Abstract:
A position sensitive detector and method are disclosed wherein an image of incident radiation is detected and amplified and an accelerated charged image is produced corresponding spatially to the incident radiation. The accelerated charged image is successively impinged on successive pairs of interdigitated electrodes having different electrode patterns and the output of the electrodes is compared for determining the location of the charges making up the charged image.

Description:
spatially to the incident radiation and/or charged particles. The accelerated charged image is successively impinged on at least one of a pair of interdigitated electrodes at a series of stages downstream in the accelerated direction of the charged image. The interdigitated electrodes at each successive impingement have different electrode patterns. After the stages, the charged image is collected; the electrodes impinged upon are compared; and the locations of the charges making up the charged image are determined. 
     In accordance with one aspect of this invention orthogonal pairs of electrodes at each stage are provided to determine both the X and Y location of the charges of the charged image. 
     In accordance with another aspect of this invention the electrode patterns include a broad pattern bisecting the accelerated charged image and finer patterns bisecting the electrodes of the next broader pattern. In a preferred aspect of the invention the finest pattern is arranged at the beginning stage. 
     In accordance with still another aspect of this invention the charged image is amplified at each of the stages of impingement. 
     In accordance with still another aspect of this invention a microchannel plate with orthogonal pairs of electrodes on its surfaces is provided at each stage for impingement and amplification of the charged image. 
     These features and aspects of the present invention will become more apparent upon a perusal of the following specification and claims taken in conjunction with the accompanying drawing wherein similar characters of reference represent similar elements in each of the several views. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic drawing, partly in a foreshortened elevational sectional form and partly in block diagram form, of the preferred embodiment of the invention. 
     FIG. 2 is a schematic view showing electrode patterns and the Gray code for the pixels determinable with the patterns. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings there is shown a position sensitive detector 10 in accordance with the preferred embodiment of the present invention. The detector 10 includes a vacuum chamber housing 11 for receiving an image of incident radiation, charged particles and/or energetic neutral particles at one end on the input face of a dual microchannel plate 20. From the microchannel plate 20 the image is passed through a series of microchannel plates 30a, b, c, d . . . successively positioned downstream from the dual microchannel plate 20 toward a collection plate 40. Since the PSD 10 can include the appropriate desired number of microchannel plates 30 in the series, the PSD is shown foreshortened for purposes of illustration. 
     The dual microchannel plate 20 includes a chevron configuration of cascaded microchannel electron multiplier plates 22 and 23 such as a fused array of lead glass tubes or channels. Typical tube interior diameters are 10 μm with 15μm spacing between centers. The plate thickness (equivalently, the tube length) is approximately 0.5 mm, corresponding to a channel aspect ratio of 50. The interior of the tubes have a high secondary electron coefficient and are slightly conductive. A suitable photocathode can be provided preceding the dual microchannel plate 20 to convert photons to an electron burst and/or a thin foil can be provided to convert charged or neutral particles to secondary electrons. 
     A metallic electrode is vacuum evaporated on to both faces of each plate 22 and 23. By applying voltage across the microchannel plate surfaces, electric field is produced along each tube so that an electron entering a tube is accelerated toward the other end. When an electron strikes the wall, several electrons are emitted and each of these repeats the process. Single plate 22 can output 10 3  -10 4  electrons per incident electron. The second microchannel plate 23 accepting the output of the first plate 22 can provide an additional 10 3  gain. The lateral position of the incident electrons is preserved. 
     The microchannel plates 30a-30c include input surfaces 31a-31c and an output surfaces 35a-35c, respectively. As shown in FIG. 2, each input surface 31a-31c includes a pair of thin film interdigitated electrodes 32a-32c and 33a-33c, respectively. Leads 32a&#39;-32c&#39; connect electrodes 32a-32c to amplifiers 32a&#34;-32c&#34;, respectively, and leads 33a&#39;-33c&#39; connect electrodes 33a-33c to amplifiers 33a&#34;-33c&#34;, respectively. The amplifiers for the two electrodes on each surface are connected to comparators 34a-34c. 
     The microchannel plates 30a-30c have output surfaces 35a-35c which include interdigitated electrodes corresponding to those on the respective input surfaces and similar amplifiers are connected to similar comparators. Only the amplifiers 32a&#34; and 33a&#34; and the comparator 34a for the input surface 31a of microchannel plate 30a are illustrated. The electrodes on the input surface and on the output surface when projected onto a common plane are orthogonal to one another. 
     The electrodes 32a-32c on input surfaces 31a-31c serve to determine the &#34;X&#34; position of an electron burst, and the electrodes on the output surfaces 35a-35c serve to determine the &#34;Y&#34; position. 
     The collection plate 40 is connected to an amplifier discriminator 41. The amplifier/comparator combination for all interdigitated electrodes and the discriminator 41 are connected to a buffer 42, then to a Gray to binary code converter 43 and then to a computer to compute the positions of all electron bursts. 
     In operation, the incident particle strikes the front surface of the dual microchannel plate 20 causing a burst of 10 6  -10 7  electrons to exit the rear surface of the second plate 23 for then passage through the microchannel plates 30a-30c and finally for striking tho metal collector plate 40. The time duration of the burst pulse striking the collector plate 40 is approximately 300 psec. 
     The electrode pairs fall in a sequence with successively finer patterns and each is connected to a comparator circuit. As a charge burst traverses the stack, it encounters at each surface 1 electrode of the pair, determining 1 digit of a Gray binary code. Each microchannel plate restores lost charge and the burst finally strikes the collection plate providing a strobe pulse to read out the comparator. For 2n pixels, only n comparators and n/2 microchannel plates are required. 
     As the burst exits the rear electron of the second plate 23, a recharge pulse is available while permitting the electron burst to proceed unimpacted. As the electron burst leaves the rear surface of each plate 30a-30c, a recharge pulse is generated in one of the two electrodes. The same electrons then strike the succeeding microchannel plate and generate a burst pulse on its front face, and the process repeats. Each of the microchannel plates 30a-30c is operated with sufficient gain to overcome electron losses. 
     On the microchannels 30a-30c the recharge pulse will occur in the electrode half on which electrons strike the front or input plate surface, and a recharge pulse will occur in the electrode half from which the burst exits. The output state of the comparator circuits 34a-34c will correspond to the activated electrode. If the magnitude of the charge burst striking the collection plate 40 exceeds the threshold of discriminator 41, the comparator output state is recorded. For providing location information two binary decisions are provided, and they are made with separate signals. The collector 40 pulse passed through the amplifier discriminator 41 serves to decide if an event has occurred, and the comparators 34a-34c decide the lateral position. 
     Each microchannel plate surface provides one digit of binary code describing the pixel position. While the electrode pattern will follow the conventional binary counting system, the performance of the system can be significantly improve by employing the binary codes developed by Gray. FIG. 2 shows a one-dimensional 8 pixel encoder code with its interdigitated electrode patterns on plates 30a-30c, noting the most significant bit pattern and the least significant bit pattern. 
     The Gray binary system has the property that adjacent codes only differ in one digit and has important application in this invention. Any electron burst may straddle a divider between electrode pairs leading to an indeterminate comparator output with the Gray system. Since every microchannel plate surface corresponds to a digit, an electron burst of width 1 pixel can only encounter a divider strip at one surface in the stack, and so outputs are never indeterminate by more than 1 pixel. In fact, electron bursts up to 3 pixels wide will unconditionally generate correct codes. 
     In accordance with the preferred embodiment of the invention with a 25 mm square active area, 256 pixels means 100 μm per pixel The finest pattern has a center distance of 200 μm. A divider strip with 50μm will utilize 75 percent of the available charge and yet be practical to fabricate. The interelectrode capacitance of comparable wedge and strip anodes is approximately 100 pF; for the pulse length of the preferred embodiment, this corresponds to a few ohms impedance. Coating the microchannel plate surface and its metallization with a weakly-conductive overlay will ameliorate charging in the divider strip without affecting the impedance. 
     The burst will increase in diameter as it traverses the microchannels stack. For perfect comparators and symmetric spreading function, the burst diameter must always be 1 pixel less than the pattern repeat distance to insure correct coding. It is therefore advantageous to place the finest patterns closer to the amplifier chevron dual microchannel plate 20. Preferably, the burst diameter should not exceed 10 pixels. The least spreading is accomplished by butting each microchannel plate against its neighbor, but this complicates connection of the electrodes. Spreading will satisfactorily be small for spacing between the microchannel plates 30a-30c of approximately 100 μm. 
     The electron burst will also spread in the time domain. A 5 microchannel plate stack operated at moderate gain will produce a pulse width of about 500 psec. A reduced gain stack in accordance with the preferred embodiment will generate pulses of about 1 to 3 nsec duration. Furthermore, pulses from succeeding microchannel plate surfaces will occur later in time. The transit time from electron velocities can be estimated since an electron with 10 eV kinetic energy travels 1 cm in 5 nsec. For this magnitude of transit time, differences can be compensated with cable length and will not impact pulse pair resolution. 
     While the preferred embodiment of this invention incorporates successive microchannel plates, other structures can be utilized. For example, an array of holes can be drilled in silicon which is then provided with polyamide films that will produce secondary electrons. 
     While the invention has been described in terms of a preferred embodiment, it will be apparent to those skilled in the art that numerous modifications can be made thereto without departing from the spirit and scope of the invention. It is intended that these modifications fall within the spirit and scope of the appended claims.