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  • 1. Ballabriga, R
    et al.
    Campbell, M
    Heijne, E H M
    Llopart Cudié, Xavier
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Tlustos, L
    The Medipix3 prototype, a pixel readout chip working in single photon counting mode with improved spectrometric performance2006In: procedings of Nuclear Science Symposium IEEE 2006, Ocotber 2006, San Diego USA, 2006Conference paper (Refereed)
  • 2. Campbell, M
    et al.
    Chefdeville, M
    Colas, P
    Colijn, A P
    Fornaini, A
    Giomataris, Y
    van der Graaf, H
    Heijne, E H M
    Kluit, P
    Llopart Cudié, Xavier
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Schmitz, J
    Timmermans, J
    Visschers, J L
    Detection of single electrons by means of a Micromegas-covered Medipix2 pixel CMOS readout circuit2005In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 540, no 2-3, p. 295-304Article in journal (Refereed)
    Abstract [en]

    A small drift chamber was read out by means of a MediPix2 readout chip as a direct anode. A Micromegas foil was placed above the chip, and electron multiplication occurred in the gap. With a He/isobutane 80/20 mixture, gas multiplication factors up to tens of thousands were achieved, resulting in an efficiency for detecting single electrons of better than 90%. We recorded many frames containing 2D images with tracks from cosmic muons. Along these tracks, electron clusters were observed, as well as δ-rays.

  • 3.
    Chmeissani, M
    et al.
    IFAE Barcelona .
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Gal, O
    CEA France.
    Llopart, X
    CERN.
    Ludwig, J
    Albert-Ludwigs-Universität-Freiburg.
    Ponchut, C
    ESRF.
    Russo, P
    INFN, Italy.
    Tlustos, L
    CERN.
    First experimental tests with a CdTe photon counting pixel detector hybridized with a Medipix2 readout chip2004In: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 51, no 5, p. 2379-2385Article in journal (Refereed)
    Abstract [en]

    We present preliminary tests of hybrid pixel detectors consisting of the Medipix2 readout chip bump-bonded to a 1-mm-thick CdTe pixel detector. This room temperature imaging system for single photon counting has been developed within the Medipix2 European Collaboration for various imaging applications with X-rays and gamma rays, including dental radiography, mammography, synchrotron radiation, nuclear medicine, and radiation monitoring in nuclear facilities. The Medipix2 + CdTe hybrid detector features 256 × 256 square pixels, a pitch of 55 μm, a sensitive area of 14×14 mm2. We analyzed the quality of the detector and bump-bonding and the response to nuclear radiation of the first CdTe hybrids. The CdTe pixel detectors, with Pt ohmic contacts, showed an ohmic response when negatively biased up to less than 60 V (electrons collection mode). Tests were also performed in holes collection mode, where a nonresistive behavior was observed above +15 V. We performed a series of imaging tests at low voltage bias with gamma radioactive sources and with an X-ray tube. Under uniform irradiation, we observed for all detectors the presence of numerous, stable structures in the form of small circles of about 200 μm diameter, with the central pixels showing a reduced counting efficiency with respect to the periphery (in electrons counting regime). Also long filament structures have been observed. Further investigations will reveal whether they are due to an intrinsic detector response (e.g., due to Te inclusions) or to the bump-bonding process.

  • 4. Colas, P
    et al.
    Colijn, A P
    Fornaini, A
    Giomataris, Y
    van der Graaf, H
    Heijne, E H M
    Llopart Cudié, Xavier
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Schmitz, J
    Timmermans, J
    Visschers, J L
    The readout of a GEM or Micromegas-equipped TPC by means of the Medipix2 CMOS sensor as direct anode2004In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 535, no 1-2, p. 506-510Article in journal (Refereed)
    Abstract [en]

    We have applied the Medipix2 pixel CMOS chip as direct anode readout for a TPC. For the gas amplification two options have been investigated: (i) a three-stage GEM system and (ii) a Micromegas mesh. The structure of the cloud of primary electrons, left after interactions of 55Fe quanta with the gas is visible with unprecedented precision. This proof-of-principle is an essential step in our project to realize a monolithic pixel sensor with integrated Micromegas, to be developed specially for the readout of TPCs, and applicable for drift chambers in general.

  • 5.
    Llopart Cudié, Xavier
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Design and characterization of 64K pixels chips working in single photon processing mode2007Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Progress in CMOS technology and in fine pitch bump bonding has made

    possible the development of high granularity single photon counting detectors for

    X-ray imaging. This thesis studies the design and characterization of three pulse

    processing chips with 65536 square pixels of 55 μm x 55 μm designed in a

    commercial 0.25 μm 6-metal CMOS technology. The 3 chips share the same

    architecture and dimensions and are named Medipix2, Mpix2MXR20 and Timepix.

    The Medipix2 chip is a pixel detector readout chip consisting of 256 x 256

    identical elements, each working in single photon counting mode for positive or

    negative input charge signals. The preamplifier feedback provides compensation

    for detector leakage current on a pixel by pixel basis. Two identical pulse height

    discriminators are used to define an energy window. Every event falling inside the

    energy window is counted with a 13-bit pseudo-random counter. The counter logic,

    based in a shift register, also behaves as the input/output register for the pixel. Each

    cell also has an 8-bit configuration register which allows masking, test-enabling

    and 3-bit individual threshold adjust for each discriminator. The chip can be

    configured in serial mode and readout either serially or in parallel. Measurements

    show an electronic noise ~160 e- rms with a gain of ~9 mV/ke-. The threshold

    spread after equalization of ~120 e- rms brings the full chip minimum detectable

    charge to ~1100 e-. The analog static power consumption is ~8 μW per pixel with

    Vdda=2.2 V.

    The Mpix2MXR20 is an upgraded version of the Medipix2. The main

    changes in the pixel consist of: an improved tolerance to radiation, improved pixel

    to pixel threshold uniformity, and a 14-bit counter with overflow control. The chip

    periphery includes new threshold DACs with smaller step size, improved linearity,

    and better temperature dependence.

    Timepix is an evolution of the Mpix2MXR20 which provides independently

    in each pixel information of arrival time, time-over-threshold or event counting.

    Timepix uses as a time reference an external clock (Ref_Clk) up to 100 MHz which

    is distributed all over the pixel matrix during acquisition mode. The preamplifier is

    improved and there is a single discriminator with 4-bit threshold adjustment in

    order to reduce the minimum detectable charge limit. Measurements show an

    electrical noise ~100 e- rms and a gain of ~16.5 mV/ke-. The threshold spread after

    equalization of ~35 e- rms brings the full chip minimum detectable charge either to

    ~650 e- with a naked chip (i.e. gas detectors) or ~750 e- when bump-bonded to a

    detector. The pixel static power consumption is ~13.5 μW per pixel with

    Vdda=2.2 V and Ref_Clk=80 MHz.

    This family of chips have been used for a wide variety of applications.

    During these studies a number of limitations have come to light. Among those are

    limited energy resolution and surface area. Future developments, such as

    Medipix3, will aim to address those limitations by carefully exploiting

    developments in microelectronics.

  • 6.
    Llopart Cudié, Xavier
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    First test measurements of a 64k pixel redout chip working in single photon counting mode2003In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 509, no 1-3, p. 157-163Article in journal (Refereed)
    Abstract [en]

    The Medipix2 chip is a pixel detector readout chip consisting of 256×256 identical elements, each working in single photon counting mode for positive or negative input charge signals. The chip is designed and manufactured in a six-metal 0.25 μm CMOS technology. This paper describes several electrical measurements which have been carried out on the chip prior to detector bump bonding using a dedicated readout system. Threshold linearity and variation has been measured for both electron and hole collection. The noise is 100e RMS and the threshold can be adjusted to 120e RMS for both polarities. The minimum operating threshold is 1000e.

  • 7.
    Llopart Cudié, Xavier
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Ballabriga, R
    Campbell, M
    Tlustos, L
    Wong, Winnie
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Timepix, a 65k programmable pixel readout chip for arrival time, energy and/or photon counting measurements2007In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 581, no 1-2, p. 485-494Article in journal (Refereed)
  • 8.
    Llopart Cudié, Xavier
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Campbell, M
    Dinapoli, R
    San Segundo, D
    Pernigotti, E
    Medipix2, a 64k pixel readout chip with 55 μm square elements working in single photon counting mode2002In: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 49, p. 2279-2283Article in journal (Refereed)
    Abstract [en]

    The Medipix2 chip is a pixel detector readout chip consisting of 256×256 identical elements, each working in single photon counting mode for positive or negative input charge signals. Each pixel cell contains around 500 transistors and occupies a total surface area of 55 μm×55 μm. A 20 μm width octagonal opening connects the detector and the preamplifier input via bump-bonding. The preamplifier feedback provides compensation for detector leakage current on a pixel by pixel basis. Two identical pulse height discriminators are used to create a pulse if the preamplifier output falls within a defined energy window. These digital pulses are then counted with a 13-bit pseudo-random counter. The counter logic, based in a shift register, also behaves as the input/output register for the pixel. Each cell also has an 8-bit configuration register which allows masking, test-enabling and 3-bit individual threshold adjust for each discriminator. The chip can be configured in serial mode and read out either serially or in parallel. The chip is designed and manufactured in a 6-metal 0.25 μm CMOS technology. Preliminary measurements show very good agreement with simulations.

  • 9. McMullan, G
    et al.
    Cattermole, D M
    Chen, S
    Henderson, R
    Llopart Cudié, Xavier
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Summerfield, C
    Trustos, L
    Faruqi, A R
    Electron imaging with medipix2 hybrid pixel detector2007In: Ultramicroscopy, ISSN 0304-3991, E-ISSN 1879-2723, Vol. 107, no 4-5, p. 401-413Article in journal (Refereed)
    Abstract [en]

    The electron imaging performance of Medipix2 is described. Medipix2 is a hybrid pixel detector composed of two layers. It has a sensor layer and a layer of readout electronics, in which each 55 μm×55 μm pixel has upper and lower energy discrimination and MHz rate counting. The sensor layer consists of a 300 μm slab of pixellated monolithic silicon and this is bonded to the readout chip. Experimental measurement of the detective quantum efficiency, DQE(0) at 120 keV shows that it can reach 85% independent of electron exposure, since the detector has zero noise, and the DQE(Nyquist) can reach 35% of that expected for a perfect detector (4/π2). Experimental measurement of the modulation transfer function (MTF) at Nyquist resolution for 120 keV electrons using a 60 keV lower energy threshold, yields a value that is 50% of that expected for a perfect detector (2/π). Finally, Monte Carlo simulations of electron tracks and energy deposited in adjacent pixels have been performed and used to calculate expected values for the MTF and DQE as a function of the threshold energy. The good agreement between theory and experiment allows suggestions for further improvements to be made with confidence. The present detector is already very useful for experiments that require a high DQE at very low doses.

  • 10. Mikuec, B
    et al.
    Campbell, M
    Heijne, E
    Llopart Cudié, Xavier
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    X-ray imaging using single photon processing with semiconductor pixel detectors2003In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 511, no 1-2, p. 282-286Article in journal (Refereed)
    Abstract [en]

    More than 10 years experience with semiconductor pixel detectors for vertex detection in high-energy physics experiments together with the steady progress in CMOS technology opened the way for the development of single photon processing pixel detectors for various applications including medical X-ray imaging. The state of the art of such pixel devices consists of pixel dimensions as small as 55×55 μm2, electronic noise per pixel <100 e− rms, signal-to-noise discrimination levels around 1000 e− with a spread <50 e− and a dynamic range up to 32 bits/pixel. Moreover, the high granularity of hybrid pixel detectors makes it possible to probe inhomogeneities of the attached semiconductor sensor.

     

     

  • 11. Tlustos, L
    et al.
    Ballabriga, R
    Campbell, M
    Heijne, E H M
    Kincade, K
    Llopart Cudié, Xavier
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Stejskal, P
    Imaging properties of the Medipix2 system exploiting single and dual enery thresholds2006In: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 53, p. 1323-1328Article in journal (Refereed)
    Abstract [en]

    Low noise, high resolution and high dose efficiency are the common requirements for most X-ray imaging applications. Especially in medical applications the dose efficiency is a necessity for detector systems. We present the imaging performance of the Medipix2 readout chip bump bonded to a 300 mu m thick Si detector as a function of the detection threshold, a free parameter not available in conventional integrating imaging systems. Spatial resolution has been measured using the modulation transfer function (MTF) and it varies between 8.2 Ip/mm and 11.0 Ip/mm at 70%. An associated measurement of noise power spectrum (NPS) permits us to derive the detective quantum efficiency (DQE) which can be as a high as 25.5 % for a broadband incoming spectrum. The influence of charge diffusion in the sensor together with threshold variation in the readout chip is discussed. Although the Medipix2 system is used in photon counting mode with a single threshold in energy, the system is also capable of counting within a given energy window of down to ~1.4 keV. First measurements and images using this feature reveal capabilities that allow to identify fluorescence and other sources of disturbance.

  • 12. Tlustos, L
    et al.
    Campbell, M
    Heijne, E H M
    Llopart Cudié, Xavier
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Signal variations in high granularity Si pixel detectors2004In: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 51, p. 3006-3012Article in journal (Refereed)
    Abstract [en]

    Fixed pattern noise is one of the limiting factors of image quality and degrades the achievable spatial resolution. In the case of silicon sensors non-uniformities due to doping inhomogeneities can be limited by operating the sensor in strong overdepletion. For high granularity photon counting pixel detectors an additional high frequency interpixel signal variation is an important factor for the achievable signal to noise ratio (SNR). It is common practice to apply flatfield corrections to increase the SNR of the detector system. For the case of direct conversion detectors it can be shown that the Poisson limit can be reached for floodfield irradiation. However when used for imaging with spectral X-ray sources flatfield corrections are less effective. This is partly a consequence of charge sharing between adjacent pixels, which gives rise to an effective energy spectrum seen by the readout, which is different from the spectral content of the incident beam. In this paper we present simulations and measurements of the limited applicability of flatfield corrections for spectral source imaging and investigate the origins of the high frequency interpixel noise component The model, calculations and measurements performed suggest that flatfield correction maps for photon counting detectors with a direct conversion Si sensor can be obtained from electrical characterization of the readout chip alone.

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