SNS Experimental FacilitiesOak Ridge X0000910/arb Neutron Detectors for Materials Research T.E....

SNS Experimental Facilities Oak RidgeX0000910/arb

Neutron Detectors for Materials Research

T.E. Mason

Experimental Facilities Division

Spallation Neutron SourceAcknowledgements: 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

– 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


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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* + 4He7Li + 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

n He H H MeV 3 3 1 0 76.

533318.

barns

~25,000 ions and electrons produced per neutron (~410-15 coulomb)


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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


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MAPS Detector Bank


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Scintillation Detectors

n Li He H MeV 6 4 3 4 79.

barns8.1

940


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Some Common Scintillators for Neutron Detectors

Li glass (Ce) 1.751022 0.45 % 395 nm ~7,000

LiI (Eu) 1.831022 2.8 % 470 ~51,000

ZnS (Ag) - LiF 1.181022 9.2 % 450 ~160,000

Material

Density of6Li atoms

(cm-3)

Scintillationefficiency

Photonwavelength

(nm)

Photons per neutron


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GEM Detector Module


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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


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Semiconductor Detectors

n Li He H MeV 6 4 3 4 79.

barns8.1

940


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Semiconductor Detectors cont’d

• ~1,500,000 holes and electrons produced per neutron (~2.410-13 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 doesn’t reach detector – poor pulse height discrimination


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Detection Efficiency

• Full expression: 1 e N t

• Approximate expression for low efficiency:

tN• Where:

= absorption cross-section

N = number density of absorber

t = thickness

N = 2.71019 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 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)


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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 cont’d

• 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 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


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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 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


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The scintillator screen for this 2-D detector consists of a mixtureof 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


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0

2000

4000

6000

8000

10000

0 5 10 15

GAUSSIAN FIT-FWHM = 5.41 mmDATA POINTS

FIBER NUMBER

CO

UN

TS

/10

SE

C

Neutron Beam

Coincidence tube

2-D tube

Scintillator Screen

Clear Fiber

Wavelength-shifting fiberAluminum wire

16-element WAND Prototype Schematic and Results


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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


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0

10

20

30

40

500

10

20

30

40

50

X-Axis

Y-Axis

1.40

1.05

0.70

0.35

0.00

-0.35

-0.70

-1.05

-1.40

Counts

Scattering Data from Germanium 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


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All fibers installed and connected to multi-anode photomultiplier mount

SNS Experimental FacilitiesOak Ridge X0000910/arb Neutron Detectors for Materials Research T.E....

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