Neutron Detectors for Materials Research - PowerPoint PPT Presentation

Loading...

PPT – Neutron Detectors for Materials Research PowerPoint presentation | free to view - id: 77d292-MTUxM



Loading


The Adobe Flash plugin is needed to view this content

Get the plugin now

View by Category
About This Presentation
Title:

Neutron Detectors for Materials Research

Description:

Neutron Detectors for Materials Research T.E. Mason Associate Laboratory Director Spallation Neutron Source Acknowledgements: Kent Crawford & Ron Cooper – PowerPoint PPT presentation

Number of Views:136
Avg rating:3.0/5.0
Slides: 30
Provided by: Thomas1307
Category:

less

Write a Comment
User Comments (0)
Transcript and Presenter's Notes

Title: Neutron Detectors for Materials Research


1
Neutron Detectors for Materials Research
  • T.E. Mason
  • Associate Laboratory Director
  • Spallation Neutron Source
  • Acknowledgements Kent Crawford Ron Cooper

2
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

3
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

4
Gas Detectors
25,000 ions and electrons produced per neutron
(4?10-15 coulomb)
5
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

6
MAPS Detector Bank
7
Scintillation Detectors
8
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
9
GEM Detector Module
10
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
11
Semiconductor Detectors
12
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?

13
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

14
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

15
Pulse Height Discrimination
16
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)

17
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

18
Multi-Wire Proportional Counter
  • Array of discrete detectors
  • Remove walls to get multi-wire counter

19
MWPC contd
  • Segment the cathode to get x-y position

20
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

21
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

22
Anger camera detector on SCD
  • Photomultiplier outputs are resistively encoded
    to give x and y coordinates
  • Entire assembly is in a light-tight box

23
Micro-Strip Gas Counter
  • Electrodes printed lithgraphically
  • Small features high spacial resolution, high
    field gradients charge localization and fast
    recovery

24
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

25
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.
26
16-element WAND Prototype Schematic and Results
Clear Fiber
2-D tube
Coincidence tube
Neutron Beam
Wavelength-shifting fiber
Aluminum wire
Scintillator Screen
27
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
28
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

29
All fibers installed and connected to multi-anode
photomultiplier mount
About PowerShow.com