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  • 1. Badel, X.
    et al.
    Linnros, J.
    Kleimann, P.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Koskiahde, E.
    Valpas, K.
    Nenonen, S.
    Petersson, Sture
    Department of Microelectronics, Roy. Inst. of Technology of Sweden, Kista.
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Metallized and Oxidized Silicon Macropore Arrays Filled with a Scintillator for CCD-based X-ray Imaging Detectors2004In: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 51, no 3 Part 3, p. 1001-1005Article in journal (Refereed)
    Abstract [en]

    Silicon Charge Coupled Devices (CCD)covered with a scintillating film are now available on the market for use in digital medical imaging. However, these devices could still be improved in terms of sensitivity and especially spatial resolution by coating the CCD with an array of scintillating waveguides. In this work, we fabricated such waveguides by first etching pores in silicon, then performing metallization or oxidation of the pore walls and finally filling the pores with CsI(Tl). The resulting structures were observed using SEM and tested under X-ray exposure. The detector performances were also compared with simulations, indicating that the optimal pore depth for metallized macropore arrays is about 80 um while it is around 350 um for oxidized ones. This result, together with the roughness of the metal coating, explains why lower performances were measured experimentally with the metallized macropores. Indeed, our macropore arrays had depths in the range of 210-390 um, which is favorable to oxidized structures.

  • 2.
    Badel, X
    et al.
    KTH.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Kleimann, P
    Université Claude Bernard Lyon-I, France.
    Williams, L
    Applied Scintillation Technology, United Kingdom.
    Moody, S.J.
    Applied Scintillation Technology, United Kingdom.
    Tyrrell, G.C.
    Applied Scintillation Technology, United Kingdom.
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Linnros, J
    KTH.
    Performance of Scintillating Waveguides for CCD-based X-ray Detectors2006In: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 53, no 1, p. 3-8Article in journal (Refereed)
    Abstract [en]

    Scintillating films are usually used to improve the sensitivity of CCD-based X-ray imaging detectors. For an optimal spatial resolution and detection efficiency, a tradeoff has to be made on the film thickness. However, these scintillating layers can also be structured to provide a pixellated screen. In this paper, the study of CsI(Tl)-filled pore arrays is reported. The pores are first etched in silicon, then oxidized and finally filled with CsI(Tl) to form scintillating waveguides. The dependence of the detector sensitivity on pore depth, varied from 40 to 400 µm here, follows rather well theoretical predictions. Most of the detectors produced in this work have a detective quantum efficiency of the incoming X-ray photons of about 25%; however, one detector shows that higher efficiency can be achieved. The comparison of its efficiency to a 1-mm thick CsI(Tl) crystal indicates that light collection efficiency in the guides may approach what is theoretically possible. Imaging capabilities of the detectors are demonstrated.

  • 3.
    Dubaric, Ervin
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Nilsson, Hans-Erik
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Monte Carlo simulation of the response of a pixellated 3D photodetector in silicon2002In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 487, no 1-2, p. 136-141Article in journal (Refereed)
    Abstract [en]

    The charge transport and X-ray photon absorption in three-dimensional (3D) X-ray pixel detectors have been studied using numerical simulations. The charge transport has been modelled using the drift-diffusion simulator MEDICI, while photon absorption has been studied using MCNP. The response of the entire pixel detector system in terms of charge sharing, line spread function and modulation transfer function, has been simulated using a system level Monte Carlo simulation approach. A major part of the study is devoted to the effect of charge sharing on the energy resolution in 3D-pixel detectors. The 3D configuration was found to suppress charge sharing much better than conventional planar detectors.

  • 4.
    Esebamen, Omeime
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Thungström, Göran
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Spectral Performance of Photon Counting Pixel Detector Using Attenuation Spectra for Test Samples2009In: AIP Conference Proceedings Volume 1204, New York: American Institute of Physics (AIP), 2009, p. 177-179Conference paper (Refereed)
    Abstract [en]

    When a material is placed along the path of an X-ray beam using a broad range of energy X-ray source, the energy dependence of the attenuation for the X-ray photons will be substantially dissimilar for different materials. The process at which X-ray radiation loses its penetrating strength as it travels through a material will be significantly larger for photons with energy above k-edge energy of that material than for those with slightly lower energy. Hence energy resolved X-ray imaging can be used to achieve colour images revealing the material content of the test sample. The attenuation of the spectrum done by scanning an energy window through the spectrum was measured for a number of samples of different materials. The test samples include Sn, Gd and I with K-edge energy at 29 keV, 50 keV and 33 keV, respectively, using a Feinfocus microfocus X-ray source (FTP-105.02) with Medipix2 photon counting chip.

  • 5.
    Fröjdh, Anna
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Fröjdh, Erik
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Thungström, Göran
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Processing and characterization of a MEDIPIX2-compatible silicon sensor with 220 mu m pixel size2011In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 633, no Suppl 1, p. S78-S80Article in journal (Refereed)
    Abstract [en]

    Pixellated silicon detectors with a pixel size of 220 um have been fabricated at Mid Sweden University. The detectors will be bonded to the MEDIPIX2 [1] readout chip. The purpose is to investigate the performance of an energy sensitive X-ray imaging sensor with reduced charge sharing.

    The detectors were fabricated on high purity silicon with a wafer thickness of 500 um and a resistivity of more than 15 kohmcm. One reason for the choice of material was to get experience for future work with very thick detectors requiring ultra high resistivity in order to be depleted. During the initial work in this project some issued were found concerning inter pixel resistance and the efficiency of the guard rings. This led to a study of existing papers on the subject [2,3,4,5] and to extensive simulations of the electric field and the charge transport in different parts of the device.

    A modified process has been developed using alternating p+ and n+ guard rings and an outer n+ doping. The results of the simulations and the process will be described as well as an outline for a process for fabrication of very thick detectors with limited guard ring extension.

    References[1] - X. Llopart, M. Campbell, R. Dinapoli, D. San Segundo and E. Pernigotti, IEEE Trans. Nucl. Sci., vol. 49, 2279-2283, October 2002.[2] – L. Evensen, A. Hanneborg, B Sundby Avset, M. Mese, Nuclear Instruments and Methods in Physics Research A 337 (1993) 44 – 52[3] – T. Pavalainen, T. Tuuva, K. Leinonen, Nuclear Instruments and Methods in Physics Research A 573 (2007) 277 – 279[4] – Z. Li, W. Huang, L. J. Zhao, IEEE Trans. Nucl. Sci., vol. 47, No. 3. 729 – 735 , June 2000.[5] – D. Han, C. Wang, G. Wang, S. Du, L. Shen, X. Tian, X. Zhang, IEEE Transactions on Electron devices, Vol. 50, No. 2, 537 – 540, February 2003

  • 6.
    Fröjdh, Christer
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Graafsma, Heinz
    Experimental Division, ESRF, BP2200 Avenue des Martyrs, 38043 Grenoble, Cedex, France.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Nilsson, Hans-Erik
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Ponchut, Cyril
    Experimental Division, ESRF, BP2200 Avenue des Martyrs, 38043 Grenoble, Cedex, France.
    Characterization of a pixellated CdTe detector with single-photon processing readout2006In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 563, no 563, p. 128-132Article in journal (Refereed)
    Abstract [en]

    A 1mm thick pixellated CdTe detector bonded to the MEDIPIX2 [1] readout chip has been characterized using a monoenergetic microbeam at the ESRF. This is an extension of the tests previously reported in [2]. The results show that a full energy peak can be obtained when a narrow beam is focused in the centre of the pixel. There is also evidence of significant charge diffusion and fluorescence. The results indicate that the charge sharing is the most important problem and will cause loss of the energy information in an imaging application. The second problem is the fluorescence which limits the number of counts in the full energy peak even for hits in the centre of the pixel.

  • 7.
    Fröjdh, Christer
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Krapohl, David
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Reza, Salim
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Fröjdh, Erik
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Thungström, Göran
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Spectral resolution in pixel detectors with single photon processing2013In: Proceedings of SPIE - The International Society for Optical Engineering, SPIE - International Society for Optical Engineering, 2013, p. Art. no. 88520O-Conference paper (Other academic)
    Abstract [en]

    Pixel detectors based on photon counting or single photon processing readout are becoming popular for spectral X-ray imaging. The detector is based on deep submicron electronics with functions to determine the energy of each individual photon in every pixel. The system is virtually noiseless when it comes to the number of the detected photons. However noise and variations in system parameters affect the determination of the photon energy. Several factors affect the energy resolution in the system. In the readout electronics the most important factors are the threshold dispersion, the gain variation and the electronic noise. In the sensor contributions come from charge sharing, variations in the charge collection efficiency, leakage current and the statistical nature of the charge generation, as described by the Fano factor. The MEDIPIX technology offers a powerful tool for investigating these effects since energy spectra can be captured in each pixel. In addition the TIMEPIX chip, when operated in Time over Threshold mode, offers an opportunity to analyze individual photon interactions, thus addressing charge sharing and fluorescence. Effects of charge sharing and the properties of charge summing can be investigated using MEDIPIX3RX. Experiments are performed using both Si and CdTe detectors. In this paper we discuss the various contributions to the spectral noise and how they affect detector response. The statements are supported with experimental data from MEDIPIX-type detectors.

  • 8.
    Fröjdh, Christer
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Nilsson, Hans-Erik
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Spectral response of Pixellated Semiconductor X-ray Detectors2005In: 2005 IEEE Nuclear Science Symposium Conference Record, Vols 1-5, IEEE , 2005, Vol. 5, p. 2967-2970, article id 1596954Conference paper (Refereed)
    Abstract [en]

    X-ray imaging with energy resolution can be performed using a detector matrix bonded to a photon counting CMOS readout circuit as the MEDIPIX2 chip. In previous experiments it has been shown that charge sharing between neighboring pixels plays an important role in the formation of the image and especially for the spectral information in the image. Charge sharing is caused both by the localization of the initial energy deposition and by diffusion during the transport of the charge to the readout electrode. In this work we have studied different factors that can effect the energy resolution in pixellated X-ray imaging detectors. Results are compared to experimental data.

  • 9.
    Fröjdh, Christer
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Jakubek, Jan
    Czech Technical University, Prague, Czech Republic.
    Holy, Tomas
    Czech Technical University, Prague, Czech Republic.
    Fröjdh, Anna
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Fröjdh, Erik
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Characterization of area sensitivity in 55 um pixellated CdTe X-ray imaging detectors2007In: IEEE Nuclear Science Symposium Conference Record, New York: IEEE conference proceedings, 2007, p. 1234-1236Conference paper (Refereed)
    Abstract [en]

    X-ray imaging detectors with energy resolution, based on a single photon processing CMOS readout circuit attached to a detector chip, are being developed by different groups. In order to achieve high quantum efficiency it is preferable to use high-Z semiconductor materials. However the fluorescent X-ray photons of such materials have high energies and are able to travel long distances thereby reducing both the spatial resolution and the energy resolution of the detector. In addition charge sharing in the detector and non-uniformities in both the detector and the readout electronics will affect the signal. In this work we have characterized a 1 mm thick CdTe detector with a pixel size of 55 um x 55 um, bump bonded to the MEDIPIX2 single photon processing readout chip. The area sensitivity of the detector is evaluated using a narrow X-ray beam of monoenergetic photons. From these measurements the effects of fluorescence and charge sharing can be evaluated.

  • 10.
    Fröjdh, Erik
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Fröjdh, Anna
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Spectral response of a silicon detector with 220 mu m pixel size bonded to MEDIPIX22011In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 633, no Supplement 1, p. S125-S127Article in journal (Refereed)
    Abstract [en]

    The standard detector used with the MEDIPIX2 [1] readout chip suffers from severe charge sharing that reduces the spectral performance of the device. This problem is expected to be solved in the MEDIPIX3 [2] design. In order to significantly reduce the charge sharing and to make a detector which could be expected to have a similar response to MEDIPIX3 we have fabricated detectors with a pixel size of 220 um and bonded these detectors to the MEDIPIX2 chip using only a limited number of pixels on the readout chip. This makes the active area of the pixel comparable with the area covered by the charge summing in MEDIPIX3.

    The charge collection properties of the device have been tested by scanning a narrow beam over a pixel. The spectral response has been measured by taking a flood exposure at different tube voltages and comparing the result with the spectrum obtained from exposing the centre of the pixel with a narrow beam thus eliminating the charge sharing. This work represents an improved characterisation as compared to [3].

    Some initial images of different objects have been taken by placing the device in an X-ray microscope with a nanofocus X-ray tube imaging objects with magnification to simulate the original pixel size of 55 um.

    References[1] - X. Llopart, M. Campbell, R. Dinapoli, D. San Segundo and E. Pernigotti, IEEE Trans. Nucl. Sci., vol. 49, 2279-2283, October 2002.[2] - R. Ballabriga, M. Campbell, E. H. M. Heijne, X. Llopart, L. Tlustos, IEEE Trans. Nucl. Sci., Vol. 54, No. 5, October 2007.[3] – B. Norlin, C. Fröjdh, G. Thungström, D. Greiffenberg, NSS Conference record,19-25 Oct. 2008, Pages:3464 – 3469, Digital Object Identifier 10.1109/NSSMIC.2008.4775083

  • 11.
    Fröjdh, Erik
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Gimenez, E
    Diamond Light Source, Didcot, Oxfordshire OX11 0DE, United Kingdom.
    Maneuski, D
    University of Glasgow, Glasgow G12 8QQ, United Kingdom.
    Marchal, J
    Diamond Light Source, Didcot, Oxfordshire OX11 0DE, United Kingdom.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    O'Shea, V
    University of Glasgow, Glasgow G12 8QQ, United Kingdom.
    Stewart, G
    University of Glasgow, Glasgow G12 8QQ, United Kingdom.
    Wilhelm, H
    Diamond Light Source, Didcot, Oxfordshire OX11 0DE, United Kingdom.
    Modh Zain, R
    University of Glasgow, Glasgow G12 8QQ, United Kingdom.
    Thungström, Göran
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Depth of interaction and bias voltage dependence of the spectral response in a pixellated CdTe detector operating in time-over-threshold mode subjected to monochromatic X-rays2012In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 7, no 3, p. Art. no. C03002-Article in journal (Refereed)
    Abstract [en]

    High stopping power is one of the most important figures of merit for X-ray detectors. CdTe is a promising material but suffers from: material defects, non-ideal charge transport and long range X-ray fluorescence. Those factors reduce the image quality and deteriorate spectral information. In this project we used a monochromatic pencil beam collimated through a 20Όm pinhole to measure the detector spectral response in dependance on the depth of interaction. The sensor was a 1mm thick CdTe detector with a pixel pitch of 110Όm, bump bonded to a Timepix readout chip operating in Time-Over-Threshold mode. The measurements were carried out at the Extreme Conditions beamline I15 of the Diamond Light Source. The beam was entering the sensor at an angle of ∌20 degrees to the surface and then passed through ∌25 pixels before leaving through the bottom of the sensor. The photon energy was tuned to 77keV giving a variation in the beam intensity of about three orders of magnitude along the beam path. Spectra in Time-over-Threshold (ToT) mode were recorded showing each individual interaction. The bias voltage was varied between -30V and -300V to investigate how the electric field affected the spectral information. For this setup it is worth noticing the large impact of fluorescence. At -300V the photo peak and escape peak are of similar height. For high bias voltages the spectra remains clear throughout the whole depth but for lower voltages as -50V, only the bottom part of the sensor carries spectral information. This is an effect of the low hole mobility and the longer range the electrons have to travel in a low field. © 2012 IOP Publishing Ltd and Sissa Medialab srl.

  • 12.
    Fröjdh, Erik
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design. CERN, CH-1211, Geneva, 23, Switzerland .
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Gimenez, E. N.
    Diamond Light Source, Harwell Science and Innovation Campus, Chiltern, Didcot, Oxfordshire, OX11 0QX, United Kingdom .
    Krapohl, David
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Maneuski, D.
    University of Glasgow, Glasgow, G12 8QQ, United Kingdom .
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    O'Shea, V.
    University of Glasgow, Glasgow, G12 8QQ, United Kingdom .
    Wilhelm, H.
    Diamond Light Source, Harwell Science and Innovation Campus, Chiltern, Didcot, Oxfordshire, OX11 0QX, United Kingdom .
    Tartoni, N.
    Diamond Light Source, Harwell Science and Innovation Campus, Chiltern, Didcot, Oxfordshire, OX11 0QX, United Kingdom .
    Thungström, Göran
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Zain, R. M.
    University of Glasgow, Glasgow, G12 8QQ, United Kingdom .
    Probing Defects in a Small Pixellated CdTe Sensor Using an Inclined Mono Energetic X-Ray Micro Beam2013In: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 60, no 4, p. 2864-2869Article in journal (Refereed)
    Abstract [en]

    High quantum efficiency is important in X-ray imaging applications. This means using high-Z sensor materials. Unfortunately many of these materials suffer from defects that cause non-ideal charge transport. In order to increase the understanding of these defects, we have mapped the 3D response of a number of defects in two 1 mm thick CdTe sensors with different pixel sizes (55 mu m and 110 mu m) using a monoenergetic microbeam at 79 keV. The sensors were bump bonded to Timepix read out chips. Data was collected in photon counting as well as time-over-thresholdmode. The time-over-thresholdmode is a very powerful tool to investigate charge transport properties and fluorescence in pixellated detectors since the signal from the charge that each photon deposits in each pixel can be analyzed. Results show distorted electrical field around the defects, indications of excess leakage current and large differences in behavior between electron collection and hole collection mode. The experiments were carried out on the Extreme Conditions Beamline I15 at Diamond Light Source.

  • 13.
    Fröjdh, Erik
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Gimenez, Eva
    Diamond Light Source.
    Krapohl, David
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Maneuski, Dizmitry
    Glasgow University.
    O'Shea, Val
    Glasgow University.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Wilhelm, Heribert
    Diamond Light Source.
    Tartoni, Nicola
    Diamond Light Source.
    Thungström, Göran
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Zain, Rasif
    Glasgow University.
    Probing Defects in a Small Pixellated CdTe Sensor Using an Inclined Mono Energetic X-Ray Micro Beam2012In: 2012 IEEE NUCLEAR SCIENCE SYMPOSIUM AND MEDICAL IMAGING CONFERENCE RECORD (NSS/MIC), IEEE conference proceedings, 2012, p. 4233-4236, article id 6551965Conference paper (Other academic)
    Abstract [en]

    High quantum efficiency is important in X-ray imaging applications. This means using high-Z sensor materials. Unfortunately many of these materials suffer from defects that cause non-ideal charge transport. In order to increase the understanding of these defects, we have mapped the 3D response of a number of defects in two 1 mm thick CdTe sensors with different pixel sizes (55 mu m and 110 mu m) using a monoenergetic microbeam at 79 keV. The sensors were bump bonded to Timepix read out chips. Data was collected in photon counting as well as time-over-threshold mode. The time-over-threshold mode is a very powerful tool to investigate charge transport properties and fluorescence in pixellated detectors since the signal from the charge that each photon deposits in each pixel can be analyzed. Results show distorted electrical field around the defects and indications of excess leakage current and large differences in behavior between electron collection and hole collection mode. The experiments were carried out in the Extreme Conditions Beamline I15 at Diamond Light Source.

  • 14.
    Fröjdh, Erik
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Thungström, Göran
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Mapping the x-ray response of a CdTe sensor with small pixels using an x-ray microbeam and a single photon processing readout chip2011In: Proceedings of SPIE - The International Society for Optical Engineering / [ed] Larry A. Franks, Ralph B. James, Arnold Burger, SPIE - International Society for Optical Engineering, 2011, p. Art. no. 814208-Conference paper (Refereed)
    Abstract [en]

    CdTe is a promising material for X-ray imaging since it has high stopping power for X-rays. However defects in the material, non ideal charge transport and long range X-ray fluorescence deteriorates the image quality. We have investigated the response of a CdTe sensor with very small pixels using an X-ray microbeam entering the sensor at a small incident angle. Effects of defects as well as depth of interaction can be measured by this method. Both electron and hole collection mode has been tested. The results show distorted electrical field around defects in the material and also shows the small pixel effect. It is also shown that charge summing can be used to get correct spectral information.

  • 15.
    Fröjdh, Erik
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Thungström, Göran
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Spectral Response in a Pixellated X-ray Imaging CdTe Detector with Single Photon Processing Readout2010In: Proceeddings of 2010 Nuclear Science Symposium and Medical Imaging Conference, IEEE conference proceedings, 2010, p. 1079-1080Conference paper (Other academic)
    Abstract [en]

    The image forming process in a CdTe detector is both a function of the X-ray interaction in the material, including scattering and fluorescence, and the charge transport in the material [2]-[4]. The response of individual photons has been investigated using two pixellated CdTe image detectors with pixel pitches of 55 mu m and 110 mu m. The detectors were bonded to TIMEPIX [5] readout chips and operated in time over threshold mode (ToT). We have illuminated the sensors with monoengertic photons generated by X-ray fluorescence in metal sheets and with gamma photons from Am-241 and Cs-137. Results shows a large degradation in energy resolution caused by charge sharing and fluorescence. By summing pixels together we can correct for the charge sharing and some, but not all fluorescence.

  • 16.
    Fröjdh, Erik
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Thungström, Göran
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    X-ray absorption and charge transport in a pixellated CdTe detector with single photon processing readout2011In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 6, no 2, p. Art. no. P02012-Article in journal (Refereed)
    Abstract [en]

    The image forming process in a CdTe detector is both a function of the X-ray interaction in the material, including scattering and fluorescence, and the charge transport in the material [2-4]. The response to individual photons has been investigated using a CdTe detector with a pixel size of 110 m m, bonded to a TIMEPIX [5] readout chip operating in time over threshold mode. The device has been illuminated with mono-energetic photons generated by fluorescence in different metals and by gamma emission from Am-241 and Cs-137. Each interaction will result in charge distributed in a cluster of pixels where the total charge in the cluster should sum up to the initial photon energy. By looking at the individual clusters the response from shared photons as well as fluorescence photons can be identified and separated. By using energies below and above the K-edges of Cd and Te the contribution from fluorescence can be further isolated. The response is analyzed to investigate the effects of both charge diffusion and fluorescence on the spectral response in the detector.

  • 17. Gustavsson, Ingvar
    et al.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    A flexible circuit analysis course2004In: Proceedings of the 9th World Conference on Continuing Engineering Education in Tokyo, Japan, May 2004., Tokyo, 2004, p. 289-294Conference paper (Refereed)
    Abstract [en]

    Blekinge Institute of Technology (BTH) and Mid Sweden University offer a flexible circuit analysis course for Initial Professional Development and Continuing Engineering Education. The course material is used on campus. A combination of synchronous and asynchronous distance learning activities is used. Asynchronous video presentations, on-line classes and on-line laboratory sessions are provided. This paper presents the course, its aims and components and discusses relevant implementation details.

  • 18.
    Hjelm, Mats
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Nilsson, Hans-Erik
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Badel, X
    Monte Carlo simulation of the imaging properties of scintillator-coated X-ray pixel detetectors2003In: 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. 76-85Article in journal (Refereed)
    Abstract [en]

    The spatial resolution of scintillator-coated X-ray pixel detectors is usually limited by the isotropic light spread in the scintillator. One way to overcome this limitation is to use a pixellated scintillating layer on top of the semiconductor pixel detector. Using advanced etching and filling techniques, arrays of CsI columns have been successfully fabricated and characterized. Each CsI waveguide matches one pixel of the semiconductor detector, limiting the spatial spread of light. Another concept considered in this study is to detect the light emitted from the scintillator by diodes formed in the silicon pore walls. There is so far no knowledge regarding the theoretical limits for these two approaches, which makes the evaluation of the fabrication process difficult. In this work we present numerical calculations of the signal-to-noise ratio (SNR) for detector designs based on scintillator-filled pores in silicon. The calculations are based on separate Monte Carlo (MC) simulations of X-ray absorption and light transport in scintillator waveguides. The resulting data are used in global MC simulations of flood exposures of the detector array, from which the SNR values are obtained. Results are presented for two scintillator materials, namely CsI(Tl) and GADOX.

  • 19.
    Krapohl, David
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Fröjdh, Erik
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Thungström, Göran
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Simulation of the Spectral Response of a Pixellated X-Ray Imaging Detector in Single Photon Processing Mode2010In: 2010 Nuclear Science Symposium and Medical Imaging Conference, IEEE conference proceedings, 2010Conference paper (Refereed)
    Abstract [en]

    X-ray imaging with spectral resolution, “Color X-ray imaging” is a new imaging technology that is currently attracting a lot of attention. It has however been observed that the quality of spectral response is degraded as the pixel size is reduced. This is an effect of charge sharing where the signal from a photon absorbed close to the border between two pixels is shared between pixels. This effect is caused by both diffusion during the charge transport and X-ray fluorescence in heavy detector materials. In order to understand the behavior of pixellated detectors with heavy detector materials operating in single photon processing mode, we have simulated the X-ray interaction with the sensor and the transport of the charge to the readout electrode using a Monte Carlo model for the X-ray interaction and a drift diffusion model for the charge transport. By combining these models, both signal and noise properties of the detector can be simulated.

  • 20.
    Lundgren, Jan
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    O'Nils, Mattias
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Oelmann, Bengt
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Abdalla, Suliman
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    An Area Efficient Readout Architecture for Photon Counting Color Imaging2007In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 576, no 1, p. 132-136Article in journal (Refereed)
    Abstract [en]

    The introduction of several energy levels, namely color imaging, in photon counting X-ray image sensors is a trade-off between circuit complexity and spatial resolution. In this paper we propose a pixel architecture that has full resolution for the intensity and uses sub-sampling for the energy spectrum. The results show that this sub-sampling pixel architecture produces images with an image quality which is, on average, 2.4 dB (PSNR) higher than those for a single energy range architecture and with half the circuit complexity of that for a full sampling architecture.

  • 21.
    Manuilskiy, Anatoily
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Nilsson, Hans-Erik
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Spectroscopy applications for the Medipix photon counting X-ray system2004In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 531, no 1-2, p. 251-257Article in journal (Refereed)
    Abstract [en]

    Since the Medipix system is a photon counting system, it is possible to use it as a spectroscopic device, which is sensitive to the energy of the incoming photons. The system can, for example, be used to achieve the spectrum of an unknown source. Since it is an imaging system, it can be useful for energy dependent scattering experiments. The factors limiting the spectroscopic resolution are source and channel noise and detector non-uniformity. A well-known method for correction non-uniformity is to use mono energetic source and a threshold adjustment. However, since the threshold calibration is energy dependent, its use for sources with wider energy range is limited. We have shown an easy method to create threshold calibration masks for different thresholds that include both detector and channel stationary influences with analysis of histograms of series of images. The model for histograms was created assuming a standard dental X-ray source, which allows finding mask-creating parameters like threshold range (Vth and Vthadj). This procedure can be performed for each energy of interest. Based on these arrays, mask files that narrowed the threshold distribution close to the theoretical limit, were prepared. The limit of the spectroscopic resolution for the system was measured by analysing histograms for series of flat images under identical conditions.

  • 22.
    Nilsson, Hans-Erik
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Mattsson, Claes
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Bethke, K
    PANalytical B.V, Netherlands.
    De Vries, R
    PANalytical B.V, Netherlands.
    Evaluation of the charge sharing effects on spot intensity in XRD setup using photon counting pixel detectors2005In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 563, no 1, p. 182-186Article in journal (Refereed)
    Abstract [en]

    In this study we examine how charge loss due to charge sharing in photon counting pixel detectors affects the recording of spot intensity in an X-ray Diffraction (XRD) setup. In the photon counting configuration the charge from photons that are absorbed at the boarder of a pixel will be shared between two pixels. If the threshold is high enough these photons will not be counted. However, if the threshold is low enough these photons will be counted twice. In an XRD setup the intensity and position of various spots should be recorded. Thus, the intensity measure will be affected by the setting of the threshold. In this study we used a system level Monte Carlo simulator to evaluate the variations in the intensity signals for different threshold settings and spot sizes. The simulated setup included an 8keV monochromatic source (providing a Gaussian shaped spot) and the MEDIPIX2 photon counting pixel detector (55 µm x 55 µm pixel size with 300µm silicon) at various detector biases. Our study shows that the charge sharing distortion can be compensated by numerical post processing and high resolution in both charge (charge distribution?) and position can be achieved.

  • 23.
    Nilsson, Hans-Erik
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Tlustos, Lukas
    CERN, Switzerland.
    Charge sharing suppression using pixel-to-pixel communication in photon counting X-ray imaging systems2007In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 576, no 1, p. 243-247Article in journal (Refereed)
    Abstract [en]

    In planar silicon detector structures the charge sharing between pixels is one limiting factor for colour X-ray imaging using integrated photon counting pixel detectors. 3D detector structures have been proposed as one solution to this problem. However, there are also readout system solutions to the problem i.e. introducing pixel to pixel communication and distributed charge summing in the readout electronics. In this work different charge summing schemes are evaluated using Monte Carlo simulation techniques. The increase in electronic noise introduced by the charge summing is one of the most severe problems. A proper selection of summing scheme is necessary to obtain an efficient system.

  • 24.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Characterisation and application of photon counting X-ray detector systems2007Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis concerns the development and characterisation of X-ray imaging systems based on single photon processing. “Colour” X-ray imaging opens up new perspectives within the fields of medical X-ray diagnosis and also in industrial X-ray quality control. The difference in absorption for different “colours” can be used to discern materials in the object. For instance, this information might be used to identify diseases such as brittle-bone disease. The “colour” of the X-rays can be identified if the detector system can process each X-ray photon individually. Such a detector system is called a “single photon processing” system or, less precise, a “photon counting system”.

    With modern technology it is possible to construct photon counting detector systems that can resolve details to a level of approximately 50 µm. However with such small pixels a problem will occur. In a semiconductor detector each absorbed X-ray photon creates a cloud of charge which contributes to the image. For high photon energies the size of the charge cloud is comparable to 50 µm and might be distributed between several pixels in the image. Charge sharing is a key problem since, not only is the resolution degenerated, but it also destroys the “colour” information in the image.

    This thesis presents characterisation and simulations to provide a detailed understanding of the physical processes concerning charge sharing in detectors from the MEDIPIX collaboration. Charge summing schemes utilising pixel to pixel communications are proposed. Charge sharing can also be suppressed by introducing 3D-detector structures. In the next generation of the MEDIPIX system, Medipix3, charge summing will be implemented. This system, equipped with a 3D-silicon detector, or a thin planar high-Z detector of good quality, has the potential to become a commercial product for medical imaging. This would be beneficial to the public health within the entire European Union.

  • 25.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Photon Counting X-ray Detector Systems2005Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    This licentiate thesis concerns the development and characterisation of X-ray imaging detector systems. “Colour” X-ray imaging opens up new perspectives within the fields of medical X-ray diagnosis and also in industrial X-ray quality control. The difference in absorption for different “colours” can be used to discern materials in the object. For instance, this information might be used to identify diseases such as brittle-bone disease. The “colour” of the X-rays can be identified if the detector system can process each X-ray photon individually. Such a detector system is called a “single photon processing” system or, less precise, a “photon counting system”.

    With modern technology it is possible to construct photon counting detector systems that can resolve details to a level of approximately 50 µm. However with such small pixels a problem will occur. In a semiconductor detector each absorbed X-ray photon creates a cloud of charge which contributes to the picture achieved. For high photon energies the size of the charge cloud is comparable to 50 µm and might be distributed between several pixels in the picture. Charge sharing is a key problem since, not only is the resolution degenerated, but it also destroys the “colour” information in the picture.

    The problem involving charge sharing which limits “colour” X-ray imaging is discussed in this thesis. Image quality, detector effectiveness and “colour correctness” are studied on pixellated detectors from the MEDIPIX collaboration. Characterisation measurements and simulations are compared to be able to understand the physical processes that take place in the detector. Simulations can show pointers for the future development of photon counting X-ray systems. Charge sharing can be suppressed by introducing 3D-detector structures or by developing readout systems which can correct the crosstalk between pixels.

  • 26.
    Norlin, Börje
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Characterisation of spectral performance of pixellated X-ray imaging detectors in a microscopy setup2009In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 607, no 1, p. 199-201Article in journal (Refereed)
    Abstract [en]

    In order to achieve energy resolved X-ray imaging with small pixel resolution, physical processes in the detector material such as fluorescence and charge sharing must be considered. This paper presents characterisation measurements performed with an X-ray microscopy setup for energy resolved imaging. The microscopy setup consists of a nanofocus X-ray source capable of 160 kV anode voltage, ESRF-type collimating slits and Medipix2 detectors. The detector systems developed in the Medipix collaboration are capable of energy resolved imaging. The measurements were performed by scanning an energy window through the spectrum. In this paper we have considered detectors made of Si, GaAs and CdTe for use in the microscopy setup. Both measurements and theoretical simulations are considered. For high X-ray energies, it is essential to consider fluorescence from the shielding and Compton scattering in silicon detectors.

  • 27.
    Norlin, Börje
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Energy dependence in dental imaging with Medipix22005In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 546, no 1-2, p. 19-23Article in journal (Refereed)
    Abstract [en]

    The possibility to achieve better contrast in dental images by selecting specific X-ray energies is investigated. By applying both the low and the high photon counting energy threshold of the Medipix2 chip only a narrow interval of the spectrum of a 60 keV standard dental source used is selected. The relative contrast between the parts of the tooth with the highest respectively lowest absorption increases 18 % when the energy spread of the X-rays is narrowed from the full spectrum to a 4 keV window centered around 36 keV. The small tail of high energy photons from the full source spectrum lowers the achieved relative contrast significantly. This result can be used in development of X-ray sources or in consideration of energy weighting.

  • 28.
    Norlin, Börje
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Fröjdh, Erik
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Spectral X-ray imaging with single photon processing detectors2013In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 8, p. Art. no. C02010-Article in journal (Refereed)
    Abstract [en]

    Spectral X-ray imaging with single photon processing detectors gains substantial interest for many applications. In this paper we discuss fundamental parameters as contrast to noise ratio (CNR) and spectral response as a function of the material in the object. Image properties have been simulated for different photon energies using MCNP5, assuming an ideal detector with 32 x 32 pixels. Simulations are supported by experimental results obtained with detectors from the MEDIPIX family. The CNR is strongly dependent on the number of incident photons and the number of photons absorbed in the object. The requirement for substantial absorption in the object limits the range of useful photon energies. In most cases the CNR is improved when high energy photons are removed from the spectrum. Materials can be uniquely identified or layers of different materials can be separated provided that there is a substantial difference in their spectral X-ray absorption. In most cases an absorption edge in the spectrum is needed to obtain good results. Several examples of material identification and material separation are discussed.

  • 29.
    Norlin, Börje
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Nilsson, Hans-Erik
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Spectral performance of a pixellated X-ray imaging detector with suppressed charge sharing2007In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. A 576, no 1, p. 248-250Article in journal (Refereed)
    Abstract [en]

    The ability to do X-ray imaging with energy resolution on a small pixel detector is severely limited by the charge sharing in the detector. Different schemes to overcome the problem have been proposed. Previous studies using synchrotron radiation have shown that, for a 300 µm thick Si detector, almost no charge sharing occurs for photons hitting the centre of each pixel. In this study we have used slits to focus the beam from a standard X-ray unit on the centre of a pixel on a MEDIPIX detector. The attenuation of the spectrum were measured for a number of samples of different materials with K-edge energy in the range of 30 to 50 keV. The measurements were done by scanning an energy window through the spectrum. Requirements for new X-ray imaging systems with true energy resolution are discussed based on these measurements.

  • 30.
    Norlin, Börje
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Nilsson, Hans-Erik
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Graafsma, Heinz
    Experimental Division, ESRF, Avenue des Martyrs, 38043 Grenoble Cedex, France.
    Vonk, Vederan
    Experimental Division, ESRF, Avenue des Martyrs, 38043 Grenoble Cedex, France.
    Ponchut, Cyril
    Experimental Division, ESRF, Avenue des Martyrs, 38043 Grenoble Cedex, France.
    Characterisation of the charge sharing in pixellated Si detectors with single-photon processing readout2006In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. A 563, no 1, p. 133-136Article in journal (Refereed)
    Abstract [en]

    Pixellated silicon detectors with a thickness of 300 µm and 700 µm bonded to the MEDIPIX2 readout chip have been characterised using a monoenergetic microbeam at the ESRF. The spectral response when a 10 x 10 µm2 wide 40 keV beam is centred on a single pixel is achieved. When the beam is scanned over the pixel, the charge sharing will increase when the beam approaches the border of the pixel. The experimental results have been verified by charge transport simulations and X-ray scattering simulations. Agreement between measurements and simulations can be achieved if a wider beam is assumed in the simulations. Widening of the absorption profile can to a large extent be explained by backscattering of lower energy photons by the tin/led bump-bounds below the detector. Widening of the detected beam is also an effect of angular alignment problems, especially on the 700 µm detector. Since the angel between the depth and a half pixel is only 2.2º, alignment of thick pixellated silicon detectors will be a problem to consider when designing X-ray imaging setups.

  • 31.
    Norlin, Börje
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    O'Nils, Mattias
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Fröjdh, Anna
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Fröjdh, Erik
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Thungström, Göran
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Energy Resolved X-ray Imaging as a Tool for Characterization of Paper Coating Quality2009In: IEEE Nuclear Science Symposium Conference Record 2009, IEEE conference proceedings, 2009, p. 1703-1706Conference paper (Refereed)
    Abstract [en]

    Energy resolved X-ray imaging can be used as a tool to analyze the variation in the chemical content of an object. In this work we have used energy resolved X-ray imaging to measure the variation in the chemical content of paper and paper coating. This is an important quality parameter for the paper industry. In order to separate the variation in coating thickness from the variation in paper thickness, energy resolution is used to separate the response of the coating from the response of the paper. The MEDIPIX2 single photon processing X-ray imaging system [1] has been used in the measurements.  The measurement results are compared to simulations with MCNP. The influence of charge sharing is discussed and the effects have been studied by comparing results from detectors with 220x220 µm2 pixels and detectors with 55x55 µm2 pixels. There is a trade-off between good spatial resolution obtained with detectors with small pixels and good energy resolution obtained with detectors with large pixels. The requirements on image quality, to achieve the resolution of coating distribution relevant for the application, are discussed.

  • 32.
    Norlin, Börje
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Thungström, Göran
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Greiffenberg, Dominic
    Freiburger Materialforschungszentrum (FMF), Albert-Ludwigs-Universität Freiburg, D-79104 Freiburg, Germany.
    Spectroscopic Imaging with an X-ray Microscopy Setup Using a Pixelated Detector with Single Photon Processing2009In: 2008 IEEE NUCLEAR SCIENCE SYMPOSIUM AND MEDICAL IMAGING CONFERENCE (2008 NSS/MIC), VOLS 1-9, New York: IEEE conference proceedings, 2009, p. 3464-3469, article id 4775083Conference paper (Other academic)
    Abstract [en]

    Energy resolving X-ray imaging opens up new perspectives for applications within the fields of medical X-ray diagnosis and industrial X-ray quality control. Single photon processing X-ray imaging systems with a pixel size of 55 x 55 µm2 have been developed within the MEDIPIX collaboration. These systems show excellent images without any background noise. However the spectral properties of the device are deteriorated by significant charge sharing between neighbouring pixels. This is a main objective for the recently started development of the MEDIPIX3 readout chip with charge summing between neighbouring pixels. In order to study the effects of improved spectral resolution on image quality before the MEIDPIX3 chip is available we have developed a detector with a pixel size of 220 x 220 µm2 and connected it to a MEDIPIX2 readout chip, using only a limited number of the available pixels. Using larger pixels will improve the spectral resolution since a smaller fraction of the detected photons are subject to charge sharing. By placing this detector in the X-ray microscope and using a magnification of four times we can estimate the response of a detector with 55 x 55 µm2 pixels and good spectroscopic resolution.

  • 33.
    Norlin, Börje
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Fröjdh, Erik
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Krapohl, David
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Fröjdh, Anna
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Thungström, Göran
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Spectroscopic X-Ray Imaging Using a Pixelated Detector with Single Photon Processing Readout2010In: Proceedings of 2010 IEEE NUCLEAR SCIENCE SYMPOSIUM CONFERENCE, IEEE conference proceedings, 2010, p. 1074-1078, article id 5873931Conference paper (Refereed)
    Abstract [en]

    Color informatics to analyze the object content in Xray images is an emerging technology. Identification of different elements for applications such as medical contrast agent imaging is possible using energy resolving X-ray imaging sensors. In this work RGB representations of transmission images of ground elements achieved with the MEDIPIX system are exemplified.

  • 34.
    Norlin, Börje
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Manuilskiy, Anatoliy
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Nilsson, Hans-Erik
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Material recognition with the Medipix photon counting colour X-ray system.2004In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 531, no 1-2, p. 265-269Article in journal (Refereed)
    Abstract [en]

    An energy sensitive imaging system like Medipixl has proved to be promising in distinguishing different materials in an X-ray image of an object. We propose a general method utilising X-ray energy information for materialrecognition. For objects where the thickness of the materials is unknown, a convenient material parameter to identify is K = α12, which is the ratio of the logarithms of the measured transmissions In(t1)/In(t2). If a database of the parameter K for different materials and energies is created, this method can be used for material recognition independent of the thickness of the materials. Series of images of an object consisting of aluminium and silicon were taken with different energy thresholds. The X-ray absorption for silicon and aluminium is very similar for the range 40-60 keV and only differs for lower energies. The results show that it is possible to distinguish between aluminium and silicon on images achieved by Medipixl using a standard dental source. By decreasing the spatial resolution a better contrast between the materials was achieved. The resolution of contrasts shown by the histograms was close to the limit of the system due to the statistical noise of the signal.

  • 35.
    Norlin, Börje
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Reza, Salim
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design. FS-DS, Deutsches Elektronen-Synchrotron (DESY), Hamburg, Germany.
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    X-ray fluorescence measurements of toxic metal content in ash from municipal solid waste incineration2017In: 2016 IEEE Nuclear Science Symposium, Medical Imaging Conference and Room-Temperature Semiconductor Detector Workshop (NSS/MIC/RTSD), IEEE, 2017, Vol. 2017-January, article id 8069695Conference paper (Refereed)
    Abstract [en]

    The vision of this paper is development of an online X-ray fluorescence method for monitoring of metal content in ash from municipal solid waste (MSW) incineration. With such measurements directly on site it is possible to optimize an ash washing process in incineration plants, allowing the fly ash to be stored in a landfill for non-hazardous waste. The presented X-ray fluorescence measurement assures that the measurement accuracy is sufficient for metal content monitoring. The actual measurement process is also fast enough to be possible to implement as an online measurement method. The optimal measurement setup is different for different metals. Several different metals might need environmental monitoring, which metals might vary over time due to systematic variations in waist content. Detection of a wide range of metals will require an X-ray source with variable voltage and multiple detectors.

  • 36.
    Norlin, Börje
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Reza, Salim
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Nordin, T.
    Process Technology Department, MoRe Research, Örnsköldsvik.
    Precision scan-imaging for paperboard quality inspection utilizing X-ray fluorescence2018In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 13, no 1, article id C01021Article in journal (Refereed)
    Abstract [en]

    Paperboard is typically made up of a core of cellulose fibers [C6H10O5] and a coating layer of [CaCO3]. The uniformity of these layers is a critical parameter for the printing quality. Current quality control methods include chemistry based visual inspection methods as well as X-ray based methods to measure the coating thickness. In this work we combine the X-ray fluorescence signals from the Ca atoms (3.7 keV) in the coating and from a Cu target (8.0 keV) placed behind the paper to simultaneously measure both the coating and the fibers. Cu was selected as the target material since its fluorescence signal is well separated from the Ca signal while its fluorescence's still are absorbed sufficiently in the paper. A laboratory scale setup is built using stepper motors, a silicon drift detector based spectrometer and a collimated X-ray beam. The spectroscopic image is retrieved by scanning the paperboard surface and registering the fluorescence signals from Ca and Cu. The exposure time for this type of setups can be significantly improved by implementing spectroscopic imaging sensors. The material contents in the layers can then be retrieved from the absolute and relative intensities of these two signals.

  • 37.
    Norlin, Börje
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Reza, Salim
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Krapohl, David
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Fröjdh, Erik
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design. CERN, Medipix Consortium, Geneva, Switzerland.
    Thungström, Göran
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Readout cross-talk for alpha-particle measurements in a pixelated sensor system2015In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 10, article id C05025Article in journal (Refereed)
    Abstract [en]

    Simulations in Medici are performed to quantify crosstalk and charge sharing in a hybrid pixelated silicon detector. Crosstalk and charge sharing degrades the spatial and spectral resolution of single photon processing X-ray imaging systems. For typical medical X-ray imaging applications, the process is dominated by charge sharing between the pixels in the sensor. For heavier particles each impact generates a large amount of charge and the simulation seems to over predict the charge collection efficiency. This indicates that some type of non modelled degradation of the charge transport efficiency exists, like the plasma effect where the plasma might shield the generated charges from the electric field and hence distorts the charge transport process. Based on the simulations it can be reasoned that saturation of the amplifiers in the Timepix system might generate crosstalk that increases the charge spread measured from ion impact on the sensor.

  • 38.
    O'Nils, Mattias
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Thim, Jan
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Oelmann, Bengt
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Threshold Modulation for Continuous Energy Resolution with Two Channels per Pixel in a Photon Counting X-ray Image Detector2009In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 607, no 1, p. 236-239Article in journal (Refereed)
    Abstract [en]

    The introduction of energy resolution in X-ray image detectors will lead to tradeoffs between circuit complexity and spatial/energy resolution in the pixel design. The proposed method provides continuous energy resolution with only two energy channels per pixel, which is a comparable complexity to that of a window discriminator pixel like Medipix2. The paper illustrates the method and validates the method through analytical analysis and through simulation of real and synthetic data.

  • 39.
    Rahman, Hafizur
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering.
    An, Siwen
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Persson, Erik
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering.
    Engstrand, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering.
    Maximized wood chip impregnation efficiency validated by new miniaturized X-ray fluorescence techniques2019Conference paper (Refereed)
    Abstract [en]

    Manufacturing of chemi-thermomechanical pulp (CTMP) is increasing due to increased demand for packaging materials such as cardboard as well as tissue and other hygiene products. Today high yield pulp (HYP) is produced from different wood species. It is well-known that chip-refining is normally responsible for more than 60% of the electric energy consumption in most high yield pulping process. There are opportunities to improve energy efficiency and quality stability in defibration processes by means of optimizing impregnation. Impregnation is a key unit operation in CTMP production as well as in all chemical pulping and biorefinery systems. The efficiency of the impregnation is known to be crucial (Ferritsius et al. 1985; Gorski et al. 2010). Early research showed difficulties to achieve even distribution of sulphite and sodium ions in wood chips resulting in inhomogeneous fibre properties (Bengtsson et al. 1988). Increased and homogenous sulphonation leads to reduced shive content, which is a key factor in all end product applications. To address this issue developing a new type miniaturized X-ray based technique (XRF) to measure local concentration of sulphur and sodium across wood chips and in individual fibres could become a key tool.

     

    The presence of elements as sulphur and sodium can be detected by X-ray fluorescence (XRF) or spectral absorption. At the XRF, images the surface of the sample using specific energies from K-shell or L-shell fluorescence. This method is investigated at the X-ray laboratory in Mid Sweden University research centre STC (Sensitive Things that Communicate) (Norlin et al. 2018). At the spectral absorption, images specific K-shell absorption energies in transmission X-ray images of the sample, a method widely used in medical diagnosis. This transmission method might also be further investigated for this application in the future (Frojdh et al. 2013; Reza et al. 2013). Both methods can be validated by using monoenergetic radiation from synchrotron facilities.

     

    An XRF imaging system uses a collimated X-ray source and a spectroscopic detector. The sample is scanned to make an image of the content of the substances of interest. A specific challenge in this case is that the low energy fluorescence photons from sulphur (S) and sodium (Na) are easily absorbed in air, which makes imaging in a different atmosphere necessary.

     

    The measurement setup has been simulated using MCNP (C. J. Werner, 2017) to validate the system setup and to select the correct, geometry, shielding, filtering and atmosphere for the measurement. The solution was to use a titanium box flooded with helium to minimise the absorption of fluorescence photons and to shield from scattered photons that might disturb the measurement, fig 1. A filter has been added to the X-ray source to make it nearly monoenergetic and to avoid emission of photons with energies close to the expected fluorescence. The system has been used to estimate sodium and sulphur content in low grammage handsheet (CTMP) or single wood chip samples. It is possible to build a laboratory instrument similar to the prototype setup to obtain the distribution of sodium and sulphur in XRF imaging.

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

    Figure 1: Photograph of XRF measurement setup with of moveable Helium atmosphere Ti box

    However, the technique we are developing can become useful in mills to improve and control process efficiency, product properties and to find solutions to process problems in future. In addition, a more even distribution of the sulphonation can reduce specific energy demand in chip refining at certain shive content.

     

    References

     

    1.      Bengtsson, G., Simonson, R., Heitner, C., Beatson, R., and Ferguson, C. (1988): Chemimechanical pulping of birch wood chips, Part 2: Studies on impregnation of wood blocks using scanning electron microscopy and energy dispersive x-ray analysis, Nord. Pulp Paper Res. J. 3 (3), 132-138.

    2.      C. J. Werner, (2017): MCNP User's manual, Code Version 6.2, Los Alamos National Laboratory report, LA-UR-17-29981.

    3.      Ferritsius, O., and Moldenius, S. (1985): The effect of impregnation method on CTMP properties. In International Mechanical Pulping Conference Proceedings, SPCI, Stockholm (p. 91).

    4.      Frojdh, C., Norlin, B. and Frojdh, E. (2013): Spectral X-ray imaging with single photon processing detectors, Journal of Instrumentaion, Volume 8, Article number C02010.  

    5.      Gorski, D., Hill, J., Engstrand, P., and Johansson, L. (2010): Reduction of energy consumption in TMP refining through mechanical pre-treatment of wood chips, Nord. Pulp Paper Res. J, 25(2), 156-161.

    6.      Norlin, B., Reza, S., Fröjdh, C. and Nordin, T. (2018): Precision scan-imaging for paperboard quality inspection utilizing X-ray fluorescence, Journal of Instrumentation, Volume: 13, Article number C01021.

    7.      Reza, S., Norlin, B. and Thim, J. (2013): Non-destructive method to resolve the core and the coating on paperboard by spectroscopic x-ray imaging, Nord. Pulp Paper Res. J. 28 (3), 439-442.

     

  • 40.
    Reza, Salim
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design. Deutsches Elektronen-Synchrotron (DESY).
    Chang, Haosi
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Thungström, Göran
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Detecting Cr Contamination In Water Using X-Ray Fluorescence2015In: 2015 IEEE Nuclear Science Symposium and Medical Imaging Conference, Institute of Electrical and Electronics Engineers (IEEE), 2015, article id 7581750Conference paper (Other academic)
    Abstract [en]

    With the rapid growth in population and the overwhelming demand of industrial consumer products around the world, the amount of generated wastes is also increasing. Therefore, the optimal utilization of wastes and the waste management policies are very important in order to protect the environment[1]. The most common way of waste management is to dispose them into city dumps and landfills. These disposal sites may produce toxic and green house gases and also a substantial amount of leachate, which can affect the environment[2]. Leachate is liquid, which, while percolating through wastes in a landfill, extracts soluble and suspended solids. Leachate contains toxic and harmful substances, such as Chromium (Cr), Arsenic, Lead, Mercury, Benzene, Chloroform and Methylene Chloride, and can contaminate surface water and aquifers.

  • 41.
    Reza, Salim
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Thim, Jan
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Non-Destructive Method to Resolve the Core and the Coating on Paperboard by Spectroscopic X-ray Imaging2013In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 28, no 3, p. 439-442Article in journal (Refereed)
    Abstract [en]

    Quality control is an important issue in the paperboard industry. A typical sheet of paperboard contains a core of cellulose fibers [C6H10O5], coated on one or both sides with layers of calcium carbonate [CaCO3] or Kaolin [Al2Si2O5(OH)4]. One of the major properties of a good quality paperboard is the consistency of the expected ratio between the thickness of the core and the coating layers. A measurement system to obtain this ratio could assist the paperboard industry to monitor the quality of their products in an automatic manner. In this work, the thicknesses of the core and the coating layers on a paperboard with coating layer on only one side were measured using an X-ray imaging technique. However, the limited spectral and spatial resolution offered by the measurement system being used led to the measured thicknesses of the layers being lower than their actual thicknesses in the paperboard sample. Suggestions have been made in relation to overcoming these limitations and to enhance the performance of the method. A Monte Carlo N-particle code simulation has been used in order to verify the suggested method.

  • 42.
    Reza, Salim
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Pelzer, Georg
    University of Erlangen-Nuremberg, ECAP-Erlangen Centre for Astroparticle Physics, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany .
    Weber, Thomas
    University of Erlangen-Nuremberg, ECAP-Erlangen Centre for Astroparticle Physics, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany .
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Bayer, Florian
    University of Erlangen-Nuremberg, ECAP-Erlangen Centre for Astroparticle Physics, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany .
    Anton, Gisela
    University of Erlangen-Nuremberg, ECAP-Erlangen Centre for Astroparticle Physics, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany .
    Rieger, Jens
    University of Erlangen-Nuremberg, ECAP-Erlangen Centre for Astroparticle Physics, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany .
    Thim, Jan
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Michel, Thilo
    University of Erlangen-Nuremberg, ECAP-Erlangen Centre for Astroparticle Physics, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany .
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Investigation on the directional dark-field signals from paperboards using a grating interferometer2014In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 9, p. Art. no. C04032-Article in journal (Refereed)
    Abstract [en]

    Recent advancements in the grating interferometer based Phase Contrast X-ray Imag- ing (PCXI) technique enables high quality dark-field images to be obtained using conventional X-ray tubes. The dark-field images map the scattering inhomogeneities inside objects. Since, the dark-field image is constructed by considering only those photons which are scattered while pass- ing through the objects, it can reveal useful information about the object inner structures, such as, the fibre structures inside paperboards.

    The end-use performance of paperboards, such as the printing quality and the stiffness de-pends on the uniformity in the thickness and the structures of the coating layer of the paperboards. The uniformity in the coating layer is determined by the coating techniques, the coating materials and the topography of the base sheet. In this article, the dark-field signals from four paperboard samples with different quality indices are analysed. The isotropic and the anisotropic scattering coefficients for all of the samples have been calculated. Based on the correlation between the isotropic coefficients and the quality indices of the paperboards, a new method for paperboard quality measurement has been suggested.

  • 43.
    Reza, Salim
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Wong, Winnie
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Fröjdh, Erik
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Fröjdh, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Thungstörm, Göran
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Thim, Jan
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Smart dosimetry by pattern recognition using a single photon counting detector system in time over threshold mode2012In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 7, no 1, p. Art. no. C01027-Article in journal (Refereed)
    Abstract [en]

    The function of a dosimeter is to determine the absorbed dose of radiation, for those cases in which, generally, the particular type of radiation is already known. Lately, a number of applications have emerged in which all kinds of radiation are absorbed and are sorted by pattern recognition, such as the Medipix2 application in [1]. This form of smart dosimetry enables measurements where not only the total dosage is measured, but also the contributions of different types of radiation impacting upon the detector surface. Furthermore, the use of a photon counting system, where the energy deposition can be measured in each individual pixel, ensures measurements with a high degree of accuracy in relation to the pattern recognition. In this article a Timepix [2] detector system has been used in the creation of a smart dosimeter for Alpha, Beta and Gamma radiation. When a radioactive particle hits the detector surface it generates charge clusters and those impacting upon the detector surface are read out and image processing algorithms are then used to classify each charge cluster. The individual clusters are calculated and as a result, the dosage for each type of radiation is given. In some cases, several particles can impact in roughly the same place, forming overlapping clusters. In order to handle this problem, a cluster separation method has been added to the pattern recognition algorithm. When the clusters have been separated, they are classified by shape and sorted into the correct type of radiation. The algorithms and methods used in this dosimeter have been developed so as to be simple and computationally effective, in order to enable implementation on a portable device. © 2012 IOP Publishing Ltd and SISSA.

  • 44.
    Thim, Jan
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    CMOS for Color X-Rays - Where do we go from here?: Invited paper at Emerging CMOS Technologies, Vancouver, Canada, August 20082008Conference paper (Refereed)
  • 45.
    Thim, Jan
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Noise Coupling in Digital X-Ray Imaging2009In: Medical Imaging: Principles, Detectors, and Electronics, Hoboken: Wiley-Blackwell, 2009Chapter in book (Other academic)
  • 46.
    Thim, Jan
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    O'Nils, Matthias
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Abdalla, Suliman
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Oelmann, Bengt
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Realizing increased sub-pixel spatial resolution in X-ray imaging using displaced multiple images2011In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 633, no Suppl 1, p. S247-S249Article in journal (Refereed)
    Abstract [en]

    A method, generally called oversampling, to reach sub-pixel resolution by taking slightly displaced images of an object is investigated for X-ray applications. By mounting the sensor on a high precision step motor table it is possible to increase the spatial resolution from 55 ÎŒm×55 ÎŒm to at least 20 ÎŒm×20 ÎŒm, which is required for quality assurance measurements in several industry processes. The performance compared to physically smaller pixels is shown, and the effects of charge sharing on the method are investigated. The suggested method is relatively cost effective compared to using X-ray microscopy. © 2010 Elsevier B.V. All rights reserved.

  • 47.
    Thim, Jan
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    O'Nils, Mattias
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Abdalla, Suliman
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Oelmann, Bengt
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Realizing increased sub-pixel spatial resolution in X-ray imaging using displaced multiple images2009In: 11th International Workshop on Radiation Imaging Detectors, 2009Conference paper (Refereed)
    Abstract [en]

    In X-Ray imaging with pixel detector systems, the resolution of the image taken is dependant on the pixel size in the detector readout electronics. Depending on the functionality of the readout electronics, the surface space on the readout chip for each pixel has a minimum size, which sets the spatial resolution of the taken images. For applications where it is required to image extremely small structures in a material, the spatial resolution of the X-Ray detector system sets the limit, and readout systems with high functionality cannot be considered. One way to reach sub-pixel resolution is to use a nanofocus source to achieve an X-ray microscopy setup [1]. However, this type of X-ray source is still too expensive to be an alternative for quality assurance systems used in the industry. In this paper we focus on a much simpler way of increasing spatial resolution that has proven effective in images for visible light. By mounting either the objects for imaging or the image sensor system on a step motor table and take multiple images slightly dislocated from one another, an increase in sub-pixel spatial resolution can be achieved.

    Consider the case that an image sensor system with a pixel size of 55x55 µm is available for an imaging application that requires a resolution of 20x20 µm. The application is material characterization and allows for multiple images to be taken for one sample. In this case, increasing the sub-pixel resolution by nine times (3x3) will result in a pixel size of about 18x18 µm, which would meet the requirements. This can be realized by taking nine images dislocated 1/3 of the pixel width from each other. If the upper left pixel of the centre image has coordinates (0,0) the upper left pixel of all the nine images will have coordinates (-1/3,1/3), (0,1/3), (1/3,1/3), (-1/3,0), (0,0), (1/3,0), (-1/3,-1/3), (0,-1/3) and (1/3,-1/3). The result of a direct combination of these images is illustrated in Figure 1, where one of nine images is shown at the left. Combining the images without images processing with an algorithm will yield the image in the centre, which can be compared to how the image would look in full 9x resolution (right image). As can be seen, some details are lost and the image is blurred compared to a full resolution image. However, with an image processing algorithm in the combination phase this effect can be reduced and the image quality increased.

    This paper shows simulated and measured results from using dislocation imaging in X-Ray imaging systems, where the test case system will be the MEDIPIX2 system [2]. An investigation of different image processing algorithms suitable for this type of imaging is conducted. An investigation is also done to show whether detectors with large size pixels compared to the standard size in a MEDIPIX system can be combined with the described sub-pixel scaling technique. The result of this combination is used to investigate the charge sharing effects on the MEDIPIX system.

    [1] Norlin B., Fröjdh C., Nuclear Instruments and Methods, sect. A (2009), doi:10.1016/j.nima.2009.03.155[2] Llopart X., Campbell M., Dinapoli R., san Segundo D., Pernigotti E., IEEE Transactions on Nuclear Science, Vol. 49, Issue 5, Part 1, pp. 2279-2283, October 2002.

    Figure 1. Image (left) with 50x50 pixels, with the resulting combination of nine images forming an image with a sub-pixel resolution of 150x150 pixel (centre), compared to a full resolution reference image (right).

  • 48.
    Thim, Jan
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Reza, Salim
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Nawaz, Khalid
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    O´Nils, Mattias
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Oelmann, Bengt
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Suitable Post Processing Algorithms for X-Ray Imaging using Oversampled Displaced Multiple Images2011In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 6, no 2, p. Art. no. C02001-Article in journal (Refereed)
    Abstract [en]

    X-ray imaging systems such as photon counting pixel detectors have a limited spatial resolution of the pixels, based on the complexity and processing technology of the readout electronics. For X-ray imaging situations where the features of interest are smaller than the imaging system pixel size, and the pixel size cannot be made smaller in the hardware, alternative means of resolution enhancement require to be considered. Oversampling with the usage of multiple displaced images, where the pixels of all images are mapped to a final resolution enhanced image, has proven a viable method of reaching a sub-pixel resolution exceeding the original resolution. The effectiveness of the oversampling method declines with the number of images taken, the sub-pixel resolution increases, but relative to a real reduction of imaging pixel sizes yielding a full resolution image, the perceived resolution from the sub-pixel oversampled image is lower. This is because the oversampling method introduces blurring noise into the mapped final images, and the blurring relative to full resolution images increases with the oversampling factor. One way of increasing the performance of the oversampling method is by sharpening the images in post processing. This paper focus on characterizing the performance increase of the oversampling method after the use of some suitable post processing filters, for digital X-ray images specifically. The results show that spatial domain filters and frequency domain filters of the same type yield indistinguishable results, which is to be expected. The results also show that the effectiveness of applying sharpening filters to oversampled multiple images increase with the number of images used (oversampling factor), leaving 60-80% of the original blurring noise after filtering a 6 x 6 mapped image (36 images taken), where the percentage is depending on the type of filter. This means that the effectiveness of the oversampling itself increase by using sharpening filters, and more images taken can be considered worth the effort.

  • 49.
    Thim, Jan
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Reza, Salim
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    O'Nils, Mattias
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    X-ray imaging of high velocity moving objects by scanning summation using a single photon processing system2015In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, article id C04023Article in journal (Refereed)
    Abstract [en]

    X-ray imaging has been used extensively in the manufacturing industry. In the paper and paperboard industry X-ray imaging has been used for measuring parameters such as coat weight, using mean values of X-ray absorption inline in the manufacturing machines. Recently, an interest has surfaced to image paperboard coating with pixel resolved images showing material distribution in the coating on the paperboard, and to do this inline in the paper machine. Naturally, imaging with pixel resolution in an application where the paperboard web travels with velocities in the order on 10 m/s sets harsh demands on the X-ray source and the detector system to be used. This paper presents a scanning imaging method for single photon imaging systems that lower the demands on the source flux by hundreds of times, enabling a system to be developed for high velocity industrial measurement applications. The paper presents the imaging method, a discussion of system limitations, simulations and real measurements in a laboratory environment with a moving test object of low velocity, all to verify the potential and limits of the proposed method.

  • 50.
    Thungström, Göran
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Harrnsdorf, Lars
    RTI Electronics AB, Lund University IKVM.
    Norlin, Börje
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Reza, Salim
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Krapohl, David
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Mattsson, S.
    Lund University IKVM.
    Gunnarsson, M.
    Lund University IKVM.
    Measurement of the sensitive profile in a solid state silicon detector, irradiated by X-rays2013In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 8, no 4, p. Art. no. C04004-Article in journal (Refereed)
    Abstract [en]

    A newly constructed solid state silicon dose profile detector is characterized concerning its sensitive profile. The use of the MEDIPIX2 sensor system displays an excellent method to align an image of an X-ray slit to a sample under test. The scanning from front to reverse side of the detector, show a decrease in sensitivity of 20%, which indicates a minority charge carrier lifetime of 0.18 ms and a diffusion length of 460 μm. The influence of diced edges results in a volumetric efficiency of 59%, an active volume of 1.2 mm 2 of total 2.1 mm2.

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