Abstract:
An X-ray detector is formed with a geometry in the form of a spherical polygon, including an entrance window, a grid and an anode. The spherical polygonal entrance window and the grid form a spherical polygonal drift region between them. The electric field in this region is radial and eliminates parallax broadening. A spherical polygonal amplification region between a resistive anode on an insulating support and the grid allows very high gas amplification and good protection against spark discharges. A readout electrode on the back side of the anode insulator detects induced charges and protects the readout electronics against sparks.

Description:
BACKGROUND 
   Gaseous detectors for detecting ionizing radiation are well-known.  FIG. 1  shows a schematic diagram of such a gaseous detector having a planar geometry. X-rays  122  entering the detector pass through an X-ray transparent cathode or window  112  and enter a drift region  118  located between cathode  112  and a mesh layer  124 . The drift region  118  is filled with a material that is typically a working gas, such as a quenched noble gas mixture that absorbs X-rays. When an X-ray is absorbed in the working gas, fast photoelectrons are produced along the X-ray trajectory. Secondary electrons  125  issued from the thermalization of the photoelectrons accelerate in the drift region  118  in response to an electric potential provided by voltage source  116 . The electrical potential in region  118  is selected so that it is not high enough to induce electron avalanche multiplication. After passing through the mesh  124 , the electrons enter a high field amplification region between mesh  124  and resistive anode  128 . The electric potential in this region is provided by voltage source  117 , which produces an electric potential than is higher than that produced by voltage source  116 . The electric potential in amplification region  119  is selected so that the field strength in that region is sufficient to induce electron avalanche multiplication within the working gas. 
   The electron avalanche phenomenon within the gas results in the formation of an electron cloud  126  that that is absorbed by the anode  128 . Anode  128  is typically a layer that has no defined conductive paths, but which is a reasonably homogeneous material of predetermined resistivity. As shown in  FIG. 1 , the anode is connected to ground at the edges, so the electrical energy absorbed from the electron cloud eventually dissipates. However, the anode material is resistive enough that there is a temporary accumulation of electric charge in the local region of the anode  128  upon which the electron cloud is incident. 
   Positioned adjacent to the anode  128  to the side opposite to the incoming electron cloud is a readout structure  130  which is conventionally comprised of two orthogonal serpentine delay lines strips or pixels. As the electron cloud encounters the resistive anode  128 , the deposited charge creates a capacitive coupling between the anode and the delay lines of the readout structure  130 . This capacitive coupling induces currents in certain paths of the delay lines of the readout structure  130 . These currents are detected by detection circuit  115 , and have a temporal signature indicative of the parallel paths in which they were induced. This temporal signature may be used to determine the position of the electron cloud in the detection plane. 
   Gaseous detectors of this type have a number of very attractive features for imaging ionizing radiation including a large active area, low noise and high count rate capability. However, they typically require the radiation to pass through at least a centimeter thickness of gas in drift region  118  in order to achieve good detection efficiency. The thickness of region  118 , in turn, introduces a non-desirable parallax error in the output. 
   The parallax error of a planar geometry gaseous detector, such as that shown in  FIG. 1 , is fundamentally limited by the detector geometry and the electric field in the drift region  118 . In particular, the secondary electrons  125  will drift along the electric field lines emerging from the cathode  112 . Parallax broadening occurs if the field lines do not coincide with the original X-ray photon direction. This is illustrated in  FIG. 1  where an X-ray  140  strikes the detector at an oblique angle. As mentioned above, when the X-ray is absorbed in the working gas, fast photoelectrons are produced along the X-ray trajectory. However, because the X-ray travels at an angle with respect to the electric field lines, these photoelectrons are produced at different positions along the length of the detector. Secondary electrons  145  and  155  issued from the thermalization of these photoelectrons accelerate in the drift region  118  and produce avalanches  150  and  160  in the amplification region  119 . The result is an asymmetric broadening of the diffraction spots. This undesirable effect becomes more pronounced at higher angles of incidence. To eliminate the parallax, the electric field lines along which secondary electrons move must emanate from a focal point which coincides with the position of the sample under study. 
   Some prior art attempts to overcome this problem have used a spherical conversion volume generated by a proportional wire chamber equipped with a resistive divider adapted to generate appropriate spherical equipotential surfaces within the drift space of the wire chamber. 
   Other conventional approaches use a radially symmetric change of the potential (spherical field) in the entrance window to the detector or in the cathode. For example, such a spherical field can be created by using a curved entrance window at constant potential. The problem with this approach is that the thickness of the conversion region changes considerably in the z direction, which is acceptable only for certain applications. In addition, a parallax error is still observed in these structures. 
   Replacing the parallel drift field with a radial drift field has been used in other prior art approaches. In these approaches, the parallax error is reduced, but only on a limited sensitive area of the detector, mainly near the central region. 
   In still another approach, the detector structure is based on a spherical gas electron multiplier, which serves as a transfer electrode with limited amplification to compensate for transparency losses. 
   SUMMARY 
   In accordance with the principles of the invention, an X-ray detector is formed with a geometry in the form of a spherical polygon, including an entrance window, a grid and an anode. The spherical polygonal entrance window and the grid form a spherical polygonal drift region between them. The electric field in this region is radial and eliminates parallax broadening. A spherical polygonal amplification region between a resistive anode on an insulating support and the grid allows very high gas amplification and good protection against spark discharges. A spherical readout electrode on the back side of the anode insulator detects induced charges and protects the readout electronics against sparks. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block schematic diagram of the conventional parallel plate gaseous detector for ionizing radiation. 
       FIG. 2  is a schematic cross section of an X-ray detector constructed in accordance with the principles of the current invention. 
       FIG. 3  is an enlarged section of the X-ray detector shown in  FIG. 2 . 
       FIG. 4  is a graph of field strength in a quadrant of the amplification region of the inventive detector structure. 
       FIG. 5  is an X-ray diffraction system employing the detector shown in  FIGS. 2 and 3 . 
       FIG. 6  is a cross-sectional view of an exemplary embodiment of the inventive detector showing mechanical construction details. 
   

   DETAILED DESCRIPTION 
     FIG. 2  shows a cross section of an X-ray detector  200  constructed in accordance with the principles of the present invention. The detector  200  comprises a cathode or entrance window  204  which is fabricated from a material that is substantially transparent to X-rays, such as beryllium, in the shape of a spherical polygon. A grid  206 , also in the shape of a spherical polygon, is positioned concentrically with the entrance window  204 . The entrance window  204  and the grid  206  form a drift region  210  in the form of the spherical polygon. A potential applied to the entrance window  204  causes the electric field lines at the entrance window (not shown in  FIG. 2 ) to radially point to a sample in a sample holder at position  202 . The acceleration region  212  is defined by the grid  206  and the readout structure  208  also in the shape of a spherical polygon. The readout structure  208  is shown in more detail in  FIG. 3  and discussed below. The spherical polygonal shape of the detector can cover a large spherical angular range 20 from 7 to ˜27 (180°-˜360°) which is ideal for diffraction pattern measurements. 
   When an X-ray is absorbed in the spherical drift region  210  of the detector a fast photoelectron is produced. In the energy range of interest, this fast electron travels perpendicularly the radial field while producing secondary ionization in the form of electron-positive ion pairs. The range of this fast electron sets the physical limit of the spatial resolution of the detector, which is typically of order several tens of microns at high pressure. This parallax free structure can have large drift volume and be used therefore for a good efficiency even at high x-ray energies. 
     FIG. 3  shows an enlarged view of a portion  300  of the anode and electrical readout structure. In the Figure, elements that correspond to those in  FIG. 2  have been given corresponding numeral designations. For example, entrance window  204  in  FIG. 2  corresponds to entrance window  304  in  FIG. 3 . The entrance window  303  and the grid  306  form a drift region  310  as discussed above. An acceleration region  312  is formed between the grid  306  and a readout structure  308 . The readout structure comprises a resistive anode  314  formed on an insulating substrate  316 , for example, a ceramic substrate. A readout electrode structure  318  is formed on the opposite side of the insulating substrate  316 . 
   The anode  314  is a resistive layer that has no defined conductive paths, but which is a reasonably homogeneous material of predetermined resistivity. The anode is connected to ground at the edges, so that electrical energy absorbed from the electron avalanche cloud eventually dissipates. However, the anode material is resistive enough that there is a time delay for the dissipation. That is, there is a temporary accumulation of electric charge in the local region of the anode  314  upon which an electron avalanche cloud is incident. Positioned on the opposite side of the insulating substrate  316  that supports the resistive anode  314  is a readout electrode structure  318  which comprises two orthogonal serpentine delay lines. As the electron avalanche cloud encounters the resistive anode  314 , the deposited charge creates a capacitive coupling between the anode and the delay lines of the readout electrode structure  318  through the insulating substrate  316 . This capacitive coupling induces currents in certain paths of the delay lines of the readout structure  318 . These currents are detected by a detection circuit (not shown in  FIG. 3 ) and have a temporal signature indicative of the parallel paths in which they were induced. Thus, the capacitively-induced charges may be used to determine the position of the electron cloud in the detection sphere. The readout arrangement is described in more detail in U.S. Pat. No. 6,340,819, which is incorporated in its entirety by reference. 
   The inventive spherical configuration offers several unique improvements over conventional designs. In particular, for a conventional parallel plate gaseous detector, such as that shown in  FIG. 1 , the maximal and the average electric field strength (E) in the amplification region are equal and defined by the potential difference divided by the electrode separation: 
   
     
       
         
           
             E 
             MAX 
           
           = 
           
             V 
             x 
           
         
       
     
   
   where V is the voltage applied the amplification region and x is the distance between the grid and the anode. The electric field strength is uniform in the gap between the grid and the anode. A so-called field enhancement factor can be defined as the maximum electric field divided by the average electric field. A field enhancement factor of 1.0 therefore represents no enhancement over the average field. 
   In the inventive structure, for two concentric spherical electrodes, the maximal electric field is given by the following equation: 
   
     
       
         
           
             E 
             MAX 
           
           = 
           
             
               V 
               
                 ( 
                 
                   b 
                   - 
                   a 
                 
                 ) 
               
             
             ⁢ 
             
               b 
               a 
             
           
         
       
     
   
   where V is the voltage applied between the spherical electrodes, a is the radius of the inner spherical electrode and b is the radius of the outer spherical electrode. 
   In the case of a spherical amplification structure, like that provided in the inventive structure, the inner spherical electrode corresponds to the grid and the outer spherical electrode corresponds to the anode. This structure provides a field enhancement factor (b/a) which is larger than 1.0.  FIG. 4  shows a plot  400  of the field strength in one quadrant of a spherical amplification structure. The lines, such as line  402 , represent equipotential surfaces and the spacing of the lines indicates field strength. As shown in the figure, field strength increases from the resistive anode  308  towards the grid  304 . 
   This field enhancement offers several advantages. For example, unlike the conventional parallel plate avalanche counter where there is no field enhancement, the inventive detector can be operated at a smaller voltage compared to the conventional design, thereby increasing the detector stability. 
   Further, the amplitude of the electric field increases in the direction toward the grid. This non-uniformity of the field inhibits the formation of electrical streamers, and therefore spark propagation, from the anode. 
   In addition, due to a higher field strength close to the grid, the majority of the electron avalanche multiplication takes place near the grid. Therefore, most of the avalanche signal is created close to the grid surface, which reduces the rise-time of the electronic signal. 
   Another benefit is that most of the positive ions that are created during the secondary ionization are created close to the grid to which they are attracted and cleared by collision with the grid. Consequently, they do not have to drift across the entire acceleration region and are cleared more rapidly. Therefore, the so-called “space-charge effect” is less pronounced than in conventional detectors and the local count-rate capability of the inventive detector is increased. 
     FIG. 5  shows a typical laboratory X-ray diffraction system  500  for performing single crystal diffraction experiments. The system  500  includes an X-ray source  502  that produces a primary X-ray beam  504  with the required radiation energy, focal spot size and intensity. X-ray optics  506  are provided to condition the primary X-ray beam  504  to a conditioned, or incident, beam  508  with the required wavelength, beam focus size, beam profile and divergence. A goniometer and sample holder  510  is used to establish and manipulate geometric relationships between the incident X-ray beam  508 , the crystal sample  512  and the X-ray detector  514 . The incident X-ray beam  508  strikes the crystal sample  512  and produces scattered X-rays  516  which are recorded in the detector  514 . The detector  514  may be constructed as a spherical polygon as described above with the sample  512  located at the center or origin of the sphere. 
   The system may further include a sample alignment and monitor assembly that comprises a sample illuminator  518 , typically a laser, which illuminates the sample  512  and a sample monitor  520 , typically a video camera, which generates a video image of the sample to assist users in positioning the sample in the instrument center and monitoring the sample state and position. 
     FIG. 6  is a cross-section through the center of an illustrative embodiment of the inventive detector showing mechanical construction details. As with  FIG. 3 , elements that correspond to those in  FIGS. 2 and 3  have been given corresponding numeral designations. For example, entrance window  604  in  FIG. 6  corresponds to entrance windows  204  and  304  in  FIGS. 2 and 3 , respectively. The detector elements are mounted in a metal housing  630  which has circular recesses  632  and  634  that hold the various components. For example, recess  634  holds the entrance window  604 . The entrance window  604  is mounted in recess  634  with an insulating material  636 , which may, for example, be an epoxy compound. Similarly, the grid  606  and the readout structure are mounted in recess  632  by means of insulating material  638 . The entire housing  630  is filled with a working gas mixture as discussed above. 
   While the invention has been shown and described with reference to a number of embodiments thereof, it will be recognized by those skilled in the art that various changes in form and detail may be made herein without departing from the spirit and scope of the invention as defined by the appended claims.