Large Area Imaging Detector for Neutron Scattering Based on Boron-Rich Liquid Scintillator

D. Vartsky1, M.B. Goldberg

Soreq NRC, Yavne 81800, Israel

A. Breskin, R. Chechik

Weizmann Institute of Science, Rehovot 76100, Israel

B. Guerard, J.F. Clergeau

Institut Laue Langevin, Grenoble, France

Abstract

We discuss a new thermal-neutron imaging detector that combines a neutron

converter coupled to a position-sensitive gaseous photodetector. The neutron

converter consists of a thin (0.5 mm) layer of boron-rich liquid scintillator. It is

viewed by an atmospheric -pressure, gas-avalanche photomultiplier with a bi-alkali

photocathode. Scintillation-induced photoelectrons are multiplied by a cascade of Gas

Electron Multipliers (GEM). The multi-GEM supplies the pulse-height, time and

position information for each converted neutron. Such a fast, large -area detector can

operate at high radiation flux.

Key Words: Liquid scintillator, neutron detector, neutron scattering, gaseous

photomultiplier

Pacs: 29.40.Mc, 29.40.Cs, 61.12.-q, 61.12.Ex

__________________________________________________________________

1 Corresponding author: Tel: 973-8-9434589 Fax: 973-8-9434676 E-mail: david@soreq.gov.il

1. Introduction

Slow neutron scattering is one of the key tools for studying condensed matter. The

advent of high current pulsed-accelerator based spallation sources (SNS, NPS, ESS)

applied to time-resolved high resolution experiments will create a need for new

detectors: la rge -area, real-time imaging devices with faster response and higher

detection efficiency than those presently in use. The requirements for these new

detectors are summarized in Table 1. The currently employed thermal neutron

detectors do not fulfill all these requirements. To overcome these limitations,

intensive R&D efforts are currently being invested by numerous groups [1]. Various

types of slow-neutron detectors are under development, including 3He-filled multiwire

chambers [2] and microstrip chambers [3], new inorganic scintillators [4], 10B-coated

Gas Electron Multipliers (GEM [5]) detectors [6], high-resolution hybrid detectors

with composite -foil converters and wire chambers [7] or with microstrip multipliers

[8], optically read out GEMs [9] etc.

In this article we present a different concept for an efficient, fast, large -area slow

neutron imaging detector. It is based on a boron-rich organic liquid scintillator

coupled to a position-sensitive gaseous photomultiplier (GPMT [10]).

In principle, the light produced by the liquid scintillator can be recorded by any

position-sensitive photon detector, e.g., vacuum-operated position-sensitive

photomultipliers, Hybrid Photodiodes (HPD) [11], etc. However, at present none of

these can provide a cost-effective light-recording solution that would cover the large

areas, (in excess of 2500 cm2) required for the applications under discussion. The

proposed gaseous -imaging photomultiplier, operating at atmospheric pressure with a

visible-light sensitive photocathode , has the potential for responding to the

requirements of large active area, sub-millimeter spatial resolution, fast response and

single -event sensitivity.

In this work we present first results for the newly developed scintillator; preliminary

data for the gaseous visible-photon GPMT is also shown.

2. Detector principle

The neutron detector consists of a thin (< 500 µm) neutron converter made of a 10B-

rich liquid organic scintillator, coupled to a novel position-sensitive GPMT (Fig. 1).

The light emitted from the scintillator, following the neutron capture reaction

10B(n,a)7Li , is converted at a semitransparent K-Cs-Sb photocathode coupled to a

gas-operated electron multiplier. The latter is a series of cascaded Gas Electron

Multiplier (GEM) elements [12], sealed to the photocathode within a gas-filled vessel.

The GEM consists of a 50-micron thin, metal-clad insulator foil (e.g. Kapton),

perforated with a dense array of 50-micron diameter holes. Electrons focused into a

hole are multiplied within the hole, under a potential of a few hundred volts, applied

across the GEM foil. Multiplication factors ranging from 103 – 106 are typically

attained for single- to 4-GEMs in cascade, respectively [12, 13]. The multiplied

charge from the last GEM is read out on a position-sensitive electrode; the GPMT

provides the pulse-height, time (with a sub-nanosecond resolution [14]) and spatial

localization of each converted neutron.

3. Scintillator properties

The liquid scintillator based on Tri-Methyl-Borazine, is an improved version of that

originally proposed by Ross and Holsopple [15]. It has been optimized to emit light at

a wavelength of 350-400 nm, matching the spectral range of the K-Cs-Sb

photocathode. Table 2 summarizes the properties of the scintillator. Evidently, it is

very fast and its macroscopic cross-section for thermal neutrons is large, allowing for

high probability (~ 90%) of neutron capture in a thin layer of liquid.

The scintillator was tested using an aluminum container for the liquid, sealed with an

indium gasket to a glass window. The window can be coupled with optical grease to a

conventional vacuum photomultiplier or to any position-sensitive light detector. Fig 2

shows the neutron spectrum obtained with two scintillator cocktails: The points

depicts a spectrum obtained with our homemade scintillator, which contain about 22%

boron by weight; the squares are for a commercial scintillator BICRON BC523A that

contains only 5% boron by weight.

It should be noted that, compared to the commercial BICRON BC523A scintillator

our scintillator has a boron content higher by a factor > 4 and its light output is only

14% lower. This permits us to construct an efficient neutron scintillator with a much-

reduced thickness, which is very important for reducing the efficiency to gamma rays

and eliminating the localization parallax. Fig 3 shows the stability of the scintillator

over a period of approximately one year. It appears that the light output slowly

increases with time, which might possibly be due to a slow decrease of trace

concentration of dissolved molecular oxygen (known to be a strong quencher) in the

liquid, by adsorpt ion to the container walls.

4. Light readout

The GPMT for the neutron detector is currently under extensive development, and the

results presented here, though very encouraging are rather preliminary. The reader is

referred to more extended articles on that subject [10, 12, 14, 16]. Of major concern

are the high chemical sensitivity of the bialkali photocathode to gas impurities and to

secondary effects induced by the gas -avalanche (e.g. photon- and ion-feedback). The

newly developed GPMT seems to be an adequate multiplier; it can operate at high

gains in noble-gas mixtures [12], which are compatible with the photocathodes, and it

permits the suppression of most of the feedback effects [12, 16]. Though it has been

recently shown that the photocathode can operate in a sealed detector containing

argon and Kapton-made GEMs, so far it has been demonstrated for only limited

periods of time due to micro leaks [16]. Efforts are being made to produce the GEM

elements from more inert materials (glass, ceramic, etc.). Multiplication factors in

excess of 104 have so far been achieved in a two-GEM GPMT sealed at atmospheric

pressure Ar/CH4 (95/5). The best quantum efficiency values attained in this mode are

of the order of 13% at 350 nm wavelength [16]. Time resolutions of s =1.6 and 0.33

ns, respectively were recorded in a multi-GEM multiplier, for 1 and for 150

photoelectrons [14, 16].

Figure 4 shows an example of a 60 mm diameter sealed GPMT prototype. This

GPMT size is dictated by the dimensions of our present UHV deposit ion-and-sealing

system. Work is in progress to improve the sealing procedure.

5. Summary and conclusions

A new imaging detector for thermal neutrons has been proposed, based on a very thin

liquid scintillator and a gaseous photomultiplier with semitransparent bialkali

photocathode. A boron-rich scintillator (22% boron content by weight) was

developed, with adequate light yield and fast response. The GPMT photon detector is

currently under development, the first prototypes already exhibiting QE above 10% at

400 nm and stability over a few weeks of operation. Sufficiently high gains could be

obtained with Ar/CH4 (95/5) gas.

It is expected that using the proposed configuration, it will be possible to construct

highly efficient slow-neutron detectors of large dimensions and diverse shapes. As

both the scintillator and the light readout are inherently fast, neutron time-of-flight

resolution will be primarily dictated by liquid scintillator thickness and is expected to

be less than 200 ns for thermal neutrons.

Acknowledgements

The development of the photon detector is being effected in cooperation with Mr. D.

Mörmann and Dr. M. Balcerzyk of the Weizmann Institute; we acknowledge their

contribution to this work. This research is partially supported by the Planning and

Budgeting Committee of the Council for Higher Education in Israel and by the Israel

Science Foundation. A. Breskin is the W. P. Reuther Professor of Research in the

peaceful use of atomic energy.

References

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Detectors: Status and Prospectives, Th. Wilpert, Editor., Hahn Meintner Institut,

Berlin, June 2001, ISSN1433-559X

[2] J. Fried, J.A. Harder, G.J. Mahler, D.S. Makowiecki, J.A. Mead, V. Radeka, N.A.

Schaknowski, G.C. Smith and B. Yu, Nucl. Instr. and Meth. A 478 (2002) 415.

[3] J.F. Clergeau, P. Convert, D. Feltin, H.E. Fischer, B. Guerard, T. Hansen, G.

Manzin, A. Oed and P. Palleau, Nucl. Instr. and Meth. A 471 (2001) 60.

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[5] F. Sauli, Nucl. Instr. & Meth. A386 (1997) 531.

[6] M. Klein, H. Abele, D. Fiolka and Chr.J. Schmidt, Ref. 1, pg. 453.

[7] V. Dangendorf, A. Demian, H. Friedrich, V. Wagner, A. Akkerman, A. Breskin,

R. Chechik and A. Gibrekhterman, Nucl. Instr. and Meth. A 350 (1994) 503 and ref.1,

pg.445.

[8] B. Gebauer, C. Schulz, G. Richter, F.V. Levchanovsky and A. Nikiforov, Nucl.

Instr. and Meth. A 471 (2001) 249.

[9] F.A.F. Fraga, L.M.S. Margato, S.T.G. Fetal, M.M.F.R. Fraga, R. Fereira

Marques, A.J.P.L. Policarpo, B. Guerard, A. Oed, G. Manzini, and T. van Vuure,

Nucl. Instr. and Meth. A478 (2002) 357.

[10] A. Breskin, T. Boutboul, A. Bouzulutskov, R. Chechik, G. Garty, E. Shefer and

B.K. Singh, Nucl. Instr. and Meth. A442 (2000) 58 and references therein.

[11] Delft Electronics Products BV., Holland, Catalogue 2002.

[12] A. Buzulutskov, A. Breskin, R. Chechik, G. Garty, F. Sauli and L. Shekhtman,

Nucl. Instr. And Meth. A442 (2000) 68 and A443 (2000) 164.

[13] Bachmann, A. Bressan, L. Ropelewski, F. Sauli, A. Sharma and D. Moermann,

Nucl. Instr. And Meth. A438 (1999) 376 and references therein.

[14] A. Breskin, A. Buzulutskov, and R. Chechik, Nucl. Instr. And Meth. A438

(2002) 670.

[15] H.H. Ross and H.L. Holsopple, Nucl. Instr. and Meth. 33 (1965) 194.

[16] D. Moermann, M. Balcerzyk, A. Breskin, R. Chechik, B.K. Singh and A.

Buzulutskov. GEM-based gaseous photomultipliers for UV and visible photon

imaging. These proceedings.

Figure captions

Figure 1. A schematic presentation of the novel neutron detector concept: Neutrons

are converted in a thin liquid scintillator and the light is imaged by a gas avalanche

photon detector based on a GEM cascade. The scintillator is separated from the

gaseous detector by a glass window, on which the photocathode is deposited.

Figure 2. Neutron spectra obtained with our scintillator cocktail, containing about

22% boron by weight (dots) and with the commercial scintillator BICRON BC523A

(squares) that contains only 5% boron by weight.

Figure 3. The stability of our scintillator, measured over a period of about one year.

Circles and squares denote for 0.4 and 6 mm thick scintillators, respectively.

Figure 4. A prototype of the sealed visible-light gaseous photomultiplier, 60 mm in

diameter, filled with 1 atmosphere Ar.

Table 1 Requirements from neutron detectors

Parameter Requirement

Type of operation Real-time

Spatial resolution (mm) ~ 1

Detection efficiency (1.8 Å) > 70%

Efficiency for gamma rays < 10-7

Detector thickness As thin as possible

Counting rate/pixel 104/s

Global counting rate 107/s

Timing resolution 1-10 µs

Dimensions At least 50x50 cm2

Table 2 Properties of the boron-rich liquid scintillator

Parameter Property

Scintillator base N,N,N, trimethylborazine

Formula C3H12B3N3

Density 0.87 g/cc

Boron content ~ 22%

Macroscopic cross-section 43 cm-1

Light output 400 photons/neutron

Light pulse duration 10 ns

Scintillator cost ~ $1.0/cc

Figure 1.

neutron

scintillation photons

photoelectrons

10B liquid scintillator

bialkali photocathode

GEM-based electron multiplier

2D read-out

window

gas

avalanche

0 100 200 300 4000

500

1000

1500

2000

coun

ts

channel number

Figure 2.

0 100 200 3000

50

100

150

Neu

tron

pea

k po

sitio

n (c

h. N

o.)

Time (days)

Figure 3

Figure 4