1. Advanced semiconductor detectors of neutrons
Institute of Experimental and Applied Physics Czech Technical University in Prague
Josef Uher
Seminar

2. Outline Neutron detection principle
Limitations of single planar structure
3D detectors
Simulation results
Converter filling technology
Experimental tests
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3. The standard planar detector silicon detector
Semiconductor itself can not detect neutrons directly!
The bias (V) needed to deplete a detector is proportional to the square of the distance between the electrodes. For a planar detector, this is equal to the thickness of the detector. Similarly, the collection time (Tc) is proportional to the distance between the electrodes and, assuming a single trap level with a lifetime τ, the trapping is related to exp(-tc/τ).
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4. 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 eV):
10B+n  a (1.47 MeV) + 7Li (0.84 MeV) + g (0.48MeV) (93.7%)
10B+n  a (1.78 MeV) + 7Li (1.01 MeV) (6.3%)
6Li reaction (cross section 940 barns at eV) :
6Li + n  a (2.05 MeV) + 3H (2.72 MeV)
Converter (LiF)
Detector a T
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5. Planar geometry – comparison of efficiency (amorphous B vs. LiF)
6LiF, enrichment 90%
Amorphous 10B, enrichment 80%
LiF: ranges in Si are RT=44.1um, Ra=8.6um, in LiF: 48um / 8um (1.8g/cm3)
B: ranges in Si are RLi=3um / 2.7um, Ra=5.4um / 5.2um, in B: 5um / 8um (1 g/cm3)
Threshold 50 keV
Efficiencies are comparable. Higher cross section of 10B does not spawn a significant increase of efficiency.
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6. 2D neutron array modification
Neutron beam
converter T
back side contact n+
“Standard” 2D type n a p+
sensitive volume
“Egg plate” 2D type
(with enlarged surface to increase
the detector efficiency)
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7. Neutron array modification
Neutron beam
back side
contact grid
“Channel” 2D type
(maximized filling)
low n+ n p+
“3D inverse” structure
(there are pillars instead of pores)
bottom view
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8. Examples of created structures
Photo-electrochemical etching
(KTH, Stockholm)
An example of structures created in KTH Stockholm and University of Glasgow. (just examples)
Laser ablation
(University of Glasgow)
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9. “3D inverse” structure Processed using saw for chip separation
300x300mm
60mm deep
100x100mm
60mm deep
200x200mm
60mm deep
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10. 3D geometry arrays - comparison of cylindrical vs. cubic 6LiF converter
Fixed wall thickness – variance in the converter / cell size
Cube
Cylinder
Ratio of captured neutrons:
LiF: Box … 77.8%, cylinder … 61.0%, planar … 28.8%
B: Box … 85.8%, cylinder … 65.9%, planar … 25.8%
Maximal efficiency: ~32%
Maximal efficiency: ~27%
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11. 3D geometry arrays - comparison of cylindrical vs. cubic 10B converter
Fixed wall thickness – variance in the converter / cell size
Cube
Cylinder
Maximal efficiency: ~36%
Maximal efficiency: ~31%
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12. 3D geometry arrays - comparison of LiF vs. amorphous B converter
Fixed wall thickness – variance in the converter / cell size
LiF B
Hustotou je ovlivněn makroskopický účinný průřez
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 (!))
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13. Deposited Energy Distribution
The edge between
the converter and
the semiconductor
LiF, r=1.2 gcm-3
40mm diameter
LiF, r=2.64 gcm-3
40mm diameter
The thermal neutron beam diameter is 2mm and it is penetrating
the LiF converter in the center.
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14. Deposited Energy Distribution
The edge between
the converter and
the semiconductor
LiF, r=1.4 gcm-3
100mm diameter
LiF, r=2.64 gcm-3
100mm diameter
The thermal neutron beam diameter is 2mm and it is displaced
45mm from the LiF converter center.
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15. Planar and 3D geometry spectra comparison
LiF density: 2.0 g/cm3
Surface density 2 mg/cm2
Layer thickness 10 mm
Diameter 58 mm
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16. Pores filling using pressure
Chip
Empty pores
Lead hob
Metal pad
Final preparation
ready for pressing
Poured powder
BaSO4 = barium sulfide
Lis = presser
force = 80kN
Pressing
Covered by foil
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17. Pores filling using pressure
BaSO4
BaSO4
BaSO4
LiF
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18. Pores filling using pressure
Roentgenogram of filled structures
BaSO4
LiF
In case of BaSO4 are pores uniformly filled
In case of LiF there is a higher noise due to lower Z of converter
Average attenuation => estimation of filling depth. Pores depth is 150um
Estimated average filling depth is 150mm
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19. 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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20. Experimental samples Charge collection tests
Detection efficiency tests
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21. Pyramidal dips
Characterization of the structure using alphas from 241Am source.
Spectrum - angle 0 deg
Spectrum - angle 70 deg
Position of the first peak
Interpretation of such integral measurement is difficult. Further measurements are necessary.
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22. The Medipix 2 imaging detector
Pixel array: 256x256
Pixel size: 55x55 mm2
Total sensitive area: ~2 cm2
Electronics for each pixel: preamplifier, two discriminators (energy window), 13-bit counter
Read out: serial - 9 ms, parallel ms (clock 100MHz)
Serial readout speed: 6 fps
Integrated source of variable detector bias ( V)
4kB EEPROM for configuration
Temperature monitoring
Cables with length up to 5m
Ability to flash a firmware
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23. Measurements Parameters of the Thermal Neutron Beam:
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
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24. Comparison of Medipix-2 and other neutron imaging detectors
Tested:
CCD camera with scintilator mixed with 6Li (pixel size mm)
Imaging plate (excitation by neutrons, deexcitation by laser scanner followed by light emission, pixel size 50mm)
Medipix-2
s=0.83 pixel
=46 mm
CCD camera
s=2.5 pixel
=350 mm
s=1.06 pixel
=53 mm
Imaging plate
s=0.93 pixel
=158 mm
Medipix-1
The resolution the best in the World.
Efficiency – 3D structures
Area – quads (8cm^2)
Medipix-2 has the highest dynamic range and resolution.
The only disadvantages are lower efficiency (2-3%) and sensitive area.
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25. Sample objects – blank cartridge
Medipix-2
Roentgenography
Photograph
Medipix-1
Neutronography
Imaging
plate
CCD
Medipix-1
Medipix-2
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26. Sample objects – fishing line
Fishing line diameter 100 mm
Medipix-1
Imaging plate
Medipix-2
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27. 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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