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Neutron Detectors for Materials Research. T.E. Mason Experimental Facilities Division Spallation Neutron Source Acknowledgements: Kent Crawford & Ron Cooper. Neutron Detectors. What does it mean to “detect” a neutron?

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neutron detectors for materials research

Neutron Detectors for Materials Research

T.E. Mason

Experimental Facilities Division

Spallation Neutron Source

Acknowledgements: Kent Crawford & Ron Cooper

neutron detectors
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
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
Gas Detectors

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

gas detectors cont d
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
some common scintillators for neutron detectors
Material

Density of

6Li atoms

(cm-3)

Scintillation

efficiency

Photon

wavelength (nm)

Photons per neutron

Some Common Scintillators for Neutron Detectors

0.45 %

395 nm

~7,000

Li glass (Ce)

1.751022

2.8 %

470

~51,000

LiI (Eu)

1.831022

9.2 %

~160,000

ZnS (Ag) - LiF

1.181022

450

anger camera
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

2000-03449/arb

semiconductor detectors cont d
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
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
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
pulse height discrimination cont d
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
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
Multi-Wire Proportional Counter
  • Array of discrete detectors
  • Remove walls to get multi-wire counter
mwpc cont d
MWPC cont’d
  • Segment the cathode to get x-y position
resistive encoding of a multi wire detector
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
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
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
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
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
slide25
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%.

16 element wand prototype schematic and results
16-element WAND Prototype Schematic and Results

Clear Fiber

2-D tube

Coincidence tube

Neutron Beam

Wavelength-shifting fiber

Aluminum wire

Scintillator Screen

slide27
Principle of Crossed-Fiber

Position-Sensitive Scintillation Detector

Outputs to multi-anode photomultiplier tube

1-mm Square Wavelength-shifting fibers

Scintillator screen

Outputs to coincidence single-anode photomultiplier tube

neutron scattering from germanium crystal using crossed fiber detector
Neutron Scattering from Germanium Crystal Using Crossed-fiber Detector
  • Normalized scattering from 1-cm high germanium crystal
  • En ~ 0.056 eV
  • Detector 50-cm from crystal
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