Abstract:
An X-ray detector ( 401, 501, 601 ) has a detecting element that comprises a semiconductor heterostructure where an undoped Germanium layer ( 402, 502 ) is enclosed between two oppositely doped Gallium Arsenide layers ( 403, 404, 503, 505 ).

Description:
TECHNICAL FIELD  
         [0001]    The invention concerns generally the technology of producing digital images of objects illuminated with X-rays. Especially the invention concerns the composition, structure and manufacturing method of a detector that is used for producing a digital image of the illuminated object.  
         BACKGROUND OF THE INVENTION  
         [0002]    [0002]FIG. 1 illustrates schematically a conventional way of producing an X-ray image in electronic form. An X-ray tube  101  emits X-rays that propagate through an object under study (such as a part of a human body)  102  and enter an image intensifier  103 . Optical equipment  104  convey the output signal of the image intensifier  103  onto a CCD array  105 , from which the two-dimensional image is read and stored into a memory  106  and/or shown on a display  107 . The drawbacks of the conventional approach arise from the large number of different devices and components that take part in converting the spatial distribution of differently attenuated x-rays into an actual image. Taking the signal through a variety of devices and components reduces the efficiency of collecting information from the object under study and introduces noise.  
           [0003]    [0003]FIG. 2 illustrates a more recently adopted approach where detected X-rays are converted into an image through fewer intermediate phases. The whole arrangement of image intensifier, optics and CCD is now replaced with a single digital detector  201  the output signal of which is ready to be taken into a memory  106  and/or a display  107 . The structure and operation of the digital detector  201  are thus such that the spatial distribution of X-ray intensity that hits the detector  201  can be more or less directly converted into an array of digital pixel values. Truly digital imaging of this kind can make the process of producing X-ray images in electronic form remarkably simpler and more efficient compared to the conventional approach of FIG. 1.  
           [0004]    There remains the question of how should the digital detector  201  be constructed. At the priority date of this description the most well-known example of digital X-ray imaging detectors is the CsI/Si (Cesium Iodide/Silicon) detector of GE Medical Systems Corporation. FIG. 3 is a simplified cross-section of a CsI/Si detector panel  301 . A CsI scintillator layer  302  is arranged to meet the incoming X-ray photons. Absorption of a photon into the CsI layer  302  causes light to be emitted locally into an amorphous Si panel  303  that is located immediately next to the CsI layer  302  and constitutes a two-dimensional photodiode/transistor array. It absorbs the light and converts it into electronic charge. Each photodiode in the amorphous Si panel  303  represents a pixel in the image to be generated. An arrangement of low-noise readout electronics  304  behind the amorphous Si panel  303  is employed to collect the accumulated charge from each photodiode and convert the individual quantities of collected charge into corresponding digital value. The two-dimensional constellation of digital values represents an image that can be stored into memory and/or shown on a display.  
           [0005]    A critical factor of a detector of the type shown in FIG. 3 is the scattering of light between the CsI scintillator layer  302  and the amorphous Si panel  303 . The objective of producing sharp images calls for keeping each burst of scintillation photons confined into an as small spatial sector as possible. GE Medical Systems Corporation announces having developed a “needle-like” structure for the CsI layer that should prevent scattering to a large extent. However, producing such a structure with good yield and highly homogenous gain throughout the detecting surface may be problematic. Homogeneity of the detector response is of crucial importance for example in medical imaging applications where far-reaching decisions are made on the basis of what fundamentally is a detected spatial distribution of received X-ray intensity. Another disadvantage of the structure of FIG. 3 are that only approximately one half of the visible light photons can be collected. Amorphous Si is not known to be a very good photodetector; it is used in this structure mainly because it allows building a detector with a relatively large area. Siemens Corporation has a corresponding product on the market with the trade name TRIXELL.  
           [0006]    Other known techniques for obtaining digital X-ray images include slot scanning, where a linear detector is mechanically moved across the illumination beam; using tiled CCD arrays coupled to a scintillator plate via fiber optics; computed radiography where electrons are trapped on photostimulated plates that are then exposed to generate image data; and direct conversion. The last-mentioned has traditionally meant that two-dimensional Selenium detector panels are used for receiving the X-ray photons, which get absorbed and give rise to local accumulation of charges in the bulk of the Se substrate. Readout electronics are then employed to collect the accumulated charge and to convert the collected charge values into a two-dimensional image. The drawbacks of the Se-based direct conversion detector arrangements have been associated with questionable reliability as well as a relatively low DQE (Detective Quantum Efficiency) values, which cause degradation to image quality and preclude the use of Se-based direct conversion detectors in advanced applications of X-ray diagnostics and therapy.  
         SUMMARY OF THE INVENTION  
         [0007]    It is an objective of the present invention to present an X-ray detector that is suitable for direct digital imaging, has a high efficiency in terms of utilizing the information carried by X-ray photons incident on the detector, and is suitable for mass production in respect of production yield, manufacturing cost and reliability in use. Further objectives of the invention are that the X-ray detector should have good absorption properties for X-rays and potential for good energy resolution, as well as small leakage current through the detector.  
           [0008]    The objectives of the invention are met by using a high-purity Germanium layer as a central part of the detector and doped Gallium Arsenide layers on each side of said Germanium layer.  
           [0009]    An X-ray detector according to the invention is characterized by the features that are recited in the characterizing part of the appended independent patent claim directed to an X-ray detector.  
           [0010]    The invention also applies to an X-ray imaging arrangement that is characterized by the features that are recited in the characterizing part of the appended independent patent claim directed to an X-ray imaging arrangement.  
           [0011]    Additionally the invention applies to a method for producing an X-ray detector. The method is characterized by the features that are recited in the characterizing part of the appended independent patent claim directed to a method.  
           [0012]    Direct conversion detectors invariably involve the conversion of the energy of an incoming photon into a certain spatial distribution of electrical charge. Controlling and detecting the space charge region with reasonable accuracy requires the use of a high-quality semiconductor material, the properties of which are homogenous enough so that detection probability and accuracy is independent of the location at which the photon happened to hit the detector.  
           [0013]    Germanium (or Ge for short) has been widely regarded as an unsuitable material for direct conversion detector applications at least in room temperature, because the excitation energy of electrons in Ge is small, only 0.65 eV, and consequently the leakage current through the material is prohibitively high. Otherwise Ge would have many advantageous properties. It is relatively straighforward to produce Ge wafers of extremely high purity and of desired thickness. The absorption cross-section of Ge is large enough for X-ray photons of the energy range used in typical imaging applications. For example absorption in Si is far too low, and consequently Si can be used for detecting only very low energy radiation. Ge is readily available at reasonable cost and it can advantageously provide very good energy resolution. For example Cadmium Telluride (CdTe) is very costly and suitable only for detectors with very small dimensions, with little or no potential for the application of microelectronic technology thereon.  
           [0014]    According to the present invention, a high-purity Ge layer is complemented with doped Gallium Arsenide (GaAs) layers on each side. A negatively doped GaAs layer is located on one side of the Ge layer and a positively doped GaAs layer comes on the other side of the Ge layer. The resulting structure resembles a GaAs diode in electric operation, which means for example a relatively low leakage current even in room temperature. One of the GaAs layers is patterned in a suitable way to give rise to pixels. The Ge layer acts as a photoelectric absorber of the X-ray photons: each X-ray photon that gets absorbed in the Ge gives rise to a local cloud of electric charge, which is collected into the pixels and read with suitable integrated readout electronics. The circuit that takes care of the reading of charge from the pixels can be bonded onto one surface of the detector, or it may even be processed directly onto one of the doped GaAs layers.  
           [0015]    The Ge layer is typically very thick compared to the GaAs layers: the thickness of the Ge layer is usually in the range between 0.2 and 2 mm, whereas the thickness of the GaAs layers is only in micrometer range, like between 1 and 5 micrometers, although the exact thickness value of the GaAs layers is not that important. The Ge layer should be thick enough to absorb a large majority of incident X-ray photons. On the other hand it is not advantageous to make the Ge layer any thicker than what is required for good absorption, because the thicker the Ge layer is, the larger is the minimum reasonable pixel size and the more vulnerable the structure is to interfering effects caused by impurities in the Ge material. In general the resistivity of the pure Ge material is high, and effects caused by residual impurities are negligible.  
           [0016]    The low excitation energy of Ge gives rise to large statistical fluctuations in the number of free charge carriers in room temperature. This in turn means that a detector according to the invention is hardly suitable for spectroscopic analysis of the incoming X-rays unless the detector is heavily cooled. The resulting lack of information about quantum energies has no significance in most medical imaging applications, where essentially monochromatic X-rays are used for illumination anyway and all useful information resides in the spatial intensity distribution of X-rays that have propagated through the object under study. Additionally it is relatively straightforward to built a cooling arrangement to cool down the detector into a temperature range where thermal excitation starts to loose significance, in which case the detector according to the invention can also be used in spectroscopical applications.  
           [0017]    The advantageousness of combining just GaAs with Ge comes from the near sameness of certain lattice constants of the two materials. Close lattice constants mean that producing a nice and regularly grown epitaxial layer of one material on top of the other in the manufacturing process is easy. Additionally recombination at the material interface can be made negligible. The high purity of the Ge layer means that the detection response of the detector can be made extremely homogenous throughout the detector area. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0018]    The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.  
         [0019]    [0019]FIG. 1 illustrates a conventional X-ray imaging arrangement,  
         [0020]    [0020]FIG. 2 illustrates a more advanced known X-ray imaging arrangement,  
         [0021]    [0021]FIG. 3 is a cross section of a detector used in the arrangement of FIG. 2,  
         [0022]    [0022]FIG. 4 is a cross section of a detector according to an embodiment of the invention,  
         [0023]    [0023]FIG. 5 illustrates bonding a readout circuit onto a detector according to an embodiment of the invention,  
         [0024]    [0024]FIG. 6 illustrates integrating a readout circuit with a detector according to an embodiment of the invention,  
         [0025]    [0025]FIGS. 7 a  and  7   b  illustrate a method according to an embodiment of the invention,  
         [0026]    [0026]FIG. 8 illustrates an X-ray imaging arrangement according to an embodiment of the invention,  
         [0027]    [0027]FIG. 9 illustrates an X-ray imaging arrangement according to another embodiment of the invention,  
         [0028]    [0028]FIG. 10 illustrates an alternative pixelizing approach and  
         [0029]    [0029]FIG. 11 illustrates an advantageous way of implementing readout electronics. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]    The exemplary embodiments of the invention presented in this patent application are not to be interpreted to pose limitations to the applicability of the appended claims. The verb “to comprise” is used in this patent application as an open limitation that does not exclude the existence of also unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated.  
         [0031]    [0031]FIG. 4 is a schematic cross section through a stack of semiconductor layers that together constitute the radiation-sensitive parts of an X-ray detector  401 . Basically the detector&#39;s semiconductor stack consists of a pure Ge layer  402  sandwiched between two oppositely doped GaAs layers  403  and  404 . “Oppositely doped” means that one of the GaAs layers  403  and  404  is an n-type semiconductor layer and comprises a surplus of negative charge carriers (electrons), while the other is a p-type semiconductor layer and comprises a surplus of positive charge carriers (holes). One of a pair of electrodes  405  and  406  is coupled to each of the doped GaAs layers  403  and  404 . The Ge layer  402  is “pure” in the sense that its purity is as high as is commercially reasonably achievable.  
         [0032]    Electrically the structure shown in FIG. 4 operates much like an ordinary GaAs pn-diode, so that it can for example be biased either in the forward or in the reverse direction. In order to be useful as an X-ray detector it is biased in the reverse direction by applying a suitable reverse bias voltage between the electrodes  405  and  406 .  
         [0033]    The GaAs layers  403  and  404  are thin enough so that they do not absorb significant amounts of X-rays incident on the detector, whereas the thickness of the Ge layer  402  is typically 200 micrometers or more. To be more exact, the thickness of the Ge layer should be selected depending on the energy of the X-ray photons that are to be detected. Typical photon energies are in the order of a few keVs, tens of keVs or even some hundreds of keVs: for example mammographic imaging applications frequently use energies of 17-25 keV while dental imaging applications use 50-60 keV and thorax imaging could use 100 keV or more. A sufficient thickness of the Ge layer is such that it serves to absorb a major portion (more than 90%) of the incident X-ray photons of interest. It is not recommendable to make the Ge layer any thicker than is necessary for achieving the absorption objective, because the thicker the Ge layer is the more its electrical characteristics tend to be dominated by residual impurities left in the Ge lattice. There is no practical upper limit for the thickness of the Ge layer, but the thicker it is, the larger must the pixels be if pixelization is required; more aspects of pixelization are described later. A sufficient thickness for the Ge layer is in many cases less than 2 mm, but even thicknesses of several centimeters like in certain neutron detectors can be realized.  
         [0034]    When an X-ray photon hits the detector, it causes a photoelectric effect in a Ge atom, producing a photoelectron that in turn excites a number of outer electrons from other atoms from their valence bands into the conduction band. Each excited electron leaves behind a positive hole. The resulting cloud of free charge is concentrated within a relatively small spatial area, in the order of some micrometers. The reverse-direction bias voltage across the detector drives the free charge carriers towards the GaAs layers. By suitable detection techniques it is possible to detect both the amount of produced free charge as well as the (two-dimensional) location at which it was produced. The amount of free charge produced by a single absorbed X-ray photon is proportional to the incident energy of the photon, and the location where the free charge appeared reveals the position where the photon hit the Ge layer. If the detector is at room temperature, the statistical fluctuation of the number of thermally excited free electrons is so large that it tends to mask the exact relationship between the amount of photoelectrically induced charge and the energy of the incident photon, but even in room temperature the spatial information is preserved to a reasonably large extent. Energy resolution may be possible in a coarse scale: intrinsic concentration of carriers is in the order of 2·10 13  1/cm, which in room temperature and an exemplary pixel volume of 100×100×100 microns means 2·10 13  electrons and a statistical fluctuation of 4500 electrons, which is equivalent to one incident photon of 20 keV. The interfering effect of thermal excitation can be reduced by cooling the detector.  
         [0035]    [0035]FIG. 5 is a cross section through a detector  501  according to an embodiment of the invention. The body of the detector is a high-purity Ge layer  502 . On one planar surface of the Ge layer  502 , which here is the bottom surface, there has been produced an n-type GaAs layer  503 , which is further covered by an ohmic contact layer  504 . The other planar surface of the Ge layer  502 , which here is the top surface, carries a p-type GaAs layer that has been patterned to form a continuous array of distinct p-type GaAs pixels  505 . The dimensions D of each pixel in the planar direction of the plate-like detector are typically in the order of 50-100 micrometers. An arrangement of ohmic contacts  506  has been produced on top of the pixels  505  in order to facilitate pixelwise reading of accumulated free charge in the detector  501 .  
         [0036]    Also very much larger pixels are possible. The limiting case is a non-imaging detector where a single “pixel” covers the whole detector area. Pixels in the scale of several millimeters or even centimeters can be used for example in simple “pinhole camera” applications where X-ray radiation is allowed to enter through a small aperture so that it hits a detector and produces a rough image of what kind of radiation sources are located ahead in the shooting direction.  
         [0037]    At one location of the detector  501  there has been placed an integrated circuit  507  the task of which is to implement the readout functions. Bonding wires  508  couple the integrated circuit  507  to all necessary ohmic contacts on the surfaces of the detector  501 . The integrated circuit  507  is typically an ASIC (Application Specific Integrated Circuit) that has been specifically designed for this purpose.  
         [0038]    [0038]FIG. 6 illustrates a detector  601  that is a variation of the detector  501  shown in FIG. 5. The only difference is in the implementation of the readout circuitry. Instead of bonding a separately manufactured ASIC onto the detector plate, in FIG. 6 a readout circuit  602  has been processed directly into the GaAs semiconductor substrate constituted by one of the GaAs layers in the detector plate. Using one of the GaAs layers in the detector plate as the substrate for an integrated circuit requires the GaAs material to be relatively heavily doped, which criterion is met since it is typical also to the GaAs—Ge—GaAs semiconductor heterostructure used according to the invention that the dopant concentration in the GaAs layers is high.  
         [0039]    Following either one of the principles of FIGS. 5 and 6 (or even both simultaneously) it is possible to place two or more separate circuit units onto the detector plate, if necessary.  
         [0040]    [0040]FIGS. 7 a  and  7   b  together show an exemplary step-by-step method for manufacturing a detector plate according to an embodiment of the invention. The method starts at step  701  by obtaining a high-purity Ge wafer of a desired thickness. In case the planar surfaces of the Ge wafer need polishing or other preparation before starting the process of depositing the GaAs layers, such preparative measures can be conceptually included in step  701 . In the method of FIGS. 7 a  and  7   b  we assume that the pixelized GaAs layer is the p-type layer and that it is produced first, so step  702  involves epitaxial growth of GaAs on that surface of the Ge wafer that is going to be pixelized. At step  703  acceptor ions are implanted into the epitaxially grown GaAs layer to make it appear as a p-type semiconductor. Typical acceptor implanting may involve something like 3×10 13  Mg +  ions per square centimeter implanted at an energy of 33 keV. Thermal annealing at 850° C. for the duration of 20 s follows the implanting to allow restoring crystalline defects caused by the implanting ion beam.  
         [0041]    Ion implanting is not the only known way of producing doped semiconductor layers. It is only mentioned here as en example of how the desired result can be achieved. For the purposes of present invention the importance of step  703  (and step  723  below) is in the result achieved therethrough: when completed, the GaAs layer must be suitably doped.  
         [0042]    At step  704  a photoresist of the thickness of about 1 micrometer is deposited on the ion-implanted GaAs layer. The photoresist must be selected so that it allows chemical etching. At step  705  the photoresist is exposed through a suitable mask and developed so that photoresist remains on pixels and all other places where GaAs will be needed, but essentially does not remain on the pixel separator lines. The pixel separators are etched out chemically over a depth of approximately 1-2 micrometers at step  706  by using for example a solution of NH 4 OH:H 2 O 2 :H 2 O in ratios 1:1:50 and a 20 minutes exposure time at room temperature. At step  707  the residual photoresist that remains on the pixels and other preserved GaAs areas is removed.  
         [0043]    Step  708  involves plasma deposition of a thin (about 40 nanometers) insulator substance like Si 3 N 4  over the entire surface at an elevated temperature in the order of 300° C. At step  709  another photoresist layer of about 1 micrometer is deposited, the photoresist now having to sustain plasma etching. At step  710  the photoresist is exposed and developed, this time to open over the pixels and other locations where the insulator layer must be removed. Selective removing of the insulator over the exposed areas through plasma etching follows at step  711 . Exemplary process parameters for the plasma etching step are 50 W, 0.04 Torr, 50 cm −3 s −1 . Plasma etching shoud stop when the GaAs surface is reached.  
         [0044]    At step  712  ohmic contacts are deposited on the exposed GaAs surfaces of the pixels by thermal evaporation. A typical composition of an ohmic contact deposited through thermal evaporation is 10 nanometers of Pt, followed by 30 nanometers of Ti, another 10 nanometers of Pt and finally 300 nanometers of Au. Removing the remaining photoresist at step  713  finalizes the manufacturing of the pixelized p-type GaAs surface.  
         [0045]    Preparing the other planar surface of the detector plate, which in this example is the unpatterned n-type GaAs side, is more straightforward. If the remaining free surface of the Ge wafer needs polishing for example to exactly determine the thickness of the Ge layer, it is accomplished at step  721 . Step  722  involves epitaxial growth of GaAs on the surface. At step  723  the GaAs layer is implanted with donor ions and annealed. At step  724  an even back ohmic contact is deposited, consisting for example of 15 nanometers of Ni, followed by 19.5 nanometers of Ge, 39 nanometers of Au, 50 nanometers of Ti and finally 200 nm of Au. Final thermal annealing at 400° C. for the duration of 40 s under a H 2  flux, with the detector plate lying on the p-type side, finalizes the manufacturing process at step  725 .  
         [0046]    [0046]FIG. 8 illustrates an X-ray imaging arrangement according to an embodiment of the invention. An X-ray tube  801  is used as a source for X-rays in the range of some tens of keVs. A detector arrangement  802  is arranged to receive the X-rays that propagated through an object under study. The detector arrangement  802  comprises the pixelized GaAs—Ge—GaAs detector plate as well as readout, amplification and A/D converter circuitry either bonded to it or processed directly onto one of the GaAs layers. The detector arrangement  802  is coupled to a central processing unit  803  for delivering the A/D-converted measurement results thereto. It would also be possible to place the amplifying and A/D-conversion circuitry at least partly into the central processing unit  803 , but placing it as near as possible to the location of the actual detection helps to eliminate noise from the measurement results.  
         [0047]    The central processing unit  803  is coupled to a memory  804  for storing digital images received from the detector arrangement as well as for reading previously stored digital images from the memory. The central processing unit has also a user interface that comprises a display  805  for displaying digital images and a keyboard  806  for receiving key commands from a human user. In order to control the process of X-ray imaging the central processing unit  803  is coupled to a high voltage source  807  that generates the voltage(s) required in the X-ray tube  801  and to an operating voltage source  808  that provides operating voltages to the detector arrangement  802 .  
         [0048]    The purpose of use of the X-ray imaging arrangement dictates among other things the physical size of the pixelized area in the detector arrangement  802 . Generally the size of the pixelized area is directly proportional to the size of the object to be studied. Monolithic detectors are in many cases the most advantageous in terms of image quality, because with a monolithic detector it is easy to obtain a highly homogenous response over the whole imaging area. Using a 6-inch or even an 8-inch Ge wafer as a starting point for building a detector would make is possible to use a monolithic direct conversion detector in a mammographic X-ray imaging application. If even larger imaging areas are required, it is possible to use an even larger monolithic wafer (disc diameters of 300 or even 450 mm have been suggested) or to tile several monolithic detector arrangements side by side. Careful calibrating is needed to ensure homogeneity of imaging response if several different detector arrangements are used. Having readout, amplification and A/D-conversion circuitry integrated on the same substrate with the actual detector (either by bonding or by directly processing) allows automatic compensation: it is possible to measure the response over a detector and to program the integrated electronics so that they automatically compensate for any possibly detected unhomogeneity in imaging response.  
         [0049]    In an ultimately simplified case the detector arrangement  802  would not need to comprise pixels at all, if at least one of the following conditions is met:  
         [0050]    only the intensity of radiation that passed through the object under study is of importance, not its spatial distribution,  
         [0051]    the size of the detector arrangement  802  is small compared to the size of the details in the object under study, so that one image obtained at a correct location is enough to reveal the information of interest,  
         [0052]    the imaging arrangement comprises means for moving at least one of the X-ray tube  801  and the detector arrangement  802  in respect of the object under study, so that a more extensive image can be obtained by scanning.  
         [0053]    [0053]FIG. 9 illustrates another imaging arrangement that is additionally meant to be used for X-ray spectroscopy, which means that the arrangement should be able to detect not only the locations of received X-ray photons on the detector plate but also their energies. The detector arrangement  901  could be similar to that used in the imaging arrangement of FIG. 8, but most probably it is advantageous to use different readout, amplification and A/D-conversion circuitry that is optimized for spectroscopy. In order to reduce the interfering effect of thermally excited electrons in the Ge layer the detector arrangement  901  is within the influence of a cooling arrangement  902 , which may include e.g. thermoelectric (Peltier) cooling and/or cooling through the use of liquified gas. The central processing unit  903  is arranged to receive both spatial and spectroscopic information from the detector arrangement  901  and to store it into a memory  904 . We assume that the imaging arrangement of FIG. 9 is built for remote-controlled operation, which means that a telemetric transceiver  905  replaces any local user interfaces. An operating voltage source  906  operates under the control of the central processing unit  903  to provide operating voltages to the detector arrangement  901 . A temperature sensor  907  is provided within the cooling arrangement  902  for providing the central processing unit with information about the temperature of the detector arrangement  901 .  
         [0054]    [0054]FIG. 10 illustrates an alternative approach to the task of providing an X-ray detector according to the invention with spatial resolution capability. The high-purity Ge layer  1001  is only shown schematically with dotted lines. One planar surface of the Ge layer  1001  comprises mutually parallel p-doped GaAs strips, of which strips  1011  and  1012  as well as a number of strips between them are shown. These strips have a certain longitudinal direction. The other planar planar surface of the Ge layer  1001  comprises mutually parallel n-doped GaAs strips, of which strips  1021  and  1022  as well as a number of strips between them are shown. These strips have also a certain longitudinal direction, which however is different than that of the strips  1011  to  1012  on the first surface. The thickness of all layers is heavily exaggerated in FIG. 10 in order to enhance graphical clarity.  
         [0055]    [0055]FIG. 11 shows how the detector structure of FIG. 10 can be used in an X-ray imaging arrangement. The GaAs strips in the “double-striped” detector arrangement  1101  are connected to readout electronics so that the strips on one side of the Ge layer are connected to one bias and readout circuit  1102  and the strips on the other side of the Ge layer are connected to another bias and readout circuit  1103  (naturally all strips could as well be connected to a common bias and readout circuit, or to common bias circuit and to common readout circuit, as long as knowledge about which signal came from which strip is maintained). Each of the bias and readout circuits  1102  and  1103  detects the hit of an X-ray photon as a transient potential swing in one of the GaAs strips connected thereto. The corresponding readout signal is taken to signal processing electronics where a correlation between signals from both sides of the detector arrangement is detected. The fact that the orientation of the GaAs strips is different on different sides of the detector means that a simultaneous signal from a certain pair of strips can only be the result of an X-ray photon hitting the detector at the intersection of those strips.  
         [0056]    An arrangement of differently oriented straight GaAs lines on different sides of the Ge layer is not the only possible geometry that can be used in the double-sided pixelizing approach. From the general field of imaging detectors also other geometries are known. It is straightforward as such to apply some known pixelizing geometry to the basic idea of having oppositely doped GaAs layers on the opposite sides of a Ge layer.  
         [0057]    Previously the advantages of bonding a readout chip onto a detector plate or even integrating the readout circuitry directly onto the semiconductor material of a detector plate were discussed. On the other hand we must remember that not placing bias and readout circuits into direct contact with the detector means that the heat dissipated in the electronic circuits does not warm up the detector. For example in the arrangement of FIG. 11 it may well be most advantageous to thermally isolate the bias and readout circuits  1102  and  1103  from the detector arrangement  1101 . All embodiments of the invention can be easily implemented so that the readout electronics are thermally isolated from the detector materials.