Possibilities of TOF measurements on NPI neutron generators

Mitja Majerle

Department of Nuclear Reactions

Nuclear Physics Institute ASCR

AER working group F meeting, Liblice, 11.4.2012

Contens

Cyclotron, neutron generators Time structure of the pulse Scintillator efficiency to neutrons and photons

(NE213) Preliminary TOF spectra Analysis, simulations Conclusion, new possibilities

AER working group F meeting, Liblice, 11.4.2012

Cyclotron, neutron generators

Isochronous cyclotron U-120M Protons 20-37.5 MeV Neutron generators, p+Li/C, p+Be, p+D2O for quasi

monoenergetic and white neutron spectra

AER working group F meeting, Liblice, 11.4.2012

Neutron spectra p+Li/C, p+Be

AER working group F meeting, Liblice, 11.4.2012

TOF measurement

cyclotron

neutrongenerator

p+n,

TOF base (ca. 4 m)

500 MHz (=2ns) continuous acquisition

Energy spectrum TOF spectrum

Time [ns]

AER working group F meeting, Liblice, 11.4.2012

Pulse time structure (on NE213) Macrostructure – RF needs to be switched on and off to avoid

discharges (frequency 100-150 Hz), duty-cycle 5-80% Microstructure - Several bunches of protons at different

cyclotron orbits, exiting with RF frequency (20-30 MHz)

Neutron pulses

Time

RF signal

0.6msDetectorresponse

AER working group F meeting, Liblice, 11.4.2012

Pulses timescale in detail

Micropulse duration is ca 50 ns. Pulses from NE213 scintillator (50) are 10-20 ns long. 1000-4000 pulses from scintillator per macropulse. The idea is to record the time of all pulses and to put it in the

histogram – this should give some TOF structure.

Start of macropulse

Micropulse duration 50 ns.2 s

Time

Detectorresponse

AER working group F meeting, Liblice, 11.4.2012

Scintillator response to neutrons and photons Photons in light

material – Compton edge

Neutrons – elastic scattering – proton

Proton/electron – different light output

n/g discrimination might be possible on falling edge

MCNPX simulation

Neutron and photon pulses on stilbene, photomultiplier 1 M

10 MeV n

4 MeV

Energy

Det. response

AER working group F meeting, Liblice, 11.4.2012

First neutron TOF spectrum,1 macropulse, 200 MHz sample fq.

Gammas from p+Li/C

Neutrons, place of QM peak from p+Li/C

Time Of Flight [ns]

AER working group F meeting, Liblice, 11.4.2012

Sorting by energy, p+Li/C

All pulses Only pulses with high energy

Time Of Flight [ns] Time Of Flight [ns]

AER working group F meeting, Liblice, 11.4.2012

TOF spectrum structure

Peak of gammas is not narrow ~ FWHM 5 ns, is there place for improvements or this is pulse structure ?

We see only QM peak and few MeV, then frame overlap (40 ns cycle) – closer distance=worse accuracy ?

n/g discrimination works, or can we discriminate only on the basis of deposited energy ? Several photons deposit energy in the same pulse ?

..

AER working group F meeting, Liblice, 11.4.2012

Analysis details – exact time

Exact time of the pulse Maximum Gauss+exp(-ax) Convolution gauss*exp(-ax)

Combining macropulses, RF frequency drift, 500 MHz sampling ? Gamma peak width was not decreased – few options to try, but it seems that

this is beam time structure what we see -> we should go further away for better accuracy (TOF time >> 5 ns FWHM).

Time [ns]

Det. response

AER working group F meeting, Liblice, 11.4.2012

Analysis details - distance

TOF spectrum of p(20 MeV)+Be sampled, two distances 1.5 m and 4.2 m

Closer distance – more background 20%, simulations suggest less, photons ?

n/g discrimination tested on p+Li/C, correct?/interpretation?=work in progress, trying new methods

background

p+Li, n/g discrimination

Energyn/g

AER working group F meeting, Liblice, 11.4.2012

MCNPX simulations

Time and energy spectra of photons and neutrons can be simulated

From time spectra and simulated NE213 efficiencies the response can be sampled

TOF gamma, p+Be TOF neutron, p+Be

Time

300 ns

AER working group F meeting, Liblice, 11.4.2012

MCNPX vs. experiment, p+Be

1.00E-04

1.00E-03

1.00E-02

0 2 4 6 8 10 12 14

10

15

20

Simulated TOF/dep.ene. Sampled TOF/dep.ene.

AER working group F meeting, Liblice, 11.4.2012

MCNPX vs. experiment, p+Li/C

Simulated TOF

Simulated, 5 ns gauss – acquisition problems ?

Sampled TOF

Time [ns]

Counts

AER working group F meeting, Liblice, 11.4.2012

Conclusion Measuring neutron TOF spectra with NE213 is feasible and showed

limited success already Measurements performed within 10 minutes 2 experiments with TOF sampling, p+Li/C, p+Be – we have learned

some specifics of setups, background conditions, DAQ settings .. Shape of some “abnormal” macropulses (hiding interesting

information on background):

A lot of work on sampled spectra ahead: n/g discrimination, estimation of background, routine use of simulations at experiment planning, shape of NE213 pulses – to fit or not ? …

AER working group F meeting, Liblice, 11.4.2012

Beyond NE213

Current pulse time structure -> compromise between energy range+background and accuracy (distance from target)

Max. distance within hall is ca 5 m, background and accuracy are acceptable there, if only we could get rid of frame overlap.

Detector with fast response, but also some energy resolution – proton recoil telescope (not sensible to gammas)

AER working group F meeting, Liblice, 11.4.2012

Medley at TSL

R. Bevilacqua et al. / Nuclear Instruments and Methods in Physics Research A 646 (2011) 100–107