1. Somewhat different applications for photon counting pixel detectors 11th ICATPP Conference Villa Olmo, Como, October 9 2009 Thilo Michel, G. Anton, J. Durst, P. Takoukam, E. Guni, M. Böhnel, U. Gebert, T. Rügheimer, M. Firsching

2. Outline Material resolved X-ray imaging Radiation monitoring X-ray polarimetry Hybrid Photon Detector (HPD) Summary 

3. The Medipix2/Timepix detectors: hybrid photon counting pixel detectors ASIC/Sensor: –Development: International Collaboration with seat at CERN –Bump-bonded with Pb/ Sn –65536 pixels –Pixel pitch: 55 µm –Size of the matrix: 14 mm (approx. 2 cm 2 ) –0.25 µm CMOS technology Sensor: –Materials: Si, GaAs, CdTe –Bias voltage: e.g. 150 V (300 µm Si) 14 mm E

4. Timepix: counting or Time-Over-Threshold or Time-To- Shutter in each pixel individually Time-over-ThresholdTime-To-Shutter Reaction in sensor Output preamp. Discrimi- nator Counter ToT = f(E depos ) = 10 nsec*N clockpulses t t t t TtS = 10 nsec*N clockpulses t t t t DAQ t StartStop DAQ t StartStop Reaction in sensor Output preamp. Discrimi- nator Counter

5. Charge sharing affects the energy resolution

6. Simulation of the response matrix: deposited energy versus primary energy Simulated set of response functions

7. Principle of material resolved imaging X-Ray transmission through a compound object mass attenuation coefficient areal density index for basis materials Detector source Aim: determine the densities of (base) materials in the object

8. Material reconstruction without explicit spectrum reconstruction Number of counts in energy deposition interval Sum over energy depositions in interval Sum over primary energies in spectrum Sum over materials Vary to get best agreement of M b and measurement in a  2 -fit

9. Test in a small animal scanner Measurements performed with a small animal scanner of the University of Canterbury fulfilling ethics requirements Threshold adjustment with flatfield images at the K-edge of iodine Measurement and reconstruction of the X-ray tube spectrum with Medipix2 Data acquisition: 4 images with different thresholds (12, 17, 33 und 42 keV) per detector position (3) and projection (360) Details Clinical contrast agent with iodine

10. Material reconstructed projections Photon counting image Non-iodine (water) image Iodine image

11. Material reconstruction in CT: Brightness = density of base materials (g/cm 3 ) Photon counting images @ 4 thresholds water / non iodine iodine Brightness = density of materials

12. Outline Material resolved X-ray imaging Radiation monitoring X-ray polarimetry Hybrid Photon Detector (HPD) Summary 

13. Methods for the reconstruction of incident X-ray spectra at very high flux Response spectrum of polychromatic irradiation Matrix-Inversion-Method Best estimate for impinging spectrum is given by: Spectrum-Stripping-Method Subtract the monoenergetic response functions successively: Medipix2 response spectrum Impinging spectrum Energy [keV] Number of counts or photons R(E i |, E j ) : the probability that a photon of energy E j causes an energy deposition of E i | …

14. Threshold scan of the spectrum of an Am-241 photon source Energy [keV] Intensity [a.u.]

15. Derivative of the threshold scan Energy in keV Intensity [a.u.]

16. Reconstructed spectrum of the Am-241 photon source using the (iterative) spectrum stripping method Reconstruction bin width 0.5 keV Energy resolution (RMS) better than 1 keV 0.25 Counts per pixel per second at the lowest energy threshold (5keV) 8 days acquisition time (Medipix operates very stable) Number of photons per 0.5 keV 14.6 keV 18.9 keV 22.0 keV 26.3 keV 59.5 keV Energy in keV

17. Measurement of an X-ray tube spectrum at high flux Number of photons per 2 keV Energy in keV Literature (scaled) Measured

18. Method for dosimetry with pixel detectors Monochromatic irradiation with E j Calibrate with j max different photon energiesApply pseudoinverse of the „counts matrix“ Known doses Number of counts in each energy deposition bin Unknown calibration factors from counts to dose Measure/simulate the number of counts in energy deposition intervals Best estimation (maximum-likelihood) for calibration factors Energy deposition [keV] Counts N 1 (60)N 2 (60)N 3 (60)

19. Experimental test of the method with the Medipix2 Measurement details Threshold adjustment registers detuned: 8 thresholds simultaneously (15 – 71 keV) Calibration with 9 different X-ray spectra (filtered/unfiltered) Air-Kerma free-in-air measured with reference dosemeter Test with different X-ray spectra Result of dose (kerma) reconstruction for X-ray spectra...... N 1 (RQR 40 keV) N 8 (RQR 40 keV) Good reconstruction of air kerma free-in-air Relative error of recon. Kerma [%] Tube voltage [kV]

20. Measurement of the temporal structure of pulsed radiation fields with the Timepix Measurements performed in collaboration with the PTB Portable 150 kVp X-ray generator Timepix operated in Time-To-Shutter mode with 90 MHz clock frequency Approximately 12 m away from tube Analysis of single hits Time after X-ray flash in ns Number of conts in a.u.

21. Outline Material resolved X-ray imaging Radiation monitoring X-ray polarimetry Hybrid Photon Detector (HPD) Summary 

22. Measurement of the degree of linear X-ray polarization using photoelectric effect Polarized irradiation This example: The probability to trigger adjacent pixels in one column (N c ) is higher than to trigger two adjacent pixels in one row (N R ) Measuring quantity: the asymmetry E NCNC NRNR Unpolarized irradiation Differential cross section for K-shell photoeffect: Only the first microns carry the information about the original direction of emission Diffusion „disturbs“ the polarization signal Total path length: R(60keV) = 23 µm

23. Experimental setup Lead/ tungsten collimator Target (PMMA) Production of linearly polarized photons by 90° Compton scattering Timepix on rotation device X-ray tube (100 kVp) + Fe filter P = 98.6 %

24. The measured asymmetry is modulated with the angle  of the columns to the plane of polarization A pol = (0.96 +- 0.02) % A app = (0.19 +- 0.01) % A pol = (0.888 +- 0.042) % Simulation (ROSI): Results Counting mode Measurement: T. Michel, J. Durst (2008): “Evaluation of a hybrid photon counting pixel detector for X-ray polarimetry'', Nucl. Instr. and Meth. A 594: 188-195

25. Measurement of the degree of polarization in dependence on energy deposition in double hit events Subtraction of target and no-target measurements Polarization asymmetry Apparative asymmetry Simulation and measure- ment in agreement A pol = 0.2 % @ 29 keV A pol = 3.4 % @ 78 keV Time-over-threshold mode

26. Principle of polarimetry using Compton scattering Compton electron (1st hit) Compton photon (2nd hit)  Impinging photon Timepix-ASIC  E Scattered photon From top Cross section Cross Section of Compton scattering

27. Measurement result after search of clusters, coincidences, distance cuts, random subtraction, acceptance correction Modulation factor µ: Modulation curve: Number of events Scattering angle  °] T. Michel, J. Durst, J. Jakubek (2008): “X-ray polarimetry by means of Compton scattering in the sensor of a hybrid photon counting X-ray detector'', Nucl. Instr. and Meth. A, accepted manuscript

28. Simulation result Number of events Scattering angle  °] Simulation: ROSI (EGS4-based) Incoming radiation completely polarized Modulation factor: Degree of polarization of detected events:P = 97.1% Result:

29. Outline Material resolved X-ray imaging Radiation monitoring X-ray polarimetry Hybrid Photon Detector (HPD) Summary 

30. Measurements with an HPD test set-up at CERN  High voltage discharge lamp  Vacuum vessel with  Deflection mirror (position adjustable)  CsI photocathode  Accelerating electric field (max. 25 kV)  Timepix chipboard (mounted upside down)  Vacuum: about 10 -5 mbar Experimental set-up at CERN: Timing: Precision of the timing information of light signals (time resolution): 3 ns Due to the statistical light flashes of the lamp Trigger the end of the acquisition 3µs after the lamp flash In collaboration with:  Jacques Séguinot  Christian Joram  André Braem In collaboration with:  Jacques Séguinot  Christian Joram  André Braem

31. Results of the timing measurements Time resolution (singles):  single = 10.4 ± 0.1 ns All hits Only single hits 20 keV

32. An ion feedback time stamp Number of hits / 10 ns Time stamp in 10 ns Fotography of the ion feedback

33. Investigating the imaging properties of the test set-up (proximity-focusing electron optics)

34. Position resolution as a function of acceleration voltage d = 41mm E i = 0.45eV Energy in in keV Width of Point-Spread-Fuction in µm

35. Outline Material resolved X-ray imaging Radiation monitoring X-ray polarimetry Hybrid Photon Detector (HPD) Summary 

36. The Medipix/Timepix can be used in a lot of applications, because they can measure.... N(E  ) (x,y) N(E  ) and N(t) P(x,y) t(x,y)

37. Acknowledgement ECAP: G. Anton, J. Durst, P. Takoukam-Talla, E. Guni, A. Loehr, M. Böhnel, U. Gebert, T. Rügheimer, P. Bartl, B. Kreisler, F. Bayer, P. Sievers, F. Lück, I. Münster, A. Ritter, T. Weber, W. Haas, M. Firsching (now at Fraunhofer) Collaborators: J. Jakubek (CTU Prague), S. Pospisil (CTU Prague), A. Butler (U Canterbury), J. Seguinot (CERN), C. Joram (CERN), A. Braehm (CERN), M. Campbell (CERN), P. Ambrosi (PTB), U. Ankerhold (PTB), O. Hupe (PTB) …and the whole Medipix collaboration