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This presentation explores the potential of time-of-flight (TOF) measurements on NPI neutron generators using NE213 scintillators. The preliminary TOF spectra, analysis, and simulations are discussed, along with new possibilities for further research. Presented at AER working group F meeting in Liblice, 11.4.2012.
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Possibilities of TOF measurements on NPI neutron generators Mitja Majerle Department of Nuclear Reactions Nuclear Physics Institute ASCR
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 neutron generator p+ n,g TOF base (ca. 4 m) cyclotron 500 MHz (=2ns) continuous acquisition TOF spectrum Energy 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) RF signal Neutron pulses 0.6ms Detector response Time AER working group F meeting, Liblice, 11.4.2012
Pulses timescale in detail Start of macropulse Detector response Micropulse duration 50 ns. 2 ms Time • Micropulse duration is ca 50 ns. • Pulses from NE213 scintillator (50W) 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. AER working group F meeting, Liblice, 11.4.2012
Scintillator response to neutrons and photons Det. response MCNPX simulation • Photons in light material – Compton edge • Neutrons – elastic scattering – proton • Proton/electron – different light output • n/g discrimination might be possible on falling edge 4 MeV g 10 MeV n Energy Neutron and photon pulses on stilbene, photomultiplier 1 MW 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 Only pulses with high energy All pulses 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 Det. response Time [ns] • 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). 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 n/g Energy AER working group F meeting, Liblice, 11.4.2012
MCNPX simulations TOF neutron, p+Be TOF gamma, p+Be 300 ns Time • Time and energy spectra of photons and neutrons can be simulated • From time spectra and simulated NE213 efficiencies the response can be sampled AER working group F meeting, Liblice, 11.4.2012
MCNPX vs. experiment, p+Be Sampled TOF/dep.ene. Simulated TOF/dep.ene. AER working group F meeting, Liblice, 11.4.2012
MCNPX vs. experiment, p+Li/C Simulated TOF Counts Simulated, 5 ns gauss – acquisition problems ? Sampled TOF Time [ns] 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 AER working group F meeting, Liblice, 11.4.2012