Neutron Detectors for Materials Research

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Neutron Detectors for Materials Research

• T.E. Mason
• Associate Laboratory Director
• Spallation Neutron Source
• Acknowledgements Kent Crawford Ron Cooper

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Neutron Detectors

• What does it mean to detect a neutron?
• Need to produce some sort of measurable
  quantitative (countable) electrical signal
• Cant 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

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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

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Gas Detectors
25,000 ions and electrons produced per neutron
(4?10-15 coulomb)
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Gas Detectors contd

• 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

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MAPS Detector Bank
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Scintillation Detectors
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Some Common Scintillators for Neutron Detectors
 
 
 
 
 
Li glass (Ce)
1.75?1022
0.45
395 nm
7,000
2.8
470
51,000
LiI (Eu)
1.83?1022
9.2
160,000
ZnS (Ag) - LiF
1.18?1022
450
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GEM Detector Module
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Anger camera

• Prototype scintillator-based area-position-sensiti
  ve 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
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Semiconductor Detectors
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Semiconductor Detectors contd

• 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?

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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 doesnt reach
  detector
• poor pulse height discrimination

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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

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Pulse Height Discrimination
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Pulse Height Discrimination contd

• 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)

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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

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Multi-Wire Proportional Counter

• Array of discrete detectors

• Remove walls to get multi-wire counter

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MWPC contd

• Segment the cathode to get x-y position

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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

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Resistive Encoding of a Multi-wire Detector
contd

• 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

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Anger camera detector on SCD

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

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Micro-Strip Gas Counter

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

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Crossed-Fiber Scintillation Detector Design
Parameters (ORNL IC)

• 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 lt 5-mm
• Shaping time 300 nsec
• Count rate capability 1 MHz
• Time-of-Flight Resolution 1 msec

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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 6LiFZnS(Ag) was
determined to be 13. 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.
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16-element WAND Prototype Schematic and Results
Clear Fiber
2-D tube
Coincidence tube
Neutron Beam
Wavelength-shifting fiber
Aluminum wire
Scintillator Screen
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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
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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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All fibers installed and connected to multi-anode
photomultiplier mount