ieap – ctu prague seminar advanced semiconductor detectors of neutrons institute of experimental...

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

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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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3D geometry arrays- comparison of cylindrical vs. cubic 10B converter


CylinderCube

Maximal efficiency: ~36% Maximal efficiency: ~31%

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3D geometry arrays- comparison of LiF vs. amorphous B converter


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

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Roen

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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

ieap – ctu prague seminar advanced semiconductor detectors of neutrons institute of experimental...

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