Constellation Technology Corp.

Phys. structure of pixelated detectors provides a unique tool to evaluate the effects of different types of defects in the semiconductor material that is used to fabricate the detectors.The spectroscopic performance measured for individual pixels or groups of pixels can be used to correlate point defects or fields of inhomogeneities within the material with the charge collected from photoelec. events.A block of single crystal mercuric iodide of approx. 18 × 18 mm² area and between 6 and 10 mm thick is preparedThe homogeneity of this material is then investigated with light in the transparent region for HgI₂ using an optical microscope.Several types of defects can be identified in this way by the scattering of light, for example, single large inclusions or voids and areas of haziness consisting of fields of small inclusions.Standard procedures are used to fabricate from this block a pixelated detector with a 121-pixel anode structure.The performance of each pixel is measured, and differences in charge collection are correlated with the optical data.Measurement data are presented, and possible mechanisms of the interactions between the defects and the charge carriers are discussed.

The radiation detection efficiency and spectral resolution of mercuric iodide detectors can be improved significantly by increasing the volume of the detectors and by using a pixellated anode structure.Detector bodies with a thickness of nominally 10 mm and an active area of approx. 14 mm x 14 mm have been used for these experimentsThe detectors were cut from single crystals grown by the phys. vapor transport method.The cut surfaces were polished and etched using a string saw and potassium iodide solutionsThe Palladium contacts were deposited by magnetron sputtering through stainless steel masks.The cathode contact is continuous; the anode contacts consist of an array of 11 x 11 pixels surrounded by a guard ring.The resistance between a pixel and its surrounding contacts should be larger than 0.25 Gohm.The detector is mounted on a substrate that makes it possible to connect the anode pixels to an ASIC, and is conditioned so that it is stable for all pixels at a bias of -3000 V.Under these conditions the spectral resolution for Cs-137 gamma rays (662 keV) is approx. 5% FWHM.When depth sensing correction methods are applied, the resolution improves to about 2% FWHM or better.It is expected that the performance of the devices can be improved by the careful selection of crystal parts that are free of structural defects.Details of the fabrication technologies will be described.The effects of material inhomogeneities and transport properties of the charge carriers will be discussed.

Xe-filled ionization detectors, due to their high at. number fill gas (Z = 54), moderate densities (∼0.3-0.5 g/cm³) and good energy resolution (2-4% at 662 keV), fill an important niche between more familiar technologies such as NaI(Tl) scintillators and Ge detectors.Until recently, difficulties with obtaining sufficient Xe purity, reducing microphonic sensitivity, and developing low-noise electronics compatible with small ionization signals have hampered the development of this nuclear detection field.Constellation Technol. Corporation, whose experience with Xe detectors goes back to the mid 1990s, has made significant progress in these areas and has developed a com. line of detectors with active volumes ranging from small (35 g Xe) to large (1400 g Xe).Current applications for Constellation's detectors are principally in the area of defense (Unmanned Aerial Vehicles and Advanced Spectroscopic Portals), but as awareness of this technol. grows, it will surely find applications in a much expanded range of fields.