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

3
Cyclotron, neutron generators

• Isochronous cyclotron U-120M
• Protons 20-37.5 MeV
• Neutron generators, pLi/C, pBe, pD2O for quasi
  monoenergetic and white neutron spectra

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Neutron spectra pLi/C, pBe
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TOF measurement
neutron generator
p
n,g
TOF base (ca. 4 m)
cyclotron
500 MHz (2ns) continuous acquisition
TOF spectrum
Energy spectrum
Time ns
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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
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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.

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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
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First neutron TOF spectrum,1 macropulse, 200 MHz
sample fq.
Gammas from pLi/C
Neutrons, place of QM peak from pLi/C
Time Of Flight ns
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Sorting by energy, pLi/C
Only pulses with high energy
All pulses
Time Of Flight ns
Time Of Flight ns
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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 distanceworse
  accuracy ?
• n/g discrimination works, or can we discriminate
  only on the basis of deposited energy ? Several
  photons deposit energy in the same pulse ?
• ..

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Analysis details exact time
Det. response
Time ns

• Exact time of the pulse
• Maximum
• Gaussexp(-ax)
• Convolution gaussexp(-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 -gt we should go further
  away for better accuracy (TOF time gtgt 5 ns FWHM).

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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 pLi/C,
  correct?/interpretation?work in progress, trying
  new methods

background
pLi, n/g discrimination
n/g
Energy
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MCNPX simulations
TOF neutron, pBe
TOF gamma, pBe
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

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MCNPX vs. experiment, pBe
Sampled TOF/dep.ene.
Simulated TOF/dep.ene.
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MCNPX vs. experiment, pLi/C
Simulated TOF
Counts
Simulated, 5 ns gauss acquisition problems ?
Sampled TOF
Time ns
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Conclusion

• Measuring neutron TOF spectra with NE213 is
  feasible and showed limited success already
• Measurements performed within 10 minutes
• 2 experiments with TOF sampling, pLi/C, pBe
  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 ?

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Beyond NE213

• Current pulse time structure -gt compromise
  between energy rangebackground 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
Medley at TSL
R. Bevilacqua et al. / Nuclear Instruments and
Methods in Physics Research A 646 (2011) 100107