B. Mikulec* M. Campbell, E. Heijne, X. Llopart, L. Tlustos CERN, Medipix Collaboration * now with the University of Geneva, Switzerland X-ray Imaging Using Single Photon Processing with Semiconductor Pixel Detectors

Bettina Mikulec Vertex 2002, 8 Nov The Origins...  High energy physics: unambiguous reconstruction of particle patterns with micrometer precision low input noise due to tiny pixel capacitance WA 97, RD 19 (CERN): 208 Pb ions on Pb target 7 planes of silicon pixel ladders; 1.1 M pixels

Bettina Mikulec Vertex 2002, 8 Nov Hybrid Pixel Detectors Electronics  CMOS technology advances steadily; Moore’s law Sensors  new materials to increase stopping power and CCE; main problem: inhomogeneities!

Bettina Mikulec Vertex 2002, 8 Nov Single Photon Processing  Quantum imaging  Example: photon counting  Q has to correspond to a single particle! if  Q > process signal

Bettina Mikulec Vertex 2002, 8 Nov Quantum Imaging - Advantages  Noise suppression high signal-to-noise ratio; dose reduction low rate imaging applications  Linear and theoretically unlimited dynamic range  Potential for discrimination of strongly Compton scattered photons (for mono-energetic sources) or e.g. fluorescence X-rays  Energy weighting of photons with spectral sources possible higher dose efficiency; dose reduction

Bettina Mikulec Vertex 2002, 8 Nov Medical Imaging Detector requirements (sensor and electronics) depend on diagnostic X-ray imaging application. Example: mammography  spatial resolution 5-20 lp/mm  high contrast resolution (<3%)  uniform response  patient dose <3 mGy  imaging area: 18 x 24 (24 x 30) cm 2  compact and easy to handle  stable operation  no cooling  digital  cheap Moore and direct detection quantum processing sensors to be improved high DQE (sensor + q.p.) to be solved ???

Bettina Mikulec Vertex 2002, 8 Nov Medipix1 / Medipix2 Medipix1  square pixel size of 170 µm  64 x 64 pixels  sensitive to positive input charge  detector leakage current compensation columnwise  one discriminator  15-bit counter per pixel  count rate: ~1 MHz/pixel (35 MHz/mm 2 )  parallel I/O  1  m SACMOS technology (1.6M transistors/chip) Medipix2  square pixel size of 55 µm  256 x 256 pixels  sensitive to positive or negative input charge (free choice of different detector materials)  pixel-by-pixel detector leakage current compensation  window in energy  discriminators designed to be linear over a large range  13-bit counter per pixel  count rate: ~1 MHz/pixel (0.33 GHz/mm 2 )  3-side buttable  serial or parallel I/O  0.25  m technology (33M transistors/chip)

Bettina Mikulec Vertex 2002, 8 Nov Medipix1 / Medipix  m  m the prototype… the new generation!

Bettina Mikulec Vertex 2002, 8 Nov Medipix1 Applications Examples:  Dental radiography  Mammography  Angiography  Dynamic autoradiography  Tomosynthesis  Synchrotron applications  Electron-microscopy  Gamma camera  X-ray diffraction  Neutron detection  Dynamic defectoscopy  General research on photon counting!

Bettina Mikulec Vertex 2002, 8 Nov Applications Mammography (INFN Pisa, IFAE Barcelona): Mo tube 30 kV; Medipix1; part of a mammographic accreditation phantom Dynamic Autoradiography: (INFN Napoli): Medipix1; 14 C L-Leucine uptake from the solution into Octopus vulgaris eggs (last slice in time: 80 min)

Bettina Mikulec Vertex 2002, 8 Nov Applications Sens-A-Ray commercial dental CCD system (Regam Medical) Medipix1 160  Gy80  Gy40  Gy Dental Radiography (Univ. Glasgow, Univ. Freiburg, Mid-Sweden Univ.):

Bettina Mikulec Vertex 2002, 8 Nov Medipix1 - SNR Pixel-to-pixel non-uniformities: optimum for counting systems: Poisson limit  N optimum SNR = N /  N determined SNR for Medipix1 taking flood fields (Mo tube) covering the entire dynamic range of the chip:  SNR uncorr (max.) ~30 using a flatfield correction  Medipix1 follows perfectly the Poisson limit! SNR uncorr Red curve = Poisson limit

Bettina Mikulec Vertex 2002, 8 Nov Medipix1 - SNR SNR uncorr  flat field corrects mainly sensor non-uniformities! 8.5 keV11.7 keV12.4 keV with adj. (35V det. bias) without adj. (35V det. bias) 7 with adj. (17V det. bias) 19.2 with adj. (80V det. bias) 30.7  differences in the raw SNR, but with flat field correction the Poisson limit is ALWAYS reached  BUT: flat field correction dependent on energy spectrum!  working in over-depletion reduces charge sharing effects SNR uncorr

Bettina Mikulec Vertex 2002, 8 Nov Medipix1 Flat Field Studies raw image flat field corrected 17 V detector bias (under-depleted) 35 V detector bias (fully depleted) ‘waves’ due to bulk doping non-uniformities wrong flat field; inverse ‘waves’, BUT: single pixel inhomogeneities smeared out  fixed pattern noise! 2 kinds of non-uniformities: ‘waves’ and fixed pattern noise

Bettina Mikulec Vertex 2002, 8 Nov Si Wave Patterns  vary detector bias voltage from under- to over-depletion  divide flat field with V 4 V 8 V12 V16 V24 V32 V48 V64 V80 V

Bettina Mikulec Vertex 2002, 8 Nov Si Wave Patterns Section of the correction map for different detector bias:  ‘waves’ move in under-depletion; stable in over-depletion  amplitude decreases with bias, but waves don’t disappear completely Remark: images can be corrected for these non-uniformities

Bettina Mikulec Vertex 2002, 8 Nov Dose Optimization  Dose optimization for specific imaging tasks: example: accumulation of single X-ray signals during X-ray of an anchovy

Bettina Mikulec Vertex 2002, 8 Nov Summary Medipix1  The Medipix1 prototype chip allows to study the photon counting approach  Comparison to charge integrating systems turned out to be sometimes difficult due to the larger pixel size of Medipix1  Most of the problems encountered were due to sensor non- uniformities (e.g. locally varying leakage currents) and bump-bonding quality  Medipix1 turned out to be a tool to study the attached sensor; even silicon sensors show non-uniformities  The flat field correction was intensively studied and allows to minimize the pixel-to-pixel variations down to the Poisson limit over the full dynamic range of the chip. The energy dependence of the flat field correction has to be further investigated.  The experience with Medipix1 lead to many improvements implemented in the Medipix2 ASIC.

Bettina Mikulec Vertex 2002, 8 Nov Medipix2 Characterization  all the reported measurements were done using the electronic calibration (injection capacitor + external voltage pulse).  The 8 fF injection capacitor nominal value has a tolerance of 10%.  The dedicated Muros2 readout system had been used

Bettina Mikulec Vertex 2002, 8 Nov Medipix2 Characterization unadjusted thresholds ~500 e - rms adjusted thresholds ~110 e - rms

Bettina Mikulec Vertex 2002, 8 Nov Medipix2 Characterization  Threshold linearity in the low threshold range:

Bettina Mikulec Vertex 2002, 8 Nov Medipix2 Characterization threshold at 2 ke - injection of 1000 pulses of 3 ke - matrix unmasked

Bettina Mikulec Vertex 2002, 8 Nov Summary of the Electrical Measurements Electron/Hole Collection Gain~12 mV/ke - Non-linearity<3% to 80 ke - Peaking time<200 ns Return to baseline <1  s for Q in <50 ke - Electronic noise  n THL ~100 e -  n THH ~100 e - Threshold dispersion  THL ~500 e -  THH ~500 e - Adjusted threshold dispersion  THL ~110 e -  THH ~110 e - Minimum threshold~1000 e - Analog power dissipation ~8  W/channel at 2.2 V supply

Bettina Mikulec Vertex 2002, 8 Nov Conclusions  Miniaturization of CMOS technology allows for small pixel sizes and increased functionality.  A new single photon processing chip Medipix2 consisting of a 256 x 256 matrix of 55  m square pixels has been produced and successfully characterized.  The potential of quantum imaging for various applications is still far from being fully explored.  Quantum imaging in the medical domain: rather complete systems are required to convince end users MTF and DQE curves as well as comparative phantom images are necessary for approval (see e.g. FDA)  A lot of progress has been made to achieve large areas; as yet no satisfactory solution for most medical applications  There is a trend in some applications towards object characterization in addition to simple transmission images  need energy information  colour X-ray imaging

Bettina Mikulec Vertex 2002, 8 Nov ‘Wishlist’ sensors: high absorption efficiency and improved homogeneity reliable ASIC-to-sensor connections tiling: large areas without dead space ASIC:  small pixel size with charge sharing solutions (modern CMOS technologies!)  low-noise front-end with appropriate sensor leakage current compensation; sensitive to electron and hole signals  very fast front-end for time-resolved studies  a precise threshold above noise  a multi-bit ADC/pixel for energy information (optimum weighting!)  large dynamic range  …??? cost!

Bettina Mikulec Vertex 2002, 8 Nov Medipix1 Flat Field Studies  vary detector bias voltage from under- to over-depletion  calculate corresponding flat field from flood images (1 st row)  divide with correction map from 100 V detector bias data (2 nd row) a phantastic tool to study sensor inhomogeneities…

Bettina Mikulec Vertex 2002, 8 Nov Medipix1 Flat Field Studies

Bettina Mikulec Vertex 2002, 8 Nov Medipix2 Characterization unadjusted thresholds ~400 e - rms adjusted thresholds ~110 e - rms mean ~1100 e - spread ~160 e - rms