GAMMA DETECTORS FOR High energy Inelastic Neutron Scattering. E.M. Schooneveld

Transcription

1 GAMMA DETECTORS FOR High energy Inelastic Neutron Scattering E.M. Schooneveld

2 Gamma detectors for HINS Collaboration: University of Rome Tor Vergata: - C. Andreani, S. Imberti, A. Pietropaolo, R. Senesi University of Milano-Bicocca: - G. Gorini, E. Perelli-Cippo, M. Tardocchi CCLRC, ISIS Facility: - T. Abdul-Redah, J. Mayers, N. Rhodes, E. Schooneveld

3 Gamma detectors for HINS Introduction Gamma detectors principles Foil cycling technique Measurements Conclusion

4 Introduction Indirect geometry spectrometer 238 U and 197 Au analyser foils Why a gamma detector? Neutron detector very inefficient at ~ 100 ev Gamma detector efficiency independent of neutron energy + Gamma detector better at even 5 ev

5 Gamma detector principles What type gamma detector to use? Factors to consider: - Energy range - Energy resolution - Cost - Reliability Energy resolution Cost Robust Two options: Semiconductor Scintillator

6 Gamma detector principles Which gamma energies to use? 100 Thermal Neutron Capture Gammas 80 (a) Au Gammas from resonant neutron 60 capture in 238 U or 197 Au Strongest lines: Au: 215, 248, 97, 168 kev U: 11, 133, 48, 612 kev Relative intensity (%) (b) U 40 ~50% gammas: E > 0.6 MeV 20 Wide range of available energies Gamma energy (MeV)

7 Gamma detector principles Select specific gamma energy lines to improve S/B ratio? Resonance peaks very small Poor peak / background ratio No significant S/B improvement Loose too much rate Same true for 238 U Count rate (A.U.) Pulse height spectrum of Au in beam On-resonance Off-resonance Resonance peaks No specific gamma line selection No need for good energy resolution Gamma energy (kev) No need for high-z detector

8 Gamma detector principles Discriminator level choice Pulse height spectrum of gamma background Measured gamma background 7.0e+4 - U-foil in front of detector - TOF just after 6.6 ev resonance - YAP scintillator Counts 6.0e+4 5.0e+4 4.0e+4 3.0e+4 2.0e+4 10 B Lot of boron in block house 1.0e Set LLD > ~550 kev Gamma energy (kev)

9 Gamma detector principles Detector configuration Requirements: - LLD > ~550 kev - No need for good energy resolution - No need for high-z detector - High reliability - Cost effective Scintillator detector most attractive Chosen YAP:Ce - Fast - High light output - Low temperature sensitivity - No neutron capture resonances below 2 kev

10 Gamma detector principles YAP detector at VLAD position, 2º Ice at 270K, 238 U Very nice spectrum

11 Gamma detector principles Gamma vs. neutron detector 3.0 ZrH 2 sample, 238 U analyser 2.5 (a) - Li-glass Advantages: - Much better P/B ratio - Better statistics - Higher efficiency for high energy resonances Can we do better? In (counts / µs / µahrs / cm 2 ) (b) - YAP YES TOF (µs)

12 Foil cycling technique Main outstanding issues Background: Peak fitting problems TOF (µs) Resolution: Same as spectrometer in low resolution mode, most important for 197 Au analyser

13 Foil cycling technique Background subtraction Three methods: 1) Fit background Doesn t cost beam time Hard to make good fit: background gamma peaks Not very user friendly

14 Foil cycling technique 2) Measure background without analyser foil Subtract background Much more user friendly 50% of time measuring background

15 Foil cycling technique 3) Prevent resonant neutrons reaching analyser foil How: - Foil of same material as analyser between sample and detector - Cycle this foil with 50% duty cycle - Subtract two data sets Much more user friendly 50% of time measuring background Gammas from cycling foil

16 Foil cycling technique Resolution improvement Same thickness as analyser foil: - Background subtraction + resolution improvement Narrower peak Considerably smaller tails Don t loose too much efficiency gamma flux (A.U.) Simple theoretical calculation, using 12.5 µm thick 197 Au foils TOF (µs) no cycling with cycling Even narrower peak with thinner cycling foil, but loose efficiency

17 Measurements 32 element detector Scintillator area: 80 mm * 25 mm Au foils, 12.5 µm thick Detector at º

18 Measurements Background subtraction 1 mm lead 5.0e-4 4.0e-4 Normal Difference Rate (A.U.) 3.0e-4 2.0e-4 1.0e e TOF (µs)

19 Measurements U-foil, 2 mm lead mm PE 1.4e-3 Rate (A.U.) 1.2e-3 1.0e-3 8.0e-4 6.0e-4 4.0e-4 2.0e-4 Normal Difference All 3 main resonance peaks usable Need thicker foils for higher energy resonances e TOF (µs) Background subtraction works very well

20 Measurements Resolution improvement 1 mm lead, Li-glass double difference 1.2e e-4 Rate YAP (A.U.) 1.0e-4 8.0e-5 6.0e-5 4.0e-5 2.0e YAP Li-glass 2.5e-4 2.0e-4 1.5e-4 1.0e-4 5.0e Rate Li-glass (A.U.) FWHM=9.5 µs FWHM = 9.2 µs -2.0e TOF YAP (µs) -5.0e-5 YAP same resolution as Li-glass in double difference

21 Measurements Statistics Li-glass double difference Same scintillator area Rate (A.U.) 5.0e-6 4.0e-6 3.0e-6 2.0e-6 1.0e Small polymer sample YAP 3 times lower count rate Better statistics Need 3 times less beam time Biggest gain for small samples Rate (A.U.) -1.0e-6 1.5e-5 Li-glass 1.0e-5 5.0e e TOF (µs)

22 Conclusion Best performance with YAP and threshold above 10 B gammas Suitable for both 197 Au and 238 U analyser foils Same resolution as double difference with neutron detectors Much better peak-background ratio considerably better statistics Gamma detectors better for high and low energy resonances Gamma detector very attractive for HINS YAP detectors on VLAD and forward bank