1. Performance of the Medipix and Timepix devices for the recognition of electron-gamma radiation fields
C. Teyssier1,3, J. Bouchami1, F. Dallaire1, J. Idarraga1, C. Leroy1,
S. Pospisil 2, J. Solc2, O. Scallon1, Z. Vykydal 2
1Université de Montréal, Montréal (Québec) H3C 3J7, Canada
2Institute of Experimental and Applied Physics of the CTU in Prague, Horská 3a/22, CZ Praha 2 – Albertov, Czech Republic
3Universite de Lyon, F-69003, Lyon, France ; Universite Lyon 1, Villeurbanne, France ; CNRS/IN2P3, UMR5822, Institut de Physique Nucleaire de Lyon, F Villeurbanne, France
Carried out within the CERN MediPix Collaboration
Cécile Teyssier
September 2010

2. Silicon pixelated detectors designed for imaging
Also used in fundamental physics research
Characterisation of a radiation field
Nature of radiation? → study of electron and photon fields
Intensity of radiation? → study of photon detector efficiency
Cécile Teyssier
September 2010

3. I/ Presentation of the detectors
Structure
Charge sharing effect
Pattern recognition
II/ Mistagging: photon or electron?
Experimental setup and method
Results for Ru source
III/ Photon detector efficiency
Experimental setup
Results
Geant4 simulation
Cécile Teyssier
September 2010

4. I/ Presentation of the detectors
Structure
Charge sharing effect
Pattern recognition
II/ Mistagging: photon or electron?
Experimental setup and method
Results for Ru source
III/ Photon detector efficiency
Experimental setup
Results
Geant4 simulation
Cécile Teyssier
September 2010

5. MPX or Medipix2-USB Device :
- bump-bonded to Medipix2 readout chip containing in each pixel cell : preamplifier, comparator and counter.
- 300 µm thick silicon pixel detector
- 256 x 256 pixels each of 55 x 55 μm2 area
Cécile Teyssier
September 2010

6. Charge sharing effect Energy Particle Threshold Cluster size 001
preamplifier
comparator
Cluster size
counter
001
Cécile Teyssier
September 2010

7. Medipix2 response to different ionizing particles
Alpha particles
137Cs (t = 0.2 sec)
Photon, Electron
241Am (t = 2 sec)
Electron
106Ru (t = 0.15 sec)
Cécile Teyssier
September 2010

8. Valid under certain experimental conditions
Pattern recognition
Valid under certain experimental conditions
Cécile Teyssier
September 2010

9. Timepix device Same structure as Medipix but equipped with a clock
Measurement of the time over threshold in each pixel (TOT)
Evaluation of energy deposit after calibration
Cécile Teyssier
September 2010

10. I/ Presentation of the detectors
Structure
Charge sharing effect
Pattern recognition
II/ Mistagging: photon or electron?
Experimental setup and method
Results for Ru source
III/ Photon detector efficiency
Experimental setup
Results
Geant4 simulation
Photons detected by 3 interactions:
Photoelectric, Compton and pair creation
→ indirect detection via produced electrons
Cécile Teyssier
September 2010

11. Why? Photons detected via 3 interactions
Photoelectric effect
Compton effect
Pair creation
Hard to differentiate electrons produced by photons from electrons of the field.
Aim → To evaluate a mistagging rate
Cécile Teyssier
September 2010

12. Experimental setup 12 Medipix2-USB 106Ru source 137Cs source
θ rotation angle of the detector with respect to the vertical axis
Vacuum 12
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September 2010

13. Activity reconstruction
First case: all the tracks are considered
Second case : tracks associated to electrons
Third case : tracks associated to photons
Fraction of solid angle
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September 2010

14. Results for the 106Ru source
Simplified spectrum:
Gamma:
512 keV 20.4% ;
622 keV 9.9%.
Electron: 2 beta decays
Mean beta- energy:
1410 keV and keV
Known activity ±5%
Configuration legend
Photon: S: Single hits, D: Double hits, T: Triple hits, Q: Quad hits, L: Long gamma
Electron: C: Curly
Cécile Teyssier
September 2010

15. Evolution of the configuration
Cécile Teyssier
September 2010

16. Summary on mistagging Mis-association stongly depends on:
Energy of the particles (other experiment)
Incidence of the particles
Configuration setup
→ mistagging rate can be evaluated but is hard to extrapolate to another mixed field
Cécile Teyssier
September 2010

17. I/ Presentation of the detectors
Structure
Charge sharing effect
Pattern recognition
II/ Mistagging: photon or electron?
Experimental setup and method
Results for Ru source
III/ Photon detector efficiency
Experimental setup
Results
Geant4 simulation
Electrons produced by photons in surrounding materials can be detected.
Aim of this study: quantify this phenomenon
Cécile Teyssier
September 2010

18. Why?
Until now, detection efficiency calculated by considering the interaction probability in the silicon layer
But photons interact with surrounding materials and produced electrons can be detected, increasing the detector efficiency.
Aim → to quantify these changes
Cécile Teyssier
September 2010

19. Experimental setup 19 Simplified gamma spectrum: 32 keV 5.8% ;
Timepix-USB
Simplified gamma spectrum:
32 keV 5.8% ;
36 keV 1.3% ;
662 keV 85.1%.
Air or vacuum
137Cs source
Threshold 7.8keV
3mm thick
Al layer
(to get rid of e-) 19
Cécile Teyssier
September 2010

20. Rate of particles interacting with the detector
Theoretical rate
For air and vacuum:
Experimental rates:
Cécile Teyssier
September 2010

21. Experimental spectrum with timepix
33keV
Compton electron from 662keV photons:
Mean energy: 253keV
Maximal energy: 478 keV
→ compatible
Cécile Teyssier
September 2010

22. Simulation Geant4 environment
Programmation of the experimental elements
Reproduction of Medipix behaviour → same type of files generated.
Silicon layer
source
Green rays: photon
Red rays: electron
Cécile Teyssier
September 2010

23. Comparison of the rates
experimental
simulated
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September 2010

24. Simulation results and comparison
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September 2010

25. Origine of detected particles - Air
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September 2010

26. Origine of detected particles - Vacuum
residual air
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September 2010

27. Summary on photon detection
indirect detection → correction factor to theoretical detector efficiency.
Other experimental setups and other photon energies → other correction factors
Promising results of the simulation
Improvements to do:
Better definition of the experimental elements in the simulation (first results)
More precise spectrum
Cécile Teyssier
September 2010

28. Conclusion Photons or electrons?
Mistagging rate depends on several parameters that makes extrapolation difficult.
Photon detection efficiency greatly improved by surrounding materials. Correction factors can be evaluated.
Geant4 simulation for Medipix gives promising results
Cécile Teyssier
September 2010

29. Thank you!
Cécile Teyssier
September 2010

30. Cécile Teyssier
September 2010

31. Cécile Teyssier
September 2010

32. Cécile Teyssier
September 2010

33. Evaluation of mistagging rate
Negative mistagging if
Positive mistagging if
i is for photon or electron
Cécile Teyssier
September 2010

34. Mistagging rate for Ru
Cécile Teyssier
September 2010

35. results for the 137Cs source
Simplified spectrum:
Gamma:
32 keV 5.8% ; 36 keV 1.3% ;
662 keV 85.1%.
Electron:
Mean beta- energy: keV
625 keV 7.8%; 656 keV 1.4%
Cécile Teyssier
September 2010

36. Evolution of the configuration
Cécile Teyssier
September 2010

37. If we considere all the tracks as electrons…
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September 2010

38. Cécile Teyssier
September 2010

39. Prague results
Cécile Teyssier
September 2010

40. Cécile Teyssier
September 2010