Planar arrays of cadmium zinc telluride photodetectors with readout electronic circuitry have been developed for use as hard-x-ray and y-ray image sensors. When a coded, x-and-y-ray-opaque aperture mask is positioned in front of such a sensor, the resulting assembly is an instrument that can be used to observe hard-x-ray and y-ray sources. In operation, the spatial pattern of x and g rays impinging on the sensor is deconvolved from the aperture pattern to obtain an image of the source. With suitable choice of the sensor pixel pitch, coded aperture pattern, and distance of the aperture in front of the detector array, it should be possible to image radiation sources at angular resolutions of 30 arc seconds and finer. In the original intended application, the instrument will be operated in outer space to measure precisely the directions to distant sources of hard-x-ray and g-ray bursts that are of cosmological interest. The instrument can also be used to image hard-x-ray and y-ray-sources in a terrestrial laboratory setting; indeed, a prototype of the instrument has been demonstrated in such a setting.

Detector Modules like this one are arranged in a square array to form a sensor for imaging of hard x rays and g rays. A pixel pitch of 100 µm in two dimensions is defined by orthogonal arrays of detector strips on the upper and lower faces of each module.

Prototype sensors containing 2 × 2 and 6 ×6 arrays of detector modules made from Cd0.9Zn0.1Te have been constructed (see figure). The dimensions of a module are 15 by 15 by 2 mm. Each face of a module is patterned with 127 metal strips, each 50 µm wide, at a pitch of 100 µm. The strip pattern is surrounded by a 450-µm guard ring. The metal strips divide the module into, and serve as electrical contacts for, a corresponding pattern of strip detectors. The strips on the front and back faces are made orthogonal to each other to establish a square pixel grid with a pitch of 100 µm.

The row and column metal strips that are collinear with each other in the various modules are electrically tied together with wire bonds to make long strips that span the entire array of modules, thereby defining a pixel coordinate grid over the whole array. The row and column strips are biased via monolithic resistors formed in voltage-divider configurations along two orthogonal edges of the array. For readout, the strips are ac-coupled, via 1,000-pF capacitors, to high-density application-specific integrated circuits.

In operation, the Cd0.9Zn0.1Te material absorbs photons with energies between 10 and 150 keV. The electrons and holes generated in the absorption of photons drift to the anode and cathode strips, respectively, where they are collected. The charges collected on orthogonal anode and cathode strips indicate the magnitudes and positions of photon-impingement events in the detector plane. Because the cloud of drifting charges induced by each photon has a finite size and can diffuse outward as the carriers drift, charge is typically induced on three adjacent strips at 100-µm pitch. The position of impingement is determined from the average of the strip numbers weighted by the charges induced on the affected strips. In addition, the charges on affected adjacent strips must be summed for accuracy in determining the photon energy. For even greater accuracy, it is necessary to sum the signals from anode strips because the charges collected on cathode strips are reduced because of the poor hole-transport properties of Cd0.9Zn0.1Te.

Using 60-keV g rays from a 241Am source, a 2 × 2 prototype sensor was tested in conjunction with a 40-µm-thick gold aperture mask patterned in a 100-µm-pitch square grid supported by a 1-mm-thick beryllium substrate. The aperture mask was placed 32 mm in front of the sensor (in the fully developed instrument, the mask would be placed 800 mm in front of the sensor). The results of the test revealed that the prototype instrument was capable of an angular resolution of about 30 arc seconds. The results give confidence in computer simulations that predict that the fully developed instrument will be able to locate about 90 g-ray bursts per year to an accuracy of ±3 arc seconds, and to produce an all-sky survey with a resolution of 30 arc seconds.

This work was done by L. Barbier, N. Gehrels, B. Teegarden, A. M. Parsons, L. M. Bartlett, P. K. Shu, and J. Tueller of Goddard Space Flight Center; C. M. Stahle of Orbital Sciences Corp.; Z. Q. Shi, K. Hu, and S. J. Snodgrass of Hughes STX Corp.; D. M. Palmer, S. D. Barthelmy, and J. Krizmanic of Universities Space Research Association; S. J. Lehtonen and K. J. Mach of Johns Hopkins University Applied Physics Laboratory; P. Kurczynski of the University of Maryland; E. Fenimore of Los Alamos National Laboratory; and D. C. Mancini of Argonne National Laboratories. For further information, access the Technical Support Package (TSP)free on-line at www.techbriefs.com/tsp under the Electronics & Computers category. GSC-14044


NASA Tech Briefs Magazine

This article first appeared in the June, 2000 issue of NASA Tech Briefs Magazine.

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