A scintillating plastic fiber tracking detector for neutron and proton imaging andspectroscopy Nucl Instrum Meth A 422 (1999) 49

ndash 12 ndash

2012 JINST 7 C04021

Figure 4 A section of the capillary array (perpendicular irradiation)

bull Full track length in microm

bull Initial recoil proton energy

bull Energy deposited in each capillary traversed by the proton

bull Number of photons created in each capillary along the track

bull Origin of proton creation (capillary core or capillary wall)

bull For perpendicular irradiation the angle α between the track projection and direction of theincident neutron

The above parameters were determined for incident neutrons with energies of 4 10 13 and 20 MeVFigures 5a and 5b show two recoil proton track-projections (different magnification scale)

generated by 20 MeV neutrons 132 and 6 capillaries long (corresponding to 1716 microm and 78 micromprojected track lengths) respectively As can be seen each track exhibits increased scintillationlight emission at one end indicating the Bragg peak Thus it is possible to determine with highaccuracy (tens of microns) the location of the neutron interaction

Figure 6 shows the relation between proton range (in microns) and energy (in MeV) vs totallight yield per track for 20 MeV incident neutrons

Using these relationships it is possible to determine the proton energy and its track length bymeasuring the total amount of light in the track

32 Reconstruction of neutron energy

The reconstruction of neutron energy from simulated data is performed for each detected neutronusing the following method

bull Count the number of capillaries traversed by the recoil proton

bull Convert this number into the projected proton track length Pr (in microm)

bull Determine the total amount of light in the track

ndash 6 ndash

2012 JINST 7 C04021

a) b)

Figure 5 Images of simulated recoil proton track-projections at different magnification scales a) 132capillaries long (1716 microm) b) 6 capillaries long (78 microm) The colour code is a measure for the amount oflight in a capillary (in number of light photons)

Figure 6 Proton range (left) and energy (right) vs total light in track

bull Use the relations of figure 6 to determine proton energy Ep and its range r from the totalamount of light in the track

bull Determine angle α for neutrons incident perpendicular to capillary axis

Using these parameters and eq (21) or in the case of perpendicular irradiation eq (22) togetherwith eq (23) the energy En of the incident neutron can be determined

Figure 7 shows the reconstructed spectra of neutrons for incident energies 4 10 13 and20 MeV The energy resolution is 17ndash2 MeV (FWHM) Noticeably the peaks are quite broad atthe base of the distribution

The main reason for this broadening is the inaccuracy in converting the number of capillariesin the projection into projection length (in microm) especially for short projections In order to reducethis error it is important that the diameter of the capillaries and the thickness of the walls be assmall as possible

ndash 7 ndash

2012 JINST 7 C04021

Figure 7 Reconstructed neutron spectra (simulated data perpendicular irradiation)

The spectra in figure 7 show only tracks where the projected length is larger than 3 capillariesThe distributions can be improved if we reject the short projections Figure 8 shows a reconstructedmonoenergetic 4 MeV neutron spectrum for tracks with projected length larger than 7 capillariesAs can be observed the spectrum is narrower and without tails However rejecting the short tracksreduces the counting efficiency at this neutron energy by a factor of 2

4 Experimental setup

The capillary detector seen in figure 9 consists of the following components capillary array filledwith liquid scintillator lens assembly time-gated image-intensifier and cooled CCD camera

41 Capillary array

In order to achieve high energy- and spatial-resolution the capillary cross-sectional dimensionsshould be as small as possible Figure 10 shows a small capillary array made from silica glass(14 cm in width 3 cm in length refractive index n=14632) made by the XOS company [12]The dimensions of a single capillary are inner average diameter 11 microm wall thickness 1 microm theusable free area (excluding the triangular patterns in figure 10) is about 72

ndash 8 ndash

2012 JINST 7 C04021

Figure 8 Reconstructed 4 MeV neutron spectrum track projection length gt 7 capillaries

Figure 9 The capillary detector setup

The array was filled by capillary action with EJ309 scintillator manufactured by ELJEN [11]presenting the following characteristics refractive index n=157 light yield = 11500 photonsMeVeedensity 0965 gcm HC ratio = 125 light output 75 of anthracene

42 Optics and track imaging

The rear end of the capillary array was viewed by a magnifying tandem lens configuration whichconsists of f=200 mm and f=50 mm lenses mounted face-to-face resulting in a magnification factorof 4 The image was focused on the photocathode of a 25 mm diameter gateable two multi-channelplates (MCP) image intensifier manufactured by Proxitronic Germany The intensified image atthe phosphor of the intensifier was viewed by a cooled CCD camera ML16083 manufactured byFinger Lakes Instrumentation The CCD size was 4096times 4096 pixels with pixel dimensions of

ndash 9 ndash

2012 JINST 7 C04021

Figure 10 Left Capillary array manufactured by XOS Right enlarged section of the capillary arrayshowing individual capillaries imaged by an electronic microscope

9times9 microm2

43 Irradiation configuration

The detector was tested in a neutron beam using the PTB cyclotron The capillary array wasirradiated from the side ie perpendicular to its central axis by neutrons created in the 9Be(dn)reaction using a 12 MeV deuteron beam that impinges on a thick Be target This reaction yieldsan intense broad-energy neutron spectrum with sufficient neutron intensity up to about 16 MeVThe deuteron beam was pulsed insim1ndash2 ns bursts at 2 MHz repetition rate The average deuteroncurrent was 14 microA Neutron energy was selected using the Time-of-Flight (TOF) technique Thiswas accomplished by time-gating the image-intensifier such that it acts as a very fast shutter (on ans time-scale) collecting light after a pre-determined time relative to the time the deuteron pulseimpinged on the Be target The image intensifier opening time was about 10 ns The detector-targetdistance was 354 m

5 Experimental results

The images were collected by opening the camera for about 1ndash2 seconds thus ensuring that onaverage an image will contain no more than 2 neutron tracks Images were obtained for 2 neutronenergies 86 plusmn12 MeV and 155 plusmn3 MeV

Figure 11a shows an entire image (all CCD pixels) obtained for neutron energy 155 MeV Thisis a net image after the readout and dark noises have been removed In addition a threshold wasapplied such that all values below 16 ADC units (ADU) were set to zero

One can observe a very large number of point-like low-light-intensity tracks and a single pro-ton continuous track (indicated by the arrow) The projection length of this track is 298 micromFigure 11b shows an enlarged image of the proton track Figure 11c shows the same image withthe light intensity threshold increased to 180 ADU

The spots are mainly due to gamma-rays and their number was found to be correlated with theintensity of gamma rays The separation between gamma-ray and neutron events is quite evidentfrom figures 11 Protons generated by neutrons in the scintillating liquid exhibit bright continuoustracks with a Bragg peak at their end Gamma-ray-induced electrons generate small faint blobs of

ndash 10 ndash

2012 JINST 7 C04021

Figure 11 CCD image of the capillary array a) entire image b) image zoomed on the proton track c)thresholded image neutron energy 155 MeV

Figure 12 Examples of 3 proton tracks obtained for 155 MeV neutrons

light that appear in figure 11a as a multitude of specks The separation of electron from protonevents can in principle be performed automatically by using light and track length thresholdingpixel connectivity analysis and the existence of a Bragg peak As is evident from figure 11c asimple thresholding procedure removes most of the gamma-ray induced events

Figure 12 shows 3 additional samples of proton tracks obtained in this run In all of them theBragg peak is clearly discernible

At this stage of the project the identification of the proton tracks has been performed by visualinspection of the CCD images This is a rather labor-consuming task so that a computerized trackrecognition procedure needs to be developed for rapid automatic identification of proton tracks ina large number of CCD images In this experiment about 60 proton tracks with projection lengthabove 7 capillaries were collected and used in the energy reconstruction The energy reconstructionprocedure is identical to that described in section 32

Figure 13 shows the reconstructed energy spectrum Clearly the event statistics is rather poornevertheless there is strong evidence for the existence of two neutron groups at about 86 MeV andat 155 MeV In this reconstruction no correction for variability in light emission over the capillarybundle cross-section was made In addition one has to take into account the non-uniformitiesof light collection and of the image-intensifier photocathode quantum-efficiency over the imagearea The correction for these effects can be made by illuminating the capillary bundle with auniform light source and registering the relative intensity for each CCD pixel This can be used forcorrection of track light intensity

ndash 11 ndash

2012 JINST 7 C04021

Figure 13 Reconstructed spectrum of 86 and 155 MeV neutrons

6 Conclusions

An imaging neutron detector consisting of micro-capillaries filled with a high refractive indexliquid scintillator has been developed The important properties of the detector are high positionresolution (tens of microns) and good rejection of gamma-ray events The detection efficiency isdependent on the size of the capillary array and in principle can be in the range of 10ndash20

The energy reconstruction method presented here based on the determination of light yieldand the projection length of each proton track resulted in 10ndash15 energy resolution in the energyrange of 4ndash20 MeV This energy resolution is inferior by a factor of 2ndash4 to that obtainable withnon-imaging organic spectrometers that use unfolding algorithms [13] and is certainly poorer thanTOF spectrometry However its energy resolution is much better than that of the Bonner multi-sphere method [13] It is planned to investigate the standard unfolding reconstruction procedureswith the capillary detector in order to achieve better energy resolution and yet maintain the highposition resolution

For high resolution neutron imaging and for neutron spectroscopy based on the method de-scribed here the direction of the incident neutron relatively to the capillary axis direction must beknown If the detector is intended for non-spectroscopic purposes but efficient gamma-ray rejec-tion is none-the-less required it is possible to use larger diameter capillaries and the direction ofthe incident neutron is of no importance

References

[1] JM Ryan et al A scintillating plastic fiber tracking detector for neutron and proton imaging andspectroscopy Nucl Instrum Meth A 422 (1999) 49

ndash 12 ndash

2012 JINST 7 C04021

a) b)

Figure 5 Images of simulated recoil proton track-projections at different magnification scales a) 132capillaries long (1716 microm) b) 6 capillaries long (78 microm) The colour code is a measure for the amount oflight in a capillary (in number of light photons)

Figure 6 Proton range (left) and energy (right) vs total light in track

bull Use the relations of figure 6 to determine proton energy Ep and its range r from the totalamount of light in the track

bull Determine angle α for neutrons incident perpendicular to capillary axis

Using these parameters and eq (21) or in the case of perpendicular irradiation eq (22) togetherwith eq (23) the energy En of the incident neutron can be determined

Figure 7 shows the reconstructed spectra of neutrons for incident energies 4 10 13 and20 MeV The energy resolution is 17ndash2 MeV (FWHM) Noticeably the peaks are quite broad atthe base of the distribution

The main reason for this broadening is the inaccuracy in converting the number of capillariesin the projection into projection length (in microm) especially for short projections In order to reducethis error it is important that the diameter of the capillaries and the thickness of the walls be assmall as possible

ndash 7 ndash

2012 JINST 7 C04021

Figure 7 Reconstructed neutron spectra (simulated data perpendicular irradiation)

The spectra in figure 7 show only tracks where the projected length is larger than 3 capillariesThe distributions can be improved if we reject the short projections Figure 8 shows a reconstructedmonoenergetic 4 MeV neutron spectrum for tracks with projected length larger than 7 capillariesAs can be observed the spectrum is narrower and without tails However rejecting the short tracksreduces the counting efficiency at this neutron energy by a factor of 2

4 Experimental setup

The capillary detector seen in figure 9 consists of the following components capillary array filledwith liquid scintillator lens assembly time-gated image-intensifier and cooled CCD camera

41 Capillary array

In order to achieve high energy- and spatial-resolution the capillary cross-sectional dimensionsshould be as small as possible Figure 10 shows a small capillary array made from silica glass(14 cm in width 3 cm in length refractive index n=14632) made by the XOS company [12]The dimensions of a single capillary are inner average diameter 11 microm wall thickness 1 microm theusable free area (excluding the triangular patterns in figure 10) is about 72

ndash 8 ndash

2012 JINST 7 C04021

Figure 8 Reconstructed 4 MeV neutron spectrum track projection length gt 7 capillaries

Figure 9 The capillary detector setup

The array was filled by capillary action with EJ309 scintillator manufactured by ELJEN [11]presenting the following characteristics refractive index n=157 light yield = 11500 photonsMeVeedensity 0965 gcm HC ratio = 125 light output 75 of anthracene

42 Optics and track imaging

The rear end of the capillary array was viewed by a magnifying tandem lens configuration whichconsists of f=200 mm and f=50 mm lenses mounted face-to-face resulting in a magnification factorof 4 The image was focused on the photocathode of a 25 mm diameter gateable two multi-channelplates (MCP) image intensifier manufactured by Proxitronic Germany The intensified image atthe phosphor of the intensifier was viewed by a cooled CCD camera ML16083 manufactured byFinger Lakes Instrumentation The CCD size was 4096times 4096 pixels with pixel dimensions of

ndash 9 ndash

2012 JINST 7 C04021

Figure 10 Left Capillary array manufactured by XOS Right enlarged section of the capillary arrayshowing individual capillaries imaged by an electronic microscope

9times9 microm2

43 Irradiation configuration

The detector was tested in a neutron beam using the PTB cyclotron The capillary array wasirradiated from the side ie perpendicular to its central axis by neutrons created in the 9Be(dn)reaction using a 12 MeV deuteron beam that impinges on a thick Be target This reaction yieldsan intense broad-energy neutron spectrum with sufficient neutron intensity up to about 16 MeVThe deuteron beam was pulsed insim1ndash2 ns bursts at 2 MHz repetition rate The average deuteroncurrent was 14 microA Neutron energy was selected using the Time-of-Flight (TOF) technique Thiswas accomplished by time-gating the image-intensifier such that it acts as a very fast shutter (on ans time-scale) collecting light after a pre-determined time relative to the time the deuteron pulseimpinged on the Be target The image intensifier opening time was about 10 ns The detector-targetdistance was 354 m

5 Experimental results

The images were collected by opening the camera for about 1ndash2 seconds thus ensuring that onaverage an image will contain no more than 2 neutron tracks Images were obtained for 2 neutronenergies 86 plusmn12 MeV and 155 plusmn3 MeV

Figure 11a shows an entire image (all CCD pixels) obtained for neutron energy 155 MeV Thisis a net image after the readout and dark noises have been removed In addition a threshold wasapplied such that all values below 16 ADC units (ADU) were set to zero

One can observe a very large number of point-like low-light-intensity tracks and a single pro-ton continuous track (indicated by the arrow) The projection length of this track is 298 micromFigure 11b shows an enlarged image of the proton track Figure 11c shows the same image withthe light intensity threshold increased to 180 ADU

The spots are mainly due to gamma-rays and their number was found to be correlated with theintensity of gamma rays The separation between gamma-ray and neutron events is quite evidentfrom figures 11 Protons generated by neutrons in the scintillating liquid exhibit bright continuoustracks with a Bragg peak at their end Gamma-ray-induced electrons generate small faint blobs of

ndash 10 ndash

2012 JINST 7 C04021

Figure 11 CCD image of the capillary array a) entire image b) image zoomed on the proton track c)thresholded image neutron energy 155 MeV

Figure 12 Examples of 3 proton tracks obtained for 155 MeV neutrons

light that appear in figure 11a as a multitude of specks The separation of electron from protonevents can in principle be performed automatically by using light and track length thresholdingpixel connectivity analysis and the existence of a Bragg peak As is evident from figure 11c asimple thresholding procedure removes most of the gamma-ray induced events

Figure 12 shows 3 additional samples of proton tracks obtained in this run In all of them theBragg peak is clearly discernible

At this stage of the project the identification of the proton tracks has been performed by visualinspection of the CCD images This is a rather labor-consuming task so that a computerized trackrecognition procedure needs to be developed for rapid automatic identification of proton tracks ina large number of CCD images In this experiment about 60 proton tracks with projection lengthabove 7 capillaries were collected and used in the energy reconstruction The energy reconstructionprocedure is identical to that described in section 32

Figure 13 shows the reconstructed energy spectrum Clearly the event statistics is rather poornevertheless there is strong evidence for the existence of two neutron groups at about 86 MeV andat 155 MeV In this reconstruction no correction for variability in light emission over the capillarybundle cross-section was made In addition one has to take into account the non-uniformitiesof light collection and of the image-intensifier photocathode quantum-efficiency over the imagearea The correction for these effects can be made by illuminating the capillary bundle with auniform light source and registering the relative intensity for each CCD pixel This can be used forcorrection of track light intensity

ndash 11 ndash

2012 JINST 7 C04021

Figure 13 Reconstructed spectrum of 86 and 155 MeV neutrons

6 Conclusions

An imaging neutron detector consisting of micro-capillaries filled with a high refractive indexliquid scintillator has been developed The important properties of the detector are high positionresolution (tens of microns) and good rejection of gamma-ray events The detection efficiency isdependent on the size of the capillary array and in principle can be in the range of 10ndash20

The energy reconstruction method presented here based on the determination of light yieldand the projection length of each proton track resulted in 10ndash15 energy resolution in the energyrange of 4ndash20 MeV This energy resolution is inferior by a factor of 2ndash4 to that obtainable withnon-imaging organic spectrometers that use unfolding algorithms [13] and is certainly poorer thanTOF spectrometry However its energy resolution is much better than that of the Bonner multi-sphere method [13] It is planned to investigate the standard unfolding reconstruction procedureswith the capillary detector in order to achieve better energy resolution and yet maintain the highposition resolution

For high resolution neutron imaging and for neutron spectroscopy based on the method de-scribed here the direction of the incident neutron relatively to the capillary axis direction must beknown If the detector is intended for non-spectroscopic purposes but efficient gamma-ray rejec-tion is none-the-less required it is possible to use larger diameter capillaries and the direction ofthe incident neutron is of no importance

References

[1] JM Ryan et al A scintillating plastic fiber tracking detector for neutron and proton imaging andspectroscopy Nucl Instrum Meth A 422 (1999) 49

ndash 12 ndash

2012 JINST 7 C04021

Figure 7 Reconstructed neutron spectra (simulated data perpendicular irradiation)

The spectra in figure 7 show only tracks where the projected length is larger than 3 capillariesThe distributions can be improved if we reject the short projections Figure 8 shows a reconstructedmonoenergetic 4 MeV neutron spectrum for tracks with projected length larger than 7 capillariesAs can be observed the spectrum is narrower and without tails However rejecting the short tracksreduces the counting efficiency at this neutron energy by a factor of 2

4 Experimental setup

The capillary detector seen in figure 9 consists of the following components capillary array filledwith liquid scintillator lens assembly time-gated image-intensifier and cooled CCD camera

41 Capillary array

In order to achieve high energy- and spatial-resolution the capillary cross-sectional dimensionsshould be as small as possible Figure 10 shows a small capillary array made from silica glass(14 cm in width 3 cm in length refractive index n=14632) made by the XOS company [12]The dimensions of a single capillary are inner average diameter 11 microm wall thickness 1 microm theusable free area (excluding the triangular patterns in figure 10) is about 72

ndash 8 ndash

2012 JINST 7 C04021

Figure 8 Reconstructed 4 MeV neutron spectrum track projection length gt 7 capillaries

Figure 9 The capillary detector setup

The array was filled by capillary action with EJ309 scintillator manufactured by ELJEN [11]presenting the following characteristics refractive index n=157 light yield = 11500 photonsMeVeedensity 0965 gcm HC ratio = 125 light output 75 of anthracene

42 Optics and track imaging

The rear end of the capillary array was viewed by a magnifying tandem lens configuration whichconsists of f=200 mm and f=50 mm lenses mounted face-to-face resulting in a magnification factorof 4 The image was focused on the photocathode of a 25 mm diameter gateable two multi-channelplates (MCP) image intensifier manufactured by Proxitronic Germany The intensified image atthe phosphor of the intensifier was viewed by a cooled CCD camera ML16083 manufactured byFinger Lakes Instrumentation The CCD size was 4096times 4096 pixels with pixel dimensions of

ndash 9 ndash

2012 JINST 7 C04021

Figure 10 Left Capillary array manufactured by XOS Right enlarged section of the capillary arrayshowing individual capillaries imaged by an electronic microscope

9times9 microm2

43 Irradiation configuration

The detector was tested in a neutron beam using the PTB cyclotron The capillary array wasirradiated from the side ie perpendicular to its central axis by neutrons created in the 9Be(dn)reaction using a 12 MeV deuteron beam that impinges on a thick Be target This reaction yieldsan intense broad-energy neutron spectrum with sufficient neutron intensity up to about 16 MeVThe deuteron beam was pulsed insim1ndash2 ns bursts at 2 MHz repetition rate The average deuteroncurrent was 14 microA Neutron energy was selected using the Time-of-Flight (TOF) technique Thiswas accomplished by time-gating the image-intensifier such that it acts as a very fast shutter (on ans time-scale) collecting light after a pre-determined time relative to the time the deuteron pulseimpinged on the Be target The image intensifier opening time was about 10 ns The detector-targetdistance was 354 m

5 Experimental results

The images were collected by opening the camera for about 1ndash2 seconds thus ensuring that onaverage an image will contain no more than 2 neutron tracks Images were obtained for 2 neutronenergies 86 plusmn12 MeV and 155 plusmn3 MeV

Figure 11a shows an entire image (all CCD pixels) obtained for neutron energy 155 MeV Thisis a net image after the readout and dark noises have been removed In addition a threshold wasapplied such that all values below 16 ADC units (ADU) were set to zero

One can observe a very large number of point-like low-light-intensity tracks and a single pro-ton continuous track (indicated by the arrow) The projection length of this track is 298 micromFigure 11b shows an enlarged image of the proton track Figure 11c shows the same image withthe light intensity threshold increased to 180 ADU

The spots are mainly due to gamma-rays and their number was found to be correlated with theintensity of gamma rays The separation between gamma-ray and neutron events is quite evidentfrom figures 11 Protons generated by neutrons in the scintillating liquid exhibit bright continuoustracks with a Bragg peak at their end Gamma-ray-induced electrons generate small faint blobs of

ndash 10 ndash

2012 JINST 7 C04021

Figure 11 CCD image of the capillary array a) entire image b) image zoomed on the proton track c)thresholded image neutron energy 155 MeV

Figure 12 Examples of 3 proton tracks obtained for 155 MeV neutrons

light that appear in figure 11a as a multitude of specks The separation of electron from protonevents can in principle be performed automatically by using light and track length thresholdingpixel connectivity analysis and the existence of a Bragg peak As is evident from figure 11c asimple thresholding procedure removes most of the gamma-ray induced events

Figure 12 shows 3 additional samples of proton tracks obtained in this run In all of them theBragg peak is clearly discernible

At this stage of the project the identification of the proton tracks has been performed by visualinspection of the CCD images This is a rather labor-consuming task so that a computerized trackrecognition procedure needs to be developed for rapid automatic identification of proton tracks ina large number of CCD images In this experiment about 60 proton tracks with projection lengthabove 7 capillaries were collected and used in the energy reconstruction The energy reconstructionprocedure is identical to that described in section 32

Figure 13 shows the reconstructed energy spectrum Clearly the event statistics is rather poornevertheless there is strong evidence for the existence of two neutron groups at about 86 MeV andat 155 MeV In this reconstruction no correction for variability in light emission over the capillarybundle cross-section was made In addition one has to take into account the non-uniformitiesof light collection and of the image-intensifier photocathode quantum-efficiency over the imagearea The correction for these effects can be made by illuminating the capillary bundle with auniform light source and registering the relative intensity for each CCD pixel This can be used forcorrection of track light intensity

ndash 11 ndash

2012 JINST 7 C04021

Figure 13 Reconstructed spectrum of 86 and 155 MeV neutrons

6 Conclusions

An imaging neutron detector consisting of micro-capillaries filled with a high refractive indexliquid scintillator has been developed The important properties of the detector are high positionresolution (tens of microns) and good rejection of gamma-ray events The detection efficiency isdependent on the size of the capillary array and in principle can be in the range of 10ndash20

The energy reconstruction method presented here based on the determination of light yieldand the projection length of each proton track resulted in 10ndash15 energy resolution in the energyrange of 4ndash20 MeV This energy resolution is inferior by a factor of 2ndash4 to that obtainable withnon-imaging organic spectrometers that use unfolding algorithms [13] and is certainly poorer thanTOF spectrometry However its energy resolution is much better than that of the Bonner multi-sphere method [13] It is planned to investigate the standard unfolding reconstruction procedureswith the capillary detector in order to achieve better energy resolution and yet maintain the highposition resolution

For high resolution neutron imaging and for neutron spectroscopy based on the method de-scribed here the direction of the incident neutron relatively to the capillary axis direction must beknown If the detector is intended for non-spectroscopic purposes but efficient gamma-ray rejec-tion is none-the-less required it is possible to use larger diameter capillaries and the direction ofthe incident neutron is of no importance

References

[1] JM Ryan et al A scintillating plastic fiber tracking detector for neutron and proton imaging andspectroscopy Nucl Instrum Meth A 422 (1999) 49

ndash 12 ndash

2012 JINST 7 C04021

Figure 8 Reconstructed 4 MeV neutron spectrum track projection length gt 7 capillaries

Figure 9 The capillary detector setup

The array was filled by capillary action with EJ309 scintillator manufactured by ELJEN [11]presenting the following characteristics refractive index n=157 light yield = 11500 photonsMeVeedensity 0965 gcm HC ratio = 125 light output 75 of anthracene

42 Optics and track imaging

The rear end of the capillary array was viewed by a magnifying tandem lens configuration whichconsists of f=200 mm and f=50 mm lenses mounted face-to-face resulting in a magnification factorof 4 The image was focused on the photocathode of a 25 mm diameter gateable two multi-channelplates (MCP) image intensifier manufactured by Proxitronic Germany The intensified image atthe phosphor of the intensifier was viewed by a cooled CCD camera ML16083 manufactured byFinger Lakes Instrumentation The CCD size was 4096times 4096 pixels with pixel dimensions of

ndash 9 ndash

2012 JINST 7 C04021

Figure 10 Left Capillary array manufactured by XOS Right enlarged section of the capillary arrayshowing individual capillaries imaged by an electronic microscope

9times9 microm2

43 Irradiation configuration

The detector was tested in a neutron beam using the PTB cyclotron The capillary array wasirradiated from the side ie perpendicular to its central axis by neutrons created in the 9Be(dn)reaction using a 12 MeV deuteron beam that impinges on a thick Be target This reaction yieldsan intense broad-energy neutron spectrum with sufficient neutron intensity up to about 16 MeVThe deuteron beam was pulsed insim1ndash2 ns bursts at 2 MHz repetition rate The average deuteroncurrent was 14 microA Neutron energy was selected using the Time-of-Flight (TOF) technique Thiswas accomplished by time-gating the image-intensifier such that it acts as a very fast shutter (on ans time-scale) collecting light after a pre-determined time relative to the time the deuteron pulseimpinged on the Be target The image intensifier opening time was about 10 ns The detector-targetdistance was 354 m

5 Experimental results

The images were collected by opening the camera for about 1ndash2 seconds thus ensuring that onaverage an image will contain no more than 2 neutron tracks Images were obtained for 2 neutronenergies 86 plusmn12 MeV and 155 plusmn3 MeV

Figure 11a shows an entire image (all CCD pixels) obtained for neutron energy 155 MeV Thisis a net image after the readout and dark noises have been removed In addition a threshold wasapplied such that all values below 16 ADC units (ADU) were set to zero

One can observe a very large number of point-like low-light-intensity tracks and a single pro-ton continuous track (indicated by the arrow) The projection length of this track is 298 micromFigure 11b shows an enlarged image of the proton track Figure 11c shows the same image withthe light intensity threshold increased to 180 ADU

The spots are mainly due to gamma-rays and their number was found to be correlated with theintensity of gamma rays The separation between gamma-ray and neutron events is quite evidentfrom figures 11 Protons generated by neutrons in the scintillating liquid exhibit bright continuoustracks with a Bragg peak at their end Gamma-ray-induced electrons generate small faint blobs of

ndash 10 ndash

2012 JINST 7 C04021

Figure 11 CCD image of the capillary array a) entire image b) image zoomed on the proton track c)thresholded image neutron energy 155 MeV

Figure 12 Examples of 3 proton tracks obtained for 155 MeV neutrons

light that appear in figure 11a as a multitude of specks The separation of electron from protonevents can in principle be performed automatically by using light and track length thresholdingpixel connectivity analysis and the existence of a Bragg peak As is evident from figure 11c asimple thresholding procedure removes most of the gamma-ray induced events

Figure 12 shows 3 additional samples of proton tracks obtained in this run In all of them theBragg peak is clearly discernible

At this stage of the project the identification of the proton tracks has been performed by visualinspection of the CCD images This is a rather labor-consuming task so that a computerized trackrecognition procedure needs to be developed for rapid automatic identification of proton tracks ina large number of CCD images In this experiment about 60 proton tracks with projection lengthabove 7 capillaries were collected and used in the energy reconstruction The energy reconstructionprocedure is identical to that described in section 32

Figure 13 shows the reconstructed energy spectrum Clearly the event statistics is rather poornevertheless there is strong evidence for the existence of two neutron groups at about 86 MeV andat 155 MeV In this reconstruction no correction for variability in light emission over the capillarybundle cross-section was made In addition one has to take into account the non-uniformitiesof light collection and of the image-intensifier photocathode quantum-efficiency over the imagearea The correction for these effects can be made by illuminating the capillary bundle with auniform light source and registering the relative intensity for each CCD pixel This can be used forcorrection of track light intensity

ndash 11 ndash

2012 JINST 7 C04021

Figure 13 Reconstructed spectrum of 86 and 155 MeV neutrons

6 Conclusions

An imaging neutron detector consisting of micro-capillaries filled with a high refractive indexliquid scintillator has been developed The important properties of the detector are high positionresolution (tens of microns) and good rejection of gamma-ray events The detection efficiency isdependent on the size of the capillary array and in principle can be in the range of 10ndash20

The energy reconstruction method presented here based on the determination of light yieldand the projection length of each proton track resulted in 10ndash15 energy resolution in the energyrange of 4ndash20 MeV This energy resolution is inferior by a factor of 2ndash4 to that obtainable withnon-imaging organic spectrometers that use unfolding algorithms [13] and is certainly poorer thanTOF spectrometry However its energy resolution is much better than that of the Bonner multi-sphere method [13] It is planned to investigate the standard unfolding reconstruction procedureswith the capillary detector in order to achieve better energy resolution and yet maintain the highposition resolution

For high resolution neutron imaging and for neutron spectroscopy based on the method de-scribed here the direction of the incident neutron relatively to the capillary axis direction must beknown If the detector is intended for non-spectroscopic purposes but efficient gamma-ray rejec-tion is none-the-less required it is possible to use larger diameter capillaries and the direction ofthe incident neutron is of no importance

References

[1] JM Ryan et al A scintillating plastic fiber tracking detector for neutron and proton imaging andspectroscopy Nucl Instrum Meth A 422 (1999) 49

ndash 12 ndash

2012 JINST 7 C04021

Figure 10 Left Capillary array manufactured by XOS Right enlarged section of the capillary arrayshowing individual capillaries imaged by an electronic microscope

9times9 microm2

43 Irradiation configuration

The detector was tested in a neutron beam using the PTB cyclotron The capillary array wasirradiated from the side ie perpendicular to its central axis by neutrons created in the 9Be(dn)reaction using a 12 MeV deuteron beam that impinges on a thick Be target This reaction yieldsan intense broad-energy neutron spectrum with sufficient neutron intensity up to about 16 MeVThe deuteron beam was pulsed insim1ndash2 ns bursts at 2 MHz repetition rate The average deuteroncurrent was 14 microA Neutron energy was selected using the Time-of-Flight (TOF) technique Thiswas accomplished by time-gating the image-intensifier such that it acts as a very fast shutter (on ans time-scale) collecting light after a pre-determined time relative to the time the deuteron pulseimpinged on the Be target The image intensifier opening time was about 10 ns The detector-targetdistance was 354 m

5 Experimental results

The images were collected by opening the camera for about 1ndash2 seconds thus ensuring that onaverage an image will contain no more than 2 neutron tracks Images were obtained for 2 neutronenergies 86 plusmn12 MeV and 155 plusmn3 MeV

Figure 11a shows an entire image (all CCD pixels) obtained for neutron energy 155 MeV Thisis a net image after the readout and dark noises have been removed In addition a threshold wasapplied such that all values below 16 ADC units (ADU) were set to zero

One can observe a very large number of point-like low-light-intensity tracks and a single pro-ton continuous track (indicated by the arrow) The projection length of this track is 298 micromFigure 11b shows an enlarged image of the proton track Figure 11c shows the same image withthe light intensity threshold increased to 180 ADU

The spots are mainly due to gamma-rays and their number was found to be correlated with theintensity of gamma rays The separation between gamma-ray and neutron events is quite evidentfrom figures 11 Protons generated by neutrons in the scintillating liquid exhibit bright continuoustracks with a Bragg peak at their end Gamma-ray-induced electrons generate small faint blobs of

ndash 10 ndash

2012 JINST 7 C04021

Figure 11 CCD image of the capillary array a) entire image b) image zoomed on the proton track c)thresholded image neutron energy 155 MeV

Figure 12 Examples of 3 proton tracks obtained for 155 MeV neutrons

light that appear in figure 11a as a multitude of specks The separation of electron from protonevents can in principle be performed automatically by using light and track length thresholdingpixel connectivity analysis and the existence of a Bragg peak As is evident from figure 11c asimple thresholding procedure removes most of the gamma-ray induced events

Figure 12 shows 3 additional samples of proton tracks obtained in this run In all of them theBragg peak is clearly discernible

At this stage of the project the identification of the proton tracks has been performed by visualinspection of the CCD images This is a rather labor-consuming task so that a computerized trackrecognition procedure needs to be developed for rapid automatic identification of proton tracks ina large number of CCD images In this experiment about 60 proton tracks with projection lengthabove 7 capillaries were collected and used in the energy reconstruction The energy reconstructionprocedure is identical to that described in section 32

Figure 13 shows the reconstructed energy spectrum Clearly the event statistics is rather poornevertheless there is strong evidence for the existence of two neutron groups at about 86 MeV andat 155 MeV In this reconstruction no correction for variability in light emission over the capillarybundle cross-section was made In addition one has to take into account the non-uniformitiesof light collection and of the image-intensifier photocathode quantum-efficiency over the imagearea The correction for these effects can be made by illuminating the capillary bundle with auniform light source and registering the relative intensity for each CCD pixel This can be used forcorrection of track light intensity

ndash 11 ndash

2012 JINST 7 C04021

Figure 13 Reconstructed spectrum of 86 and 155 MeV neutrons

6 Conclusions

An imaging neutron detector consisting of micro-capillaries filled with a high refractive indexliquid scintillator has been developed The important properties of the detector are high positionresolution (tens of microns) and good rejection of gamma-ray events The detection efficiency isdependent on the size of the capillary array and in principle can be in the range of 10ndash20

The energy reconstruction method presented here based on the determination of light yieldand the projection length of each proton track resulted in 10ndash15 energy resolution in the energyrange of 4ndash20 MeV This energy resolution is inferior by a factor of 2ndash4 to that obtainable withnon-imaging organic spectrometers that use unfolding algorithms [13] and is certainly poorer thanTOF spectrometry However its energy resolution is much better than that of the Bonner multi-sphere method [13] It is planned to investigate the standard unfolding reconstruction procedureswith the capillary detector in order to achieve better energy resolution and yet maintain the highposition resolution

For high resolution neutron imaging and for neutron spectroscopy based on the method de-scribed here the direction of the incident neutron relatively to the capillary axis direction must beknown If the detector is intended for non-spectroscopic purposes but efficient gamma-ray rejec-tion is none-the-less required it is possible to use larger diameter capillaries and the direction ofthe incident neutron is of no importance

References

[1] JM Ryan et al A scintillating plastic fiber tracking detector for neutron and proton imaging andspectroscopy Nucl Instrum Meth A 422 (1999) 49

ndash 12 ndash

2012 JINST 7 C04021

Figure 11 CCD image of the capillary array a) entire image b) image zoomed on the proton track c)thresholded image neutron energy 155 MeV

Figure 12 Examples of 3 proton tracks obtained for 155 MeV neutrons

light that appear in figure 11a as a multitude of specks The separation of electron from protonevents can in principle be performed automatically by using light and track length thresholdingpixel connectivity analysis and the existence of a Bragg peak As is evident from figure 11c asimple thresholding procedure removes most of the gamma-ray induced events

Figure 12 shows 3 additional samples of proton tracks obtained in this run In all of them theBragg peak is clearly discernible

At this stage of the project the identification of the proton tracks has been performed by visualinspection of the CCD images This is a rather labor-consuming task so that a computerized trackrecognition procedure needs to be developed for rapid automatic identification of proton tracks ina large number of CCD images In this experiment about 60 proton tracks with projection lengthabove 7 capillaries were collected and used in the energy reconstruction The energy reconstructionprocedure is identical to that described in section 32

Figure 13 shows the reconstructed energy spectrum Clearly the event statistics is rather poornevertheless there is strong evidence for the existence of two neutron groups at about 86 MeV andat 155 MeV In this reconstruction no correction for variability in light emission over the capillarybundle cross-section was made In addition one has to take into account the non-uniformitiesof light collection and of the image-intensifier photocathode quantum-efficiency over the imagearea The correction for these effects can be made by illuminating the capillary bundle with auniform light source and registering the relative intensity for each CCD pixel This can be used forcorrection of track light intensity

ndash 11 ndash

2012 JINST 7 C04021

Figure 13 Reconstructed spectrum of 86 and 155 MeV neutrons

6 Conclusions

An imaging neutron detector consisting of micro-capillaries filled with a high refractive indexliquid scintillator has been developed The important properties of the detector are high positionresolution (tens of microns) and good rejection of gamma-ray events The detection efficiency isdependent on the size of the capillary array and in principle can be in the range of 10ndash20

The energy reconstruction method presented here based on the determination of light yieldand the projection length of each proton track resulted in 10ndash15 energy resolution in the energyrange of 4ndash20 MeV This energy resolution is inferior by a factor of 2ndash4 to that obtainable withnon-imaging organic spectrometers that use unfolding algorithms [13] and is certainly poorer thanTOF spectrometry However its energy resolution is much better than that of the Bonner multi-sphere method [13] It is planned to investigate the standard unfolding reconstruction procedureswith the capillary detector in order to achieve better energy resolution and yet maintain the highposition resolution

For high resolution neutron imaging and for neutron spectroscopy based on the method de-scribed here the direction of the incident neutron relatively to the capillary axis direction must beknown If the detector is intended for non-spectroscopic purposes but efficient gamma-ray rejec-tion is none-the-less required it is possible to use larger diameter capillaries and the direction ofthe incident neutron is of no importance

References

[1] JM Ryan et al A scintillating plastic fiber tracking detector for neutron and proton imaging andspectroscopy Nucl Instrum Meth A 422 (1999) 49

ndash 12 ndash

2012 JINST 7 C04021

Figure 13 Reconstructed spectrum of 86 and 155 MeV neutrons

6 Conclusions

An imaging neutron detector consisting of micro-capillaries filled with a high refractive indexliquid scintillator has been developed The important properties of the detector are high positionresolution (tens of microns) and good rejection of gamma-ray events The detection efficiency isdependent on the size of the capillary array and in principle can be in the range of 10ndash20

The energy reconstruction method presented here based on the determination of light yieldand the projection length of each proton track resulted in 10ndash15 energy resolution in the energyrange of 4ndash20 MeV This energy resolution is inferior by a factor of 2ndash4 to that obtainable withnon-imaging organic spectrometers that use unfolding algorithms [13] and is certainly poorer thanTOF spectrometry However its energy resolution is much better than that of the Bonner multi-sphere method [13] It is planned to investigate the standard unfolding reconstruction procedureswith the capillary detector in order to achieve better energy resolution and yet maintain the highposition resolution

For high resolution neutron imaging and for neutron spectroscopy based on the method de-scribed here the direction of the incident neutron relatively to the capillary axis direction must beknown If the detector is intended for non-spectroscopic purposes but efficient gamma-ray rejec-tion is none-the-less required it is possible to use larger diameter capillaries and the direction ofthe incident neutron is of no importance

References

[1] JM Ryan et al A scintillating plastic fiber tracking detector for neutron and proton imaging andspectroscopy Nucl Instrum Meth A 422 (1999) 49

ndash 12 ndash

2012 JINST 7 C04021

[2] RS Miller et al SONTRAC An imaging spectrometer for MeV neutrons Nucl Instrum Meth A505 (2003) 36

[3] JM Ryan et al Development and performance of the Fast Neutron Imaging Telescope for SNMdetection Proc SPIE 6945 (2008) 694509

[4] J Peel et al Development of a directional scintillating fiber detector for 14 MeV neutrons NuclInstrum Meth A 556 (2006) 287

[5] httpwwwdetectorssaint-gobaincom

[6] L Disdier et al Capillary detector with deuterated scintillator for inertial confinement fusionneutron images Rev Sci Instrum 75 (2004) 2134

[7] P Annis et al High-resolution tracking using large capillary bundles filled with liquid scintillatorNucl Instrum Meth A 449 (2000) 60

[8] M Brandis et al Proof of principle of a high-spatial-resolution resonant-response gamma-raydetector for Gamma Resonance Absorption in 14N 2011 JINST 6 PO2008

[9] V Dangendorf et al Detectors for Energy Resolved Fast Neutron Imaging Nucl Instrum Meth A535 (2004) 93

[10] I Mor et al Parameters affecting image quality with time-resolved optical integrative neutron(TRION) detector Nucl Instrum Meth A 640 (2011) 192

[11] httpwwweljentechnologycom

[12] httpwwwxoscom

[13] FD Brooks and H Klein Neutron spectrometry-historical review and present status Nucl InstrumMeth A 476 (2002) 1

ndash 13 ndash