Possibilities of TOF measurements on NPI neutron generators

1. Possibilities of TOF measurements on NPI neutron generators Mitja Majerle Department of Nuclear Reactions Nuclear Physics Institute ASCR

2. 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

3. 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

4. Neutron spectra p+Li/C, p+Be AER working group F meeting, Liblice, 11.4.2012

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. MCNPX vs. experiment, p+Be Sampled TOF/dep.ene. Simulated TOF/dep.ene. AER working group F meeting, Liblice, 11.4.2012

16. 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

17. 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

18. 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

19. 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