Patent Publication Number: US-6340819-B1

Title: Readout structure and technique for electron cloud avalanche detectors

Description:
FIELD OF THE INVENTION 
     The invention relates generally to the field of electromagnetic signal detection and, more particularly, to signal detection using photon-counting detectors. 
     BACKGROUND OF THE INVENTION 
     Photon-counting or particle-counting detectors are used extensively for science, industry and medicine. One example of such a detector is a gas avalanche detector. Recently, a number of new gas avalanche detectors based on parallel grid geometries have been developed. These new designs offer very high counting rate capability as compared to conventional Multiwire Proportional Counters (MWPC). They also offer higher gain, and superior stability and robustness as compared to Microstrip Gas Counters (MSGC). Indeed, this type of detector, when using a 100-micron gap, has demonstrated counting rates on the order of 10 9  counts/mm 2 -sec, nearly a million times faster than a conventional MWPC. 
     One type of parallel grid detector uses an arrangement as shown in FIG. 1, which is a schematic side view of a prior art photon counting detector  10 . The detector is configured for use in detecting high energy particles or photons. For example, initial energy component  22  might be an x-ray used in an analysis technique such as x-ray diffraction. A cathode  12  of the detector is a conductive material that is transparent to the energy  22 . In this particular detector, a photocathode layer  23  is located on the side of the cathode  12  away from the initial direction of the x-ray. As the x-ray energy passes through the cathode and encounters the photocathode material, it is converted from x-ray energy to a small number of electrons. 
     Located opposite cathode  12  is an anode  14 . The anode is also conductive and is used for collecting electrons that originate at the cathode. One type of anode structure includes two orthogonal serpentine delay lines, as is discussed in more detail below. A voltage differential on the plates  12 ,  14  is provided by voltage sources  16 ,  17  and is typically in the range of 0.5-5 kV, the specific amount depending on the desired gain. Often, a conductive mesh  24  is placed between the cathode  12  and the anode  14 . Typically, the mesh is a simple cross-hatch of conductive material, although other structures may also be used. The mesh is electrically returned to the voltage source  16 , such that a circuit path is defined between the mesh  24  and the cathode  12 . Thus, two different voltage differentials are defined by the structure, one across the space  18  between the cathode  12  and the mesh  24 , and one across the gap between the mesh  24  and the anode  14 . In this example, an electric potential is used in the region  18  that is lower than would be required to cause an avalanche multiplication of the electrons generated at the photocathode layer  23 . In contrast, the region between the mesh  24  and the anode has a higher electric potential, which is sufficient to induce avalanche electron multiplication. 
     In the space  19  located between the anode  14  and the mesh  24  is an active gas material that, in the presence of the electric field generated by the voltage source  17 , responds to the introduction of electrons that travel from the photocathode layer  23 . With this electric field applied, the electrons from the cathode  12  will induce an avalanche secondary electron multiplication within the gas. An example of an electron multiplication within the detector  10  is given by the graphic depiction of the path  25  of an incident x-ray photon, and the ensuing electron multiplication. As shown, multiple secondary electrons are generated as the initial electron encounters the active gas. These secondary electrons themselves cause the generation of more secondary electrons, and the amplification process continues. 
     The use of a parallel grid detector allows detection of the electron cloud that results from the avalanche multiplication. For example, as is known in the art, two overlapping serpentine delay lines positioned orthogonal to each other provide a means by which the electron cloud may be located in a two-dimensional detection plane. The overlapping delay lines form a detection grid, the resolution of which is determined by the spacing between the lines, i.e., the “anode pitch.” As demonstrated in FIG. 2, limits on the anode pitch directly limit the sensitivity of the detector. 
     FIG. 2 is a schematic view of one serpentine delay line  20  that provides spatial information in one of the two dimensions of the detection grid. It will be understood that the figure is not necessarily to scale, but is intended for instructional purposes only. For each of the parallel portions of the delay line upon which an electron cloud is incident, a signal is generated that is uniquely identifiable relative to that lateral position. Since a second delay line (not shown) has parallel paths that run perpendicular to the parallel paths of the first delay line, signals on these paths provide information relative to the position of the electron cloud in the perpendicular lateral direction of the detection plane. The signals from the two delay lines are detected using a detection circuit  15  (FIG.  1 ), and are used to determine the region of the detection plane that encounters the electron cloud. 
     In FIG. 2, regions impacted by two different electron clouds, labeled “A” and “B,” are represented by circles overlapping the delay line  20 . Each of these electron clouds generates detectable signals in the delay line. As shown, electron cloud A overlaps three of the parallel paths of the delay line, thereby generating three different signals at different time delays, and therefore at different determinable spatial positions in a first lateral dimension. However, electron cloud B overlaps only one of the delay line paths. With electron cloud A of FIG. 2, several signals in each of the two dimensions of the detection plane provide sufficient spatial information to calculate a centroid with a resolution more accurate than the anode pitch. However, spatial information provided by electron cloud B is limited by the fact that it overlaps only one delay line path. Thus, it is apparent that the resolution of a detector of this type for relatively small electron clouds will be limited to the anode pitch. 
     One way to increase the resolution of a delay line detector would be to narrow the pitch between the parallel paths. However, this necessarily increases the length of the delay lines as well which, in turn, significantly increases the signal attenuation. Alternatively, the gap  19  (FIG. 1) between the anode  14  and the mesh  24  can be increased to create a larger drift region within which the electron cloud can expand. However, electron reattachment can occur in this region, the extent of which depends on the gas molecules that are present. Thus, the demand on gas purity in region  19  would be greatly increased, which can be a significant concern for sealed-tube designs that are prone to outgassing over the long term. Moreover, the spacing of the region  19  determines not only the lateral diffusion of an electron cloud, but the longitudinal diffusion as well (i.e., diffusion in the direction perpendicular to the detection plane). More longitudinal diffusion degrades the time resolution of the detector, which can limit the counting rate and, for delay line readouts, degrades the spatial resolution. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a detection apparatus for detecting an electron cloud in two dimensions includes a resistive layer with a detection plane upon which the electron cloud is incident. The resistive layer is capacitively coupled to a readout apparatus such that interaction of the electron cloud with the resistive layer induces charge in the readout apparatus. The readout apparatus identifies the locations of the charge in a plane that is parallel to the detection plane, and thereby provides an indication of the two dimensional distribution of the electron cloud. 
     The detection apparatus is preferably part of a parallel grid detector, in which a high-energy photon or particle is amplified using electron avalanche multiplication. In a preferred embodiment, the photon or particle is converted to electrons, which are then accelerated toward an avalanche region. Within the avalanche region, an active secondary electron-emitting material is located and is encountered by the electrons. An acceleration field maintained in the avalanche region is high enough to induce the avalanche of secondary electrons that result in the electron cloud. 
     In a preferred embodiment, the readout apparatus has a conductive grid, which may consist of two orthogonal serpentine delay lines. Spacing between the resistive layer and the readout apparatus may be selected with regard to the grid. For example, for a given charge, the width of the charge distribution on the readout apparatus is matched to a pitch between conductive segments of the grid. Furthermore, in the preferred embodiment, the resistivity of the layer is used to control the rate of charge dissipation on the anode layer. In particular, the resistivity of the resistive layer is selected relative to the thickness of the anode and the bandwidth of the readout electronics used. The resistivity is selected to be low enough to support the highest bandwidth (i.e., counting rate) of the detector electronics, while still being high enough that the charge can penetrate through the anode layer to the readout plane. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic cross sectional side view of a prior art parallel grid detector. 
     FIG. 2 is a schematic top view of a prior art serpentine delay line used with parallel grid detectors. 
     FIG. 3 is a schematic cross sectional side view of a parallel grid detector according to the present invention. 
     FIG. 4 is a graphical view of the time evolution of a peak charge density for one embodiment of the present invention given different anode material parameters. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Shown in FIG. 3 is a detector according to a preferred embodiment of the invention. A number of the features of this detector are the similar to the prior art detector of FIG. 1, and the same reference numerals have been used for those elements that are the same in both figures. In this embodiment, as in FIG. 1, the electrons are generated in a photocathode layer  23  from the high-energy x-rays or particles  22  incident upon it. In a preferred embodiment, the layer is used for converting high energy x-rays and is a porous layer of cesium iodide, although other photocathode materials may be used as well. A drift region  18  is located between cathode  12  and mesh layer  24 , and accelerates the electrons toward the mesh  24  via an electric potential provided by voltage source  16 . This voltage source is not high enough to induce avalanche multiplication. 
     After passing through the mesh  24 , electrons generated in the photocathode layer  23  enter the high field region between mesh  24  and resistive anode  28 . The field strength in this layer is provided by voltage source  17 , which produces a voltage potential than is higher than that produced by voltage source  16 , and that provides region  19  with a field strength sufficient to induce electron avalanche multiplication in the presence of an active material. Those skilled in the art will recognize that the voltage sources  16 ,  17  are for descriptive purposes, and that the desired voltage potentials may be provided in any of a number of known ways. 
     In the preferred embodiment, the active material in the region  19  is a gas such as a quenched noble gas mixture, although other secondary electron-emitting materials may be used as well. The avalanche phenomenon within the gas results in the formation of an electron cloud that that is absorbed the anode  28 . The anode  28  is a layer that has no defined conductive paths, but which is a reasonably homogeneous material of predetermined resistivity. As shown in FIG. 3, 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 reasonably long time delay for the dissipation. That is, there is a temporary accumulation of electric charge in the local region of the anode  28  upon which the electron cloud is incident. 
     Positioned adjacent to the anode  28  to the side of it away from the incoming electron cloud is a readout structure  30 . The readout structure is similar to the anode  14  of the prior art detector shown in FIG. 1 in that it has two orthogonal serpentine delay lines. As the electron cloud encounters the resistive anode  28 , the deposited charge creates a capacitive coupling between the anode and the delay lines of the readout structure  30 . This capacitive coupling with the delay lines has a similar effect as the direct coupling between the electron cloud and the delay lines of the structure of FIG.  1 . That is, the capacitive coupling induces currents in certain paths of the delay lines of the readout structure  30 . These currents are detected by detection circuit  15 , and have a temporal signature indicative of the parallel paths in which they were induced. Thus, as in prior art delay line detectors, the capacitively-induced charges may be used to determine the position of the electron cloud in the detection plane. 
     The charge induced on the surface of the readout structure in the embodiment of FIG. 3 is given by the following:            σ   SP          (     x   ,   y     )       =         -   ɛ                   E     =     ɛ                 dk        ∫          A              σ   RA          (       x   ′     ,     y   ′       )           [         (       x   ′     -   x     )     2     +       (       y   ′     -   y     )     2     +     d   2       ]       3   /   2                   x   ′                 y   ′                               
     where σ RA  is the charge density on or near the resistive anode, σ sp  is the charge induced on the segmented readout plane, d is the separation between the top of the resistive anode and the readout plane, x and y are coordinates in the detection plane, ε is the permittivity between the anode and the readout structure and k is Coulomb&#39;s constant. When σ RA  is a point charge at x=y=0, then the induced charge is given by:            σ   SP          (     x   ,   y     )       =       ɛσ   A         [       x   2     +     y   2     +     d   2       ]       3   /   2                         
     Thus, the width of the induced charge distribution (or, more particularly, the full-width half-maximum) is on the order of the spacing between the top of the resistive anode and the readout plane. In the preferred embodiment, the spacing d is therefore selected so that this width of the charge distribution is matched to a pitch of the delay lines used. This removes the need for finely pitched delay lines. 
     The resistance of the anode  28  is made high enough that the electric field from the avalanche charge is able to penetrate through to the readout plane  30 . Therefore, for a resistive anode  28  of thickness t, the resistivity is made to exceed a predetermined level. In the preferred embodiment, the resistivity ρ (in ohm-cm) is set such that: 
     
       
         ρ&gt;3 πμ 0 f BW t 2   
       
     
     where f BW  is the frequency bandwidth of the readout electronics. For example, if the electronics have an effective analog readout bandwidth of 100 MHz, and the resistive anode has a thickness of 1 mm, the resistivity should be made greater than or equal to 0.1 ohm-cm. 
     The charge that collects on the resistive anode  28  is dissipated by diffusing laterally and is collected at the anode edge. The lateral charge diffusion of the anode layer is given by:            ∂     σ   RA         ∂   t       =       1       R   S          C   S              Δσ   RA                       
     where R s  is the surface resistivity of the resistive anode  28  in ohms/sq, and C s  is the capacitance of the anode  28  with respect to the readout plane in F/m 2 . The fastest possible readout rates are achieved by making R s C s  as small as possible without violating the resistivity limit given above. FIG. 4 shows the time evolution of the peak charge density for various values of R s C s . As shown, FIG. 4 indicates that, for a value of R s C s  on the order of 0.001 ohm-F/m 2  or less, Poisson-distributed count rates in excess of 10 7  counts/mm 2 -sec are possible. 
     An example of a resistive anode according to the preferred embodiment uses a borosilicate glass plate. The thickness of the plate is 1-3 mm, depending on the desired anode strip spacing. The plate is coated with indium-tin-oxide on both sides at a resistivity of 100-1000 ohms/sq. Other possible embodiments include thin plates of silicon carbide, doped silicon or other semiconductors. 
     An additional benefit of the present invention is a reduced occurrence of discharge or arcing. In conventional, segmented-anode parallel grid type detectors, electric flux concentrations occur at the edges of conducting strips. Discharges can occur at these flux concentrations that can potentially damage the readout electronics or, over the long term, degrade the readout anode itself. With the smooth resistive anode of the present invention, such flux concentrations do not exist, and the probability of discharges is thus significantly lower. 
     While the invention has been shown and described with reference to a preferred embodiment 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. For example, the preferred embodiment is described in terms of x-ray detection, but is equally applicable to detection of high energy particles. Furthermore, as mentioned above, material other than gases may be used as the avalanche medium. Indeed, the detector may be used to detect electron clouds that are generated in any of a number of different ways, for example, by microchannel plate electron multipliers.