Advanced semiconductor detectors of neutrons

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

2. Outline • Neutron detection principle • Limitations of single planar structure • 3D detectors • Simulation results • Converter filling technology • Experimental tests

3. The standard planar detector silicon detector Semiconductor itself can not detect neutrons directly!

4. T Converter (LiF) a Detector a T 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) + g (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  a (2.05 MeV) + 3H (2.72 MeV)

5. 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.

6. 2D neutron array modification Neutron beam converter T back side contact n+ “Standard” 2D type n a sensitive volume p+ “Egg plate” 2D type (with enlarged surface to increase the detector efficiency)

7. Neutron array modification Neutron beam back side contact grid low n+ “Channel” 2D type (maximized filling) n p+ “3D inverse” structure (there are pillars instead of pores) bottom view

8. Examples of created structures Photo-electrochemical etching (KTH, Stockholm) Laser ablation (University of Glasgow)

9. “3D inverse” structure Processed using saw for chip separation 300x300mm 60mm deep 100x100mm 60mm deep 200x200mm 60mm deep

10. 3D geometry arrays- comparison of cylindrical vs. cubic 6LiF converter Fixed wall thickness – variance in the converter / cell size Cube Cylinder Maximal efficiency: ~32% Maximal efficiency: ~27%

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%

12. 3D geometry arrays- comparison of LiF vs. amorphous B converter Fixed wall thickness – variance in the converter / cell size B LiF 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 (!))

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.

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.

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

16. Pores filling using pressure Chip Lead hob Empty pores Metal pad Final preparation ready for pressing Poured powder Pressing Covered by foil

17. Pores filling using pressure BaSO4 BaSO4 BaSO4 LiF

18. Pores filling using pressure Roentgenogram of filled structures BaSO4 LiF Estimated average filling depth is 150mm

19. 2D stuffed detector • A next step in the development would be 2D detector diode with etched pores filled with a neutron converter.

20. Experimental samples Charge collection tests Detection efficiency tests

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.

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 - 266 ms (clock 100MHz) Serial readout speed: 6 fps Integrated source of variable detector bias (5 - 105V) 4kB EEPROM for configuration Temperature monitoring Cables with length up to 5m Ability to flash a firmware

23. Measurements Parameters of the Thermal Neutron Beam: • Horizontal channel (neutron guide) of the LVR-15 nuclear research reactorat 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

24. CCD camera Medipix-2 Medipix-1 Imaging plate s=1.06 pixel =53 mm s=0.83 pixel =46 mm s=0.93 pixel =158 mm s=2.5 pixel =350 mm 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 50mm) Medipix-2 has the highest dynamic range and resolution. The only disadvantages are lower efficiency (2-3%) and sensitive area.

25. Medipix-2 Sample objects – blank cartridge Roentgenography Photograph Medipix-1 Neutronography Imaging plate CCD Medipix-1 Medipix-2

26. Sample objects – fishing line Fishing line diameter 100 mm Medipix-1 Imaging plate Medipix-2

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