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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


Neutron Detector Screen Design I&C)

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 I&C)

Clear Fiber

2-D tube

Coincidence tube

Neutron Beam

Wavelength-shifting fiber

Aluminum wire

Scintillator Screen


Principle of Crossed-Fiber I&C)

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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