# Presentation: 181170

## Neutron Detectors for Materials Research

- T.E. Mason
- Associate Laboratory Director
- Spallation Neutron Source
- Acknowledgements: Kent Crawford & Ron Cooper

## 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

## Nuclear Reactions for Neutron Detectors 

- n + 3He  3H + 1H + 0.764 MeV
- n + 6Li  4He + 3H + 4.79 MeV
- n + 10B  7Li* + 4He7Li + 4He + 0.48 MeV  +2.3 MeV	(93%)			  7Li + 4He 		       +2.8 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

## Gas Detectors 

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

## Gas Detectors  –  cont’d 

- Ionization 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

## MAPS Detector Bank

## Scintillation Detectors

## Some Common Scintillators for Neutron Detectors

- Li glass (Ce)

- 1.751022

- 0.45 %

- 395 nm

- ~7,000

- LiI (Eu)

- 1.831022

- 2.8 %

- 470

- ~51,000

- ZnS (Ag) - LiF

- 1.181022

- 9.2 %

- 450

- ~160,000

## GEM Detector Module

## Anger camera

- 2000-03449/arb

- 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

## Semiconductor Detectors

## 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?

## 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

## Detection Efficiency

- Full expression:

- Approximate expression for low efficiency:

- Where:
-  = 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

## Pulse Height Discrimination

## 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)

## 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

## Multi-Wire Proportional Counter 

- Array of discrete detectors

- Remove walls to get multi-wire counter

## MWPC cont’d

- Segment the cathode to get  x-y  position

## 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

## 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

## Anger camera detector on SCD

- Photomultiplier outputs are resistively encoded to give x and y coordinates
- Entire assembly is in a light-tight box

## Micro-Strip Gas Counter 

- Electrodes printed lithgraphically
- Small features – high spacial resolution, high field gradients – charge localization and fast recovery

## 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 sec

## Neutron Detector Screen Design

- 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%.

**Neutron Detector Screen Design**

## 16-element WAND Prototype Schematic and Results

- Neutron Beam

- Coincidence tube

- 2-D tube

- Scintillator Screen

- Clear Fiber

- Wavelength-shifting fiber

- Aluminum wire

**16-element WAND Prototype Schematic and Results**

## Principle of Crossed-Fiber 

**Position-Sensitive Scintillation Detector**

- Outputs to multi-anode photomultiplier tube

- Outputs to coincidence single-anode photomultiplier tube

- 1-mm Square Wavelength-shifting fibers

- Scintillator screen

## Normalized scattering from 1-cm high germanium crystal
En ~ 0.056 eV
Detector 50-cm from crystal

- Normalized scattering from 1-cm high germanium crystal
- En ~ 0.056 eV
- Detector 50-cm from crystal

**Neutron Scattering from Germanium Crystal Using Crossed-fiber Detector**

## All fibers installed and connected to multi-anode photomultiplier mount