Abstract:
A high efficiency radiation detector employs longitudinally extending converter elements receiving longitudinally propagating radiation to produce high-energetic electrons received by detector structures in interstitial spaces. The secondary electron generation in this architecture allows great freedom in selection of converter materials and thickness. A variety of detector mechanisms may be used including ionization-type detectors or scintillation-type detector.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is based on provisional application 60/299,097 filed Jun. 18, 2001, and PCT application PTC/US/02/19154 filed Jun. 17, 2002 and entitled “Radiation Detector with Laterally Acting Converters” and claims the benefit thereof. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not Applicable. 
   BACKGROUND OF THE INVENTION 
   Highly efficient photon detectors play a major role in countless applications in physics, nuclear engineering and medical physics. In nuclear engineering, radioactive waste can be characterized with photon detectors using nondestructive assay techniques (PNDA). In medical physics, photon detectors are extensively used for diagnostic x-ray and CT imaging, nuclear medicine, and quite recently, radiation therapy of cancer. 
   In radiation therapy of cancer, ever more accurate delivery techniques spur the need for efficient detectors for million electron volt (MeV) photons in order to allow the imaging of the patient during radiation delivery. In particular, in Tomotherapy, a detector for MeV photons can be used for both the CT imaging and for verifying the dose received by the patients. 
   Referring now to  FIG. 1 , an ionization detector  10  may be used for the detection of radiation in the thousand electron-volt (KeV) range such as is used in conventional diagnostic x-ray and CT. The ionization detector  10  employs a set of conductive laminae  12  oriented generally along an axis  14  of the propagating radiation. The lamina  12  may be spaced apart along a transverse axis generally parallel to the radiation axis  14  in parallel configuration defining between them detector volumes  16 . The detector volumes  16  may be filled with a gas having a high atomic number, such as xenon, which may be further pressurized to increase the density of xenon atoms within the detector volume  16 . 
   An incident KeV x-ray  18  entering the detector volume  16  will have a high probability of colliding with a xenon atom (not shown) to create one or more secondary electrons  20  within the detector volume  16 . These electrons  20  produce negatively and positively charged ions within the detector volume  16 . The height of the detector volume  16  along the radiation axis  14  may be adjusted so that substantially all KeV x-rays  18  entering the detector volume  16  will experience one such collision. 
   Opposite laminae  12  surrounding the given detector volume  16  are biased with a voltage source  21  causing the migration of the ionization charge to the oriented lamina  12 . The current generated by such electron flow is measured by a sensitive ammeter circuit  22 , providing an indirect measure of the amount of incident KeV radiation  18 . 
   The laminae  12  thus first serve as collector plates for the ionization detector  10 . They also serve to block oblique KeV radiation  18 ′ scattered by the intervening patient from being imaged thus improving the sharpness and clarity of the image. The laminae  12  further serve to prevent migration of the electrons  20  between detector volumes  16  such as would produce cross talk further blurring the image. The laminae  12  are optimized in thickness in the transverse direction consistent with these roles. 
   The ionization detector  10  of  FIG. 1  would not be expected to be efficient for MeV x-rays which would be expected to pass fully through any practical thickness of xenon, generating relatively few electrons. 
   Referring now to  FIGS. 2   a  and  2   b , more efficient detection of MeV x-rays  24  may be accomplished by the use of a converter plate  26  which converts the MeV x-rays into high-energetic charged particles which are subsequently recorded electronically or photonically. In a first embodiment of  FIG. 2   a , a detector  25  uses a converter plate  26  that is an opaque, high density, high atomic number material, such as lead, placed above detector media  28  to convert each photon of MeV x-rays  24  into multiple electrons  20 . The detector media  28  may be film, an ionization-type detector  10 , a scintillation detector or other well-known detector types. 
   A high atomic number and/or high-density material is preferred for the converter plate  26  because it has a high cross-section for the interaction of high-energy photons. Generally, however, the height  30  of the converter plate  26  is limited to less than that required to filly absorb the MeV x-rays  24  correspondingly limiting the conversion efficiency of the detector  25 . The reason for this is that increasing the height  30  to provide for more absorption of MeV x-rays becomes fruitless as additional ejected electrons are balanced by increased absorption of electrons within the converter plate  26  itself. 
   Referring to  FIG. 2   b , the limitation imposed by the converter plate  26  of detector  25  of  FIG. 2   a , may be overcome by using a transparent scintillating converter plate  26 ′ as shown in  FIG. 2   b . Here the MeV x-rays  24  striking the scintillating converter plate  26 ′ produce photons  34  which pass through the transparent scintillating converter plate  26 ′ to be received by light detector  36 . The transparent scintillating converter plate  26 ′ may be made thick enough to block a greater proportion of the MeV x-rays  24  because the mobility of photons within the transparent scintillating converter plate  26 ′ is proportionally much greater than the mobility of electrons within the solid converter plate  26 . Transverse movement of the photons within the transparent scintillating converter plate  26 ′ may be blocked by opaque elements  38  which may, for example, be slices cut into the material of transparent scintillating converter plate  26 ′ and filled with a light and x-ray blocking material so as to define regular detection areas. 
   Ideally the scintillating material will have a relatively high atomic number and great transparency. Unfortunately, the manufacture of transparent scintillating converter plate  26 ′ using such high quality scintillators is significantly more expensive than the manufacture of conventional converter plate  26  shown in  FIG. 2   a  and the efficiencies of such radiation detectors remain modest. 
   What is needed is a relatively simple, inexpensive, and high efficiency radiation detector suitable for high-energy radiation. 
   BRIEF SUMMARY OF THE INVENTION 
   The present inventors have recognized that the height limitation of the converter plate, such that avoids reabsorption of electrons, may be overcome by breaking the converter plate into a plurality of axially extending converter elements. High-energetic electrons and, depending on the energy of the incident radiation, other positive and negative charge carriers, exit the converter material into the detector volumes placed between the converter elements. Converter elements may now be of arbitrary height in the longitudinal direction with electrons generated both at the top of the converter and the bottom of converter likewise liberated only a short distance, through the converter element into the detector. In this way, the problem of electrons being retained by the converter as it increases in height is substantially eliminated and converter height sufficient to convert substantially all MeV x-rays can be contemplated. 
   Specifically then, the present invention provides a radiation detector providing a plurality of converter laminae oriented to extend substantially longitudinally along the propagation axis of the radiation and spaced transversely across the axis to define a plurality of axially extending detector volumes. Laminae receive radiation longitudinally and liberate electrons into the detector volumes. Detector structure for detecting electrons liberated into the detector volumes provides substantially independent signals. 
   Thus it is one object to provide a new detector geometry that uses relatively inexpensive converter materials to provide extremely high converter efficiencies. The longitudinal thickness of the converter material is no longer limited and may be adjusted to provide for absorption of a substantially greater proportion of the radiation. 
   The detection structure may be a scintillator within the detector volume optically coupled to a photodetector or may be an ionizing gas or other material coupled to a collecting electrode assembly, the latter of which may, in part, be the laminae. 
   Thus it is another object of the invention to provide a new detector geometry suitable for use with a number of detecting mechanisms. 
   The laminae may be substantially parallel plates or may be tubes with coaxial wires where the detector volumes are the spaces between the tubes and the wires. 
   Thus it is another object of the invention to provide for the improved detector structure offering one-dimensional, two dimensional/areal or even fully general three-dimensional detector versions. 
   The tubes may contain a coaxial wire and the detector volume may be the space between the tube and wire, which are used as part of an ionization chamber. Or the tube may be filled with a scintillating material. 
   Thus it is another object of the invention to provide for either an areal scintillation or areal ionization-type detector. It another object of the invention to allow the use of relatively low quality scintillation materials, for example, those having low atomic number to produce a high efficiency detection device. 
   The longitudinal length of the laminae may be sized to substantially block the radiation and the transverse width of the laminae may be less than the average propagation distance of an electron in the material of the laminae. 
   Thus it is another object of the invention to provide for a detector assembly suitable for use with a wide range of radiation energies and converter materials. 
   The laminae may be tipped with respect to the radiation axis so as to increase the area of the detector over which radiation is intercepted by a lamina 
   Thus it is another object of the invention to provide the benefits described above while increasing the efficiency of the detector by improving the capture of radiation by laminae. 
   The laminae may be aligned with lines of radius extending from a detector focal point and the radiation source may be positioned so that the radiation emanates from a point displaced from the focal point. This displacement would allow to easily place the detector into the radiation beam without causing the detector signals to be highly sensitive to the exact position of the detector with respect to the radiation source. 
   It is yet another object of the invention to allow for the use of off-the-shelf KeV x-ray detectors for MeV detection. Defocusing the detector increases the interception of radiation by a lamina changing the mechanism of the detector from a standard ionization detector to a converter/ionization detector of the present invention. 
   The foregoing objects and advantages may not apply to all embodiments of the inventions and are not intended to define the scope of the invention, for which purpose claims are provided. In the following description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment also does not define the scope of the invention and reference must be made therefore to the claims for this purpose. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view of a prior art ionization detector for KeV x-rays taken along a plane of radiation propagation, as has been described above in the background of the invention; 
       FIGS. 2   a  and  2   b  are cross-sectional views similar to that of  FIG. 1  but of prior art ionization detectors for MeV x-rays having single transverse converter elements as have also been described above in the background of the invention; 
       FIG. 3  is a cross-sectional view of a detector of the present invention having multiple longitudinal converter elements generating high-energetic electrons exiting the converter media producing ionization charges that may be collected in an ionization-type detector; 
       FIG. 4  is a cross-sectional view of one embodiment of the detector assembly of  FIG. 3  positioned with respect to a radiation source and presenting longitudinal but tipped converter elements so as to increase the area of the radiation beam intercepted by the converter elements; 
       FIG. 5  is a detailed view of  FIG. 4  showing the path of adjacent x-rays, both of which are intercepted by tipped converter elements; 
       FIG. 6  is a simplified schematic view of two converter elements showing important dimensions for the converter elements such as depend on the material of the converter elements and their application; 
       FIG. 7  is a figure similar to that of  FIG. 4  showing a conventional CT-type KeV ionization detector modified for use with MeV x-rays by movement of the focal point of radiation such as causes ionization by high-energetic electrons exiting the converter in preference to the intended ionization by direct radiation; 
       FIG. 8  is a plot of detector efficiency as a function of angle along the detector of  FIG. 7  showing a drop off of efficiency toward the center of the detector in which the detector veins are tipped less with respect to the incident radiation; 
       FIG. 9  is fragmentary perspective view of an embodiment of the present invention for providing an area detector composed of tubes with concentric wire conductors as the converter elements; 
       FIG. 10  is a cross-sectional view through the tube and wire construction of  FIG. 9  showing a further embodiment where the gaseous ionization medium is replaced with a solid state semiconductor material; and 
       FIG. 11  is a figure similar to that of  FIG. 10  showing a further embodiment where the center wire conductor of the tube is replaced with a scintillating material to transmit light to a photo-detecting device. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to  FIG. 3 , a detector  40  of the present invention provides for a series of longitudinally extending converter elements  42  aligned generally with the local radiation axis  14  of radiation propagation. The converter elements  42  may be, for example, planar vanes or may be rods or other shapes. 
   Converter elements  42  are separated from each other in a direction transverse to the radiation axis  14  to create interconverter volumes  44  such as may be filled with an ionizing medium such as a gas including, for example, xenon. The gas may be compressed in a housing (not shown) so as to increase the odds of electron-gas interaction in the interconverter volumes  44 . 
   MeV x-rays  24  received by the detector  40  strike the converter elements  42  to produce high-energetic electrons  46  which proceed into the interconverter volumes  44 . The electrons ionize the gas in the interconverter volumes  44 . Some MeV x-rays  24 ′ will pass completely through interconverter volumes  44  without contacting the converter elements  42  and may produce some ionization. However, in the invention, this ionization will be less than the ionization caused by high-energetic electrons  46  exiting the converter. 
   Adjacent converter elements  42  may be given voltages of opposite polarity so as to provide a biasing field collecting the ionization charges whose flow may be measured using current detector circuitry well known in the art ionization detectors. 
   In this embodiment, the material of the converter element  42  is preferably a conductive metal so as to support the current flows of the ionization, however, the function of collecting charge may be separated from the function of converting x-rays to electrons and non-metallic converter elements having a conductive coating are also possible. Similarly, in this embodiment, the converter elements  42  are preferably composed of a high atomic number and/or high-density material so as to reduce their height and so as to provide efficient reduction of scattered x-rays like the laminae  12  described with respect to  FIG. 1 . Nevertheless, it will be recognized that a variety of different materials may be used depending on manufacturing convenience, the energy of the radiation, and the desire for compactness. 
   Referring now to  FIG. 4 , a detector array  50  may be created by arranging a number of converter elements  42  along an arc of constant radius about a focal spot  52 . A radiation source is placed at the focal spot  52  to as to create a fan beam of radiation whose local radiation axes  14  are lines of radius from the focal spot  52  to the detector array  50 . The converter elements  42  extending generally longitudinally with respect to the local radiation axis  14  but are also slightly tipped with respect to the local radiation axis  14 . Referring also to  FIG. 5 , this tipping of the converter elements  42  increases the area over which the radiation beam, for example, MeV x-rays  24 ′ will strike a converter element  42  and not pass unintercepted through an interconverter volume  44 . Preferably, the tipping will be equal to the width of the converter element  42  in the transverse direction over the height of the converter element in the longitudinal direction. However, more or less tipping may also be used, including none as will be described below. When the converter elements  42  are tipped, the height and width of the converter elements  42  may be adjusted to ensure that a path length  56  of MeV x-rays  24 ′ through the converter element  42  is sufficient to ensure probable absorption of the MeV x-rays  24 ′. 
   The slopped sides of the converter elements  42  such as produced by the tipping as shown in  FIG. 4  need not be monotonic but adjacent converter elements  42  may alternatively have, for example, interdigitating projections so as to preserve an interconverter volume  44  but to expose no direct through path between converter elements  42 . 
   Referring to  FIG. 6 , the preferred dimensions of the converter elements  42  will depend on the radiation energy, the material of the converter elements  42  and the desired resolution of the detector. Generally the centerline spacing  55  of the converter elements  42  will be determined by the spatial resolution desired in the resultant detector. The width  54  of the converter elements  42  will depend on their material and a tradeoff between the spacing  55  between converter elements  42  which determines the width  57  of detector material and the width  54  of the converter elements  42  which determine the amount of conversion, both which relate to conversion efficiency. Potentially the thickness of the converter element  42  may be quite small making use of breakthroughs in the production of so-called nano-wires of extremely small diameter. 
   Referring now to  FIG. 7 , a conventional CT ionization-type KeV detector  58  such as one manufactured by the General Electric Company for its KeV CT machines may be applied for use with MeV x-rays using the present invention&#39;s mechanism of generating electrons using the laminae of the detector as converter elements  42 . Absent recognition of the conversion properties of the laminae, use of such a detector for MeV radiation would be counter intuitive because of the expected low interaction of MeV radiation with the inter-laminae gas. This particular detector  58  provides in effect an array of 50, 738 converter elements  42  formed from the tungsten laminae. Up to 500-volt potential may be applied across adjacent converter elements  42  in an alternating configuration. For a fan beam detector, the height of the detector may be 3.56 cm and the detector may be 44 cm long to measure a six MeV beam. 
   Improved sensitivity may be provided by defocusing the detector  58 . As shown in  FIG. 7 , an actual focal point  60  is defined by the orientation of the laminae  12  such as divided the ionization chamber into detector volumes  16 . Focal point  60  maybe displaced typically inward by a predetermined amount  61  from the focal spot  52  of the MeV x-rays thus causing the x-rays from focal spot  52  to strike the laminae  12  at an angle increasing the absorption of radiation and their liberation of electrons. For example, the detector  58  may have a focal point of 103.6 cm and be placed 141 cm away from focal spot  52 . 
   Referring to  FIG. 8 , the centermost lamina  12  in region  64 , which despite this displacement are essentially aligned with radiation from the focal spot  52 , exhibit a decreased sensitivity in comparison with those off center lamina in regions  66  which are receiving radiation directed against their sides as well as their ends. Edge most laminae  12  in regions  68  exhibit decreased sensitivity because of shadowing caused by adjacent laminae  12 . 
   Referring now to  FIG. 9 , an areal detector  70  may be constructed along the principals described above, by using a set of longitudinally aligned tubes  72  having coaxial wires  74 . Here the interconverter volumes  44  are those spaces between the walls of the tubes  72  and the wires  74 . Inter-tube regions  75  do not serve for detection in this embodiment but are relatively minor in area. 
   In this embodiment, the coaxial wires  74  may be given a positive charge to collect negative charge carriers formed by ionization of gas held in the interconverter volumes  44  between the wires  74  and the walls of the tubes  72  or vice versa. Here both tubes  72  and wires  74  provide for conversion properties projecting liberated electrons for detection. It will be understood that the tubes  72  may be packed to define an arbitrary area and that each tube  72  and coaxial wire  74  defines a detector element. 
   Referring to  FIG. 10 , in an alternative embodiment, the space between the wire  74  and tube  72  (converter materials) may be filled with a semi-conductor material such as amorphous selenium  76  (detector material) so as to produce hole-electron pairs which may be collected by the electrodes formed by the wire  74  and tube  72 . 
   Referring now to  FIG. 11 , in yet a further embodiment, the wire  74  may be dispensed with and the tube  72  filled with a scintillator material  80  receiving the liberated electrons  46  and emitting a photon  82  for detection by a solid-state photo detector  84 . The use of the structure of tubes  72  limits the necessity that the scintillator material  80  have significant conversion properties (of converting radiation to photons) or be highly transparent (as its height may be limited by proper choice of the converter materials of the tube  72 ). This allows lower cost scintillating material to be used. It will be understood from the above description, that the above described invention employing a generating and liberating electron mechanism may be used for KeV or lower energy radiation including visible light. Generally, the dimensions of the detector structures are fully scalable with the energy of the incident radiation. Higher energy of the incident radiation translates to larger detector structures (converter material and detection material), and lower energy of the incident radiation translates to smaller detector structures. As used herein, converter materials are the materials that covert radiation photons to electrons and detector materials are materials that are used in the detection of the electrons (e.g. ionizable gasses or semiconductors). The lower limit of scalability is only determined by atomic dimensions. Thus, the converter material can be of a nanometer scale (nanostructure), e.g., having dimensions (for example the width of the converter elements) less than 100 nanometers. 
   The longitudinal converter mechanism also has potential application in the field of radiation sensitive films where converter structures, possibly in the form of freely dispersed filaments or aligned filament structures using electrostatic techniques and the like, may be embedded in the emulsion of the film itself with liberated electrons interacting with the silver compounds of the emulsion to produce a higher sensitivity in the film than that which would normally be provided by the film alone. 
   It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but that modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments also be included as come within the scope of the following claims. For example, the use of semiconductor detectors or scintillation detectors could be used with the embodiment of  FIG. 4 .