1. Neutron Detectors for Materials Research
T.E. Mason
Associate Laboratory Director
Spallation Neutron Source
Acknowledgements: Kent Crawford & Ron Cooper

2. Neutron Detectors What does it mean to “detect” a neutron?
Need to produce some sort of measurable quantitative (countable) electrical signal
Can’t directly “detect” slow neutrons
Need to use nuclear reactions to “convert” neutrons into charged particles
Then we can use one of the many types of charged particle detectors
Gas proportional counters and ionization chambers
Scintillation detectors
Semiconductor detectors

3. Nuclear Reactions for Neutron Detectors
n + 3He  3H + 1H MeV
n + 6Li  4He + 3H MeV
n + 10B  7Li* + 4He7Li + 4He MeV  +2.3 MeV (93%)  7Li + 4He MeV ( 7%)
n + 155Gd  Gd*  -ray spectrum  conversion electron spectrum
n + 157Gd  Gd*  -ray spectrum  conversion electron spectrum
n + 235U  fission fragments + ~160 MeV
n + 239Pu  fission fragments + ~160 MeV

4. Gas Detectors
~25,000 ions and electrons produced per neutron (~410-15 coulomb)

5. Gas Detectors – cont’d Ionization Mode Proportional Mode
electrons drift to anode, producing a charge pulse
Proportional Mode
if voltage is high enough, electron collisions ionize gas atoms producing even more electrons
gas amplification
gas gains of up to a few thousand are possible

6. MAPS Detector Bank

7. Scintillation Detectors

8. Some Common Scintillators for Neutron Detectors
Material
Density of
6Li atoms
(cm-3)
Scintillation
efficiency
Photon
wavelength (nm)
Photons per neutron
Li glass (Ce)
1.751022
0.45 %
395 nm
~7,000
LiI (Eu)
1.831022
2.8 %
470
~51,000
9.2 %
~160,000
ZnS (Ag) - LiF
1.181022
450

9. GEM Detector Module

10. Anger camera
Prototype scintillator-based area-position-sensitive neutron detector
Designed to allow easy expansion into a 7x7 photomultiplier array with a 15x15 cm2 active scintillator area.
Resolution is expected to be ~1.5x1.5 mm2
/arb

11. Semiconductor Detectors

12. Semiconductor Detectors cont’d
~1,500,000 holes and electrons produced per neutron (~2.410-13 coulomb)
This can be detected directly without further amplification
But standard device semiconductors do not contain enough neutron-absorbing nuclei to give reasonable neutron detection efficiency
put neutron absorber on surface of semiconductor?
develop boron phosphide semiconductor devices?

13. Coating with Neutron Absorber
Layer must be thin (a few microns) for charged particles to reach detector
detection efficiency is low
Most of the deposited energy doesn’t reach detector
poor pulse height discrimination

14. Detection Efficiency Full expression:
Approximate expression for low efficiency:
Where:
s = absorption cross-section
N = number density of absorber
t = thickness
N = 2.71019 cm-3  atm-1 for a gas
For 1-cm thick 3He at 1 atm and 1.8 Å,
 = 0.13

15. Pulse Height Discrimination

16. Pulse Height Discrimination cont’d
Can set discriminator levels to reject undesired events (fast neutrons, gammas, electronic noise)
Pulse-height discrimination can make a large improvement in background
Discrimination capabilities are an important criterion in the choice of detectors ( 3He gas detectors are very good)

17. Position Encoding Discrete - One electrode per position
Discrete detectors
Multi-wire proportional counters (MWPC)
Fiber-optic encoded scintillators (e.g. GEM detectors)
Weighted Network (e.g. MAPS LPSDs)
Rise-time encoding
Charge-division encoding
Anger camera
Integrating
Photographic film TV
CCD

18. Multi-Wire Proportional Counter
Array of discrete detectors
Remove walls to get multi-wire counter

19. MWPC cont’d
Segment the cathode to get x-y position

20. Resistive Encoding of a Multi-wire Detector
Instead of reading every cathode strip individually, the strips can be resistively coupled (cheaper & slower)
Position of the event can be determined from the fraction of the charge reaching each end of the resistive network (charge-division encoding)
Used on the GLAD and SAND linear PSDs

21. Resistive Encoding of a Multi-wire Detector cont’d
Position of the event can also be determined from the relative time of arrival of the pulse at the two ends of the resistive network (rise-time encoding)
Used on the POSY1, POSY2, SAD, and SAND PSDs
There is a pressurized gas mixture around the electrodes

22. Anger camera detector on SCD
Photomultiplier outputs are resistively encoded to give x and y coordinates
Entire assembly is in a light-tight box

23. Micro-Strip Gas Counter
Electrodes printed lithgraphically
Small features – high spacial resolution, high field gradients – charge localization and fast recovery

24. Crossed-Fiber Scintillation Detector Design Parameters (ORNL I&C)
Size: 25-cm x 25-cm
Thickness: 2-mm
Number of fibers: 48 for each axis
Multi-anode photomultiplier tube: Phillips XP1704
Coincidence tube: Hamamastu 1924
Resolution: < 5-mm
Shaping time: 300 nsec
Count rate capability: ~ 1 MHz
Time-of-Flight Resolution: 1 msec

25. Neutron Detector Screen Design
The scintillator screen for this 2-D detector consists of a mixture
of 6LiF and silver-activated ZnS powder in an epoxy binder. Neutrons incident on the screen react with the 6Li to produce a triton and an alpha particle. Collisions with these charged particles cause the ZnS(Ag) to scintillate at a wavelength of approximately 450 nm. The 450 nm photons are absorbed in the wavelength-shifting fibers where they converted to 520 nm photons emitted in modes that propagate out the ends of the fibers. The optimum mass ratio of 6LiF:ZnS(Ag) was determined to be 1:3. The screen is made by mixing the powders with uncured epoxy and pouring the mix into a mold. The powder then settles to the bottom of the mold before the binder cures. After curing the clear epoxy above the settled powder mix is removed by machining. A mixture containing 40 mg/cm2 of 6LiF and 120 mg/cm2 of ZnS(Ag) is used in this screen design. This mixture has a measured neutron conversion efficiency of over 90%.

26. 16-element WAND Prototype Schematic and Results
Clear Fiber
2-D tube
Coincidence tube
Neutron Beam
Wavelength-shifting fiber
Aluminum wire
Scintillator Screen

27. Principle of Crossed-Fiber Position-Sensitive Scintillation Detector
Outputs to multi-anode photomultiplier tube
1-mm Square Wavelength-shifting fibers
Outputs to coincidence single-anode photomultiplier tube
Scintillator screen

28. Neutron Scattering from Germanium Crystal Using Crossed-fiber Detector
Normalized scattering from 1-cm high germanium crystal
En ~ eV
Detector 50-cm from crystal

29. All fibers installed and connected to multi-anode photomultiplier mount