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Seminar IEAP – CTU Prague
Advanced semiconductor detectors of neutrons
Institute of Experimental and Applied Physics Czech Technical University in Prague
Josef Uher
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seminar IEAP – CTU Prague, Josef Uher
Outline
Neutron detection principleLimitations of single planar structure3D detectorsSimulation resultsConverter filling technologyExperimental tests
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The standard planar detector silicon detector
Semiconductor itself can not detect neutrons directly!
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Planar detector + neutron converter
Conversion of thermal neutrons to heavy charged particles in 6Li or 10B converter layer.
10B reaction (Cross section 3840 barns at 0.0253 eV): 10B+n a (1.47 MeV) + 7Li (0.84 MeV) + (0.48MeV) (93.7%)10B+n a (1.78 MeV) + 7Li (1.01 MeV) (6.3%)
6Li reaction (cross section 940 barns at 0.0253 eV) : 6Li + n (2.05 MeV) + 3H (2.72 MeV)
Converter (LiF)
Detector
T
T
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Planar geometry– comparison of efficiency (amorphous B vs. LiF)
6LiF, enrichment 90% Amorphous 10B, enrichment 80%
Threshold 50 keV
Efficiencies are comparable. Higher cross section of 10B does not spawn a significant increase of efficiency.
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2D neutron array modification
“Egg plate” 2D type(with enlarged surface to increase the detector efficiency)
converter
sensitive volume
T
Neutron beam
n+
p+
n“Standard” 2D typeback side contact
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Neutron array modification
“3D inverse” structure(there are pillars instead of pores)
low n+
p+
n“Channel” 2D type(maximized filling)
Neutron beamback sidecontact grid
bottom view
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Examples of created structures
Photo-electrochemical etching
Laser ablation
(KTH, Stockholm)
(University of Glasgow)
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“3D inverse” structureProcessed using sawfor chip separation
100x100m60m deep
200x200m60m deep
300x300m60m deep
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3D geometry arrays- comparison of cylindrical vs. cubic 6LiF converter
Fixed wall thickness – variance in the converter / cell size
CylinderCube
Maximal efficiency: ~32% Maximal efficiency: ~27%
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seminar IEAP – CTU Prague, Josef Uher
3D geometry arrays- comparison of cylindrical vs. cubic 10B converter
Fixed wall thickness – variance in the converter / cell size
CylinderCube
Maximal efficiency: ~36% Maximal efficiency: ~31%
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seminar IEAP – CTU Prague, Josef Uher
3D geometry arrays- comparison of LiF vs. amorphous B converter
Fixed wall thickness – variance in the converter / cell size
Threshold 50 keV
LiF – with increasing converter density is increasing efficiency (max ~32%, density 2.64 g/cm3 (!))
B – with increasing converter density is decreasing efficiency (max ~36%, density 1.0 g/cm3 (!))
LiF B
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Deposited Energy Distribution
The edge between
the converter andthe
semiconductor
LiF, =1.2 gcm-3
40m diameterLiF, =2.64 gcm-3
40m diameter
The thermal neutron beam diameter is 2m and it is penetratingthe LiF converter in the center.
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Deposited Energy Distribution
The edge between
the converter andthe
semiconductor
LiF, =1.4 gcm-3
100m diameterLiF, =2.64 gcm-3
100m diameter
The thermal neutron beam diameter is 2m and it is displacedm from the LiF converter center.
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Planar and 3D geometry spectra comparison
LiF density: 2.0 g/cm3
Surface density 2 mg/cm2
Layer thickness 10 mDiameter 58 m
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Pores filling using pressure
Empty pores
Poured powder
Covered by foil
Lead hob
Final preparationready for pressing
Pressing
Chip
Metal pad
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Pores filling using pressure
LiF
BaSO4
BaSO4 BaSO4
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Pores filling using pressureRoentgenogram of filled structures
LiF BaSO4
Estimated average filling depth is 150m
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2D stuffed detector
A next step in the development would be 2D detector diode with etched pores filled with a neutron converter.
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Experimental samples
Charge collection tests Detection efficiency tests
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Pyramidal dips
Spectrum - angle 0 deg
Interpretation of such integral measurement is difficult. Further measurements are necessary.
Spectrum - angle 70 deg
Characterization of the structure using alphas from 241Am source.
Position of the first peak
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The Medipix 2 imaging detector
Pixel array: 256x256Pixel size: 55x55 mm2
Total sensitive area: ~2 cm2
Electronics for each pixel: preamplifier, two discriminators (energy window), 13-bit counterRead out: serial - 9 ms, parallel - 266 ms (clock 100MHz)
Serial readout speed: 6 fpsIntegrated source of variable detector bias (5 - 105V) 4kB EEPROM for configurationTemperature monitoring Cables with length up to 5mAbility to flash a firmware
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Measurements
Horizontal channel (neutron guide) of the LVR-15 nuclear research reactor at Nuclear Physics Institute of the Czech Academy of Sciences at Rez near Prague.
Intensity about 107 neutrons/(cm2s) at reactor power of 8MW Beam Cross section: 4 mm (height) x 60 mm (width) The divergence of the neutron beam is < 0.5°
Spallation neutron source in Paul Sherrer Institut at Villigen in Switzerland
Intensity about 3·106 neutrons/(cm2s) at proton accelerator current of 1mA and proton energy of 590 MeV
Beam Cross section: 40 cm in diameter
Parameters of the Thermal Neutron Beam:
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Comparison of Medipix-2 and other neutron imaging detectors
Tested:-CCD camera with scintilator mixed with 6Li (pixel size 0.139 mm)-Imaging plate (excitation by neutrons, deexcitation by laser scanner followed by light emission, pixel size 50m)Medipix-2
=0.83 pixel =46 m
CCD camera
=2.5 pixel =350 m
=1.06 pixel =53 m
Imaging plate
=0.93 pixel =158 m
Medipix-1
Medipix-2 has the highest dynamic range and resolution.The only disadvantages are lower efficiency (2-3%) and sensitive area.
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Sample objects – blank cartridge
Photograph
CCD Medipix-1Imaging plate Medipix-2
Neu
tron
og
rap
hy
Roen
tgen
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hy
Medipix-2Medipix-1
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Sample objects – fishing line
Medipix-1 Imaging plate Medipix-2
Fishing line diameter 100 m
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Conclusion
Simulation software
More geometries has been simulated
Optimal structure parameters have been found
Filling using pressure has been tested
Testing devices have been proposed
Neutron imaging using Medipix detector has been successfully tested
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