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

2. 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

3. 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+D 2 O for quasi monoenergetic and white neutron spectra

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

5. AER working group F meeting, Liblice, 11.4.2012 TOF measurement cyclotron neutron generator p+p+ n,  TOF base (ca. 4 m) 500 MHz (=2ns) continuous acquisition Energy spectrum TOF spectrum Time [ns]

6. 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 0.6ms Detector response

7. 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 Detector response

8. 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

9. 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]

10. 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]

11. 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 ?..

12. 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

13. 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?=wor k in progress, trying new methods background p+Li, n/g discrimination Energy n/g

14. 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

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

16. 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

17. 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 ? …

18. 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)

19. 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