Abstract:
A layer of X-ray photoconductor is sandwiched between electrodes for conversion of X-radiation to an electrical signal. The photoconductor layer is biased, and films of zinc sulfide may be used between the layer and each electrode, to minimize dark current. Arrays of solid state radiation detectors utilizing the biased photoconductive conversion layer are also disclosed.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to radiation detectors and, more particularly, to a novel solid state X-ray detectors and to arrays thereof. 
     Accurate measurement of radiation flux intensity is required in many applications. In particular, the measurement of X-ray flux intensity in computerized tomography systems, and particularly in very high resolution tomography systems for non-destructive evaluation of mechanical parts, requires a very high degree of accuracy. Increased accuracy is also beneficial in more conventional X-ray imaging systems for medical, industrial or other measurement tasks. 
     Generally, X-ray flux intensity is acurately measured by utilization of one of two physical processes; a solid state scintillator element may be utilized to convert the X-radiation to emit a luminescent intensity to be measured by an additional device, such as a photomultiplier tube or a silicon photosensitive device; or a gaseous X-radiation photoconductor, such as xenon and the like, is utilized to provide a changing induced conductance therein responsive to the degree of X-radiation ionization. Both principle forms of X-ray flux intensity measuring devices are relatively complex and it is desirable to provide radiation flux detectors of increased simplicity and capable of forming compact arrays thereof, particularly for increasing the accuracy of computerized tomography systems. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance, with the invention, a solid state radiation detector utilizes a layer of a photoconductive material characterized by a change in conductance between opposed boundaries of the layer responsive to changes in incident radiation flux. An electrode is fabricated on each of the opposed boundary surfaces, with the electrode material being selected to minimize dark current flowing through the detector when the detector photoconductor layer is biased. 
     In one preferred embodiment, wherein the photoconductive material is selenium, a film of zinc sulfide is fabricated upon each of the opposed boundary surfaces prior to fabrication of the electrodes thereon to even further reduce dark current injected across the photoconductive layer. 
     A detector array utilizes a set of parallel electrodes upon one surface of a photoconductive member, with a common bias electrode, to which a source of bias potential is connected. Each of the plurality of electrodes are individually connected to one of a plurality of output conditioning amplifiers. 
     Accordingly, it is one object of the present invention to provide a novel solid state radiation-responsive photoconduction detector. 
     It is another object to provide a novel solid state radiation detector array. 
     These and other objects of the present invention will become apparent on consideration of the following detailed description, when taken in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a novel solid state radiation detector in accordance with the principles of the present invention; and 
     FIG. 2 is a perspective view of a solid state radiation detector array utilizing the invention principles. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring initially to FIG. 1, a solid state radiation detector 10 utilizes a layer 11 having a pair of opposed layer surfaces 11a and 11b. The electrical conductance measurable between the opposed surfaces is responsive to absorption of radiation quanta. In particular, for detection of X-ray flux, layer 11 is fabricated of solid materials such as lead oxide, cadmium selenide, selenium and the like. In the presently preferred embodiment, selenium is utilized as the X-ray-responsive photoconductive material, due to the very low dark conductivity thereof. Selenium, having at atomic number of only 34, is not an optimum X-ray absorber; however, the ease of deposition, as by evaporation and the like proceses, and the previously mentioned very low dark conduction (very high dark resistance, on the order of 10 15  ohm-centimeters) overcome the less-than-optimum radiation absorption characteristics of the material. The selenium layer 11 is deposited to a thickness A of about 500 microns (20 milli-inches) upon one surface of a first electrode 12 having a thickness T 1  from about 500 Angstroms to about 6 milli-inches. Advantageously, electrode 12 is a nickel-tungsten member, although, gold, indium oxide, tin oxide, indium tin oxide, nickel-coated tungsten, aluminum and nickel may be equally as well utilized. 
     It is preferable, although not necessary, that a thin layer, having a thickness T of about 1,000 angstroms, of zinc sulfide be fabricated upon electrode 12 prior to the fabrication of the photoconductive layer 11 thereon. Similarly, it is preferable, although not necessary, that a similar layer 16 of zinc sulfide be fabricated upon the remaining surface 11b of the photoconductive layer. A second electrode 18 is fabricated either directly upon photoconductive layer surface 11b or, if film 16 of zinc sulfide is utilized, upon the surface of layer 16 furthest from photoconductive layer surface 11b. Electrode 18 is fabricated of any of the aforementioned materials, or of a metalized plastic material, to a thickness T 2  of between about 500 Angstroms to about 4 milli-inches. In one preferred embodiment, second electrode 18 is fabricated of aluminum with a nickel-coated steel electrode being used as first electrode 12. 
     The resistivity of the selenium layer is about 10 15  ohm-cm and a 0.05 cm thick layer will have a dark resistance of about 2.5×10 13  ohms. The absorption coefficient of selenium, for X-radiation at about 100keV; is about 2/cm, whereby the product of the layer thickness A and the absorption coefficient is about 0.1 for 100keV. X-ray quanta. 
     The detector is positioned to receive X-ray quanta at some small angle θ to the normal N to one end plane 11c thereof, with absorption of the X-ray quanta occurring along the length B of the detector. Preferably, length B is from about 0.35 cm. to about 1 cm., with the larger values being preferred to reduce beam-hardening tracking errors. It will be seen that the response of the detector to scattered radiation quanta is minimized by reducing the angle θ at which quanta may enter the detection layer; the angle θ is related to the ratio of layer thickness A to layer length B and is optimized if surface 11c is substantially traverse to the planes of the opposed layer surfaces 11a and 11b. The width C of the quanta-receiving detector face is about two centimeters. The conversion efficiency of the detector is on the order of 2×10 -16  Coulombs per X-ray quanta, yielding a dark current shot noise level on the order of one X-ray equivalent in a one milli-second interval. 
     A potential source 20 is coupled to first electrode 12 with a positive potential of magnitude V, with respect to ground potential. Second electrode 18 is coupled to the input 25a of an operational amplifier 25 having its output 25b coupled both to input 25a through a feedback resistance R f  and to electrical ground potential through an equivalent load resistance R 1 . The electrical output signal of the detector is proportional to the magnitude of the radiation-responsive photoconductance and is taken from operational amplifier output 25b. Typically, magnitude V of potential source 20 is on the order of 5,000 volts, while the dark resistance of the detector, as previously mentioned, is on the order of 2.5×10 13  ohms, whereby a dark current of about 0.2 nanoamperes flows through the series circuit of the detector and the equivalent input resistance Req of the amplifier. 
     In operation, reception of radiation quanta, through detector surface 11c and into the volume of layer 11, increases electrical conductivity of layer 11 between electrodes 12 and 18. The increased conductive (or decreased resistance) increases the current flow through the detector layer and causes an increase in the magnitude of the output signal from the current-measuring amplifier 25. The output signal is thus varied in amplitude responsive to the intensity of radiation flux incident on the detector. 
     Referring now to FIG. 2, a solid state detector array 30 includes an insulative substrate 35 having a plurality N of electrodes 37a-37n fabricated upon a first surface 35a thereof, with each electrode being parallel to, but spaced from, each of the other electrodes. A thin film 39 of zinc sulfide may be fabricated over all of the parallel, spaced-apart electrodes 37. A layer of 42 of radiation-responsive photoconductive material is fabricated upon a portion of insulative substrate surface 35a (or upon film 39, if used) to cover the area bounded by all of the plurality of electrodes 37. The surface of photoconductive material layer 42 furthest from substrate 35 may be covered by a thin film 44 of zinc sulfide, and a conductive first electrode 46 is fabricated upon either the zinc sulfide layer 44 or, if layer 44 is not utilized, directly upon the surface of photoconductive layer 42 furthest from the substrate. Advantageously, first electrode 46 may extend across one edge of the photoconductive layer and onto a portion of the substrate 35, for mechanical stability. A potential source 20&#39;, of magnitude V, is coupled to electrode 46 to place a bias potential of positive polarity thereon to bias the photoconductive (e.g. selenium) layer 42. 
     Each detector of the array is defined by the photoconductive layer bounded by electrode 46 and by one of electrodes 37. Each elongated one of electrodes 37a-37n has a width of about 3 to 4 milli-inches with an electrode-to-electrode spacing S of about 5 milli-inches. The length L of each electrode is about 1 centimeter and the height H of the selenium photoconductor layer is on the order of 20 milli-inches. Thus, the array comprises a plurality of abutting detectors of about 5  milli-inches width, at the radiation-receiving surface. Each individual one of electrodes 37a-37n is coupled to the input of a like plurality of operational amplifiers 50a-50n, each having an associated feedback resistance R f ,l -R f ,n coupled between the input and output thereof, and an equivalent load resistance R L ,1 -R L ,n coupled from the output thereof to ground. Each of the plurality of individual output signals O 1  -O n  appear at the output of the respective operational amplifiers 50a-50n, responsive to a change in the photoconductance P of the column of photoconductive material between electrode 46 and that one of electrodes 37a-37n associated with the operational amplifier from which a particular output is taken. Thus, if X-ray quanta impinge upon detector layer face 42a at an angle θ&#39; to the normal N&#39; thereto, and within the acceptance angle of the detector, each of the radiation quanta is absorbed in the volume of photoconductive layer associated with one of electrodes 37a-37n and increases the photoconductivity between that one of electrodes 37 and electrode 46, causing an increase in that associated one of output signals O 1  -O n , respectively. 
     While the present invention is described with reference to several presently preferred embodiments, many variations or modifications will now become apparent to those skilled in the art. It is my intent, therefore, to be limited only by the scope of the appending claims, and not by the specific details set forth herein.