Instrumentation for Gamma-ray and Neutron Detectors

1. Instrumentation for Gamma-ray and Neutron Detectors Stan Hunter Code 661, 301-286-7280 stanley.d.hunter@nasa.gov Solar Gamma-Ray and Neutron Physics October 30, 2009

2. Neutrons and Gammas: • Neutral particles, indirect detectioni.e. via charged secondaries • Neutrons • Elastic scattering, (n,p) • Nuclear interactions3He(n,p)3H, 6Li(n,a)3H, 10B(n,a)7Li. • Gammas • Photoelectric absorption • Compton scattering • Pair production 10/30/2009 2

3. Measurement Parameters & Science Goals Energy / Spectrum Direction / Imaging Arrival time / Variability Gammas: Polarization 10/30/2009 3

4. Neutron Imaging • Elastic (n,p) scattering • Multiple scattering • Double scattering plus TOF, or Triple scattering • Double scattering plus momenta • Nuclear interaction • Total energy of fragments • Momentum of fragments • Momenta of fragments totally defines the kinematics of the interaction 10/30/2009 4

5. Multiple Scattering • Measure proton energy, Ep • Determine neutron energy, En = Ep • Neutron detection and spectroscopy • No imaging, Omni-directional sensitivity • Can be collimated Scintillator Incident neutron 10/30/2009 5

6. Double Scatter Plus TOF or Triple • Measure proton energy, Epi+ Interaction location, Li • 1 scatter  neutron detection 2 scatters  2 sr location, En > Ep1+Ep2 TOF or 3 scatters  ring on sky image Time of flight 10/30/2009 6

7. Double Scattering Plus Momenta • 2 scatters  2 sr location, En > Ep1+Ep2 n + Momenta of scattered protons  Energy and direction of neutron 1 n’ n” 2 • Measure proton energy, Ep1 & Ep2+ Interaction locations, L1 & L2 • 1 scatter  neutron detection 10/30/2009 7

8. Nuclear Interactions • 3He or 10BF3 tubes • Measure total energy of fragmentsEquivalent to multiple scattering,Intensity, no direct imaging • Measure momenta of fragments • Low density target medium3-D tracking of fragments • Reconstruct neutron momentum from momenta of fragments 8

9. Gamma Imaging • Photoelectric absorption • Recoil electron, photon electric field • “Imaging” with collimator • Compton scattering • Recoil electron, photon electric field • Second, third, … interactions  circles on the sky • Pair production • Recoil electron and positron,photon electric field • , direct imaging 10/30/2009 9

10. Background Reduction Imaging to reduce acceptance solid angle Differentiate source from background 10/30/2009 10

11. Neutron & Gamma Detection • Readily done with liquid scintillators and pulse shape discrimination • Maybe possible with NIC…Large difference in dE/dx makes this difficult • Two separate instruments optimized separately for n and  10/30/2009 11

12. Lunar Prospector • 3He tubes • Thermal & epi-thermal En < 0.25 • Borated-plastic (BC454) • E = 0.3 to 9 MeV • Fast neutrons, coincident detection of charged particles in the BC454 with a 478 keV gamma ray in the BGO from the 10B(n,a)7Li* reaction 10/30/2009 12

13. MESSENGER Gamma Ray & Neutron Spectrometer Two Lithium-glass plates, 10x10x0.4 cm3 Thermal and epithermal neutrons (0 to ~100 keV One borated plastic scintillator, 10x10x10 cm3 Epithermal and fast neutrons (1 eV to 7.5 MeV) Charged particle exclusion? SEP Background?

14. FNIT Instrument PMT scintillator 15 cm PMT • Radial symmetry • uniform 360˚ FoV • Liquid scintillator • high hydrogen content • n/ pulse shape discrimination • Two PMTs view each bar • position along z-axis • maximize light collection • Prototype • three scintillator tubes • simulated “full” instrument by making many data runs at different rotation angles and summing results 10/30/2009 14

15. FNIT Performance Corrected for instrument response Angular Resolutionactual - calculated= error 10/30/2009 15

16. NIC/3-DTI Theory of Operation • Ionization chamber: Large-volume time projection chamber (TPC) • Proportional counter: 2-D gas micro-well detector (MWD) readout • Low density, homogenous medium (low energy particle tracking) • 100 % active detector volume (no scattering in passive material) 3-D imaging from drift time 10/30/2009 16 16

17. Neutron Imaging z PT Pp PN y x 10/30/2009 Imaged 2 MeV neutrons at the NSWC/PIAF Neutron momentum reconstructed from p & T momenta Angular resolution 68 = ~8o = ~5.4o 17

18. Successful NIC Demonstration 3He based NIC IEEE 2009 Orlando

19. 30x30x30 cm3 NIC Prototype • Double (n,p) scatter • 1536 FEE channels • ASIC optimized for positive & negative signals • Streaming mode readout • Hardware zero suppression • No dead time • Field tests in early 2010 • D-T generator • 252Cf source • NIC data overlaid on 360o video image 10/30/2009 19

20. Gamma Ray Imager • Change gas • Requires higher gain MWDsto track electrons; lower dE/dx • Possibility of triplet detection + e- e- + e+ + e- • Measure momentum of recoil electronMuch better angular resolution possible 10/30/2009 20

21. Conclusions: n, γ Instrumentation Maximize Sensitivity: Effective area Energy resolution Angular resolution Background reduction Approach: Time for discussion…

22. Backup Slides

23. Instrumentation Goal Study the physical processes that contribute to flares Are the processes different for small and large flares? RHESSI: 2.2 MeV emission produced in different location than 0.3 MeV emission 10/30/2009 23

24. Subtlety of Double Scatter Instrument Limited knowledge of angular distribution of multiple secondary particles n  C gives multiple particles 10/30/2009 24

25. Neutron Cross-sections 10/30/2009 25