Download - O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Neutron Detectors for Materials Research T.E. Mason Associate Laboratory Director Spallation.

OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY

Neutron Detectors for Materials Research

T.E. Mason

Associate Laboratory Director

Spallation Neutron SourceAcknowledgements: Kent Crawford & Ron Cooper

2OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY

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* + 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


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)


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

n Li He H MeV 6 4 3 4 79.

barns8.1

940


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


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

n Li He H MeV 6 4 3 4 79.

barns8.1

940


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?


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: 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


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


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


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


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


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


All fibers installed and connected to multi-anode photomultiplier mount