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NEW COMPOUND SEMICONDUCTOR MATERIALS FOR NUCLEAR DETECTORS MICHAEL R. SQUILLANTE, JOHN ZHANG, CHUXIN ZHOU, PAUL BENNETT, AND LARRY MOY Radiation Monitoring Devices, Inc., 44 hunt Street, Watertown, MA 02172 ABSTRACT In many instances, the ability of scientists and engineers to make nuclear radiation measurements is only limited by the properties of available radiation detectors. Because of this, research into new semiconductor materials for radiation detection has been, and continues to be, a very active field. This article reviews recent research on several promising "new" materials. INTRODUCTION The search for new materials for use as nuclear detector materials is an exciting branch of semiconductor materials science. The Detection nuclear radiation presents a variety of unique material requirements and places severe demands on the properties of the materials to be used. Research into new semiconductor materials for radiation detection has been a very active field for several decades. A number of excellent review articles have already been published on this topic",2,3,4 . Semiconductor nuclear detectors are now experiencing a dramatic increase in interest. This increase is partly due to the availability of better quality materials and to advances in semiconductor processing technology which make possible the fabrication of new and better device structures. However, one of the most important factors that has stimulated interest in semiconductor detectors is the availability of powerful and relatively inexpensive electronic circuitry for powering and reading out semiconductor devices. In particular, these advances have influenced the current direction of research on imaging devices. This increased interest has stimulated a desire to identify and develop new materials which can solve problems that cannot be solved using the three "traditional" semiconductor detector materials: silicon, germanium and cadmium telluride. The search for better performance usually does not mean looking for a totally new material. Shortcomings in one material often lead to research in another very similar material. An example of this process is the history of research on PbI at Radiation Monitoring Devices. For twenty years, HgI2 has been the leading contender as the next practical detector material and has recently been used in several commercial products. But, in addition to its strengths as a detector material, HgI 2 has deficiencies, some of which have been solved 5, but some of which have yet to be overcome and the desire to circumvent these problems led directly to research on the very similar material, PbI2. Research on PbI has shown that this material does indeed solve many of the problems associated with HgI 2 : it can be grown from the melt and it is chemically and electrically very stable6 . It does not, however, solve all the problems since, like HgI 2, it has a layered crystal structure and is soft and fragile. These characteristics led to research on another promising heavy metal halide, TlBr, which was chosen partly because