Fundamentals of Semiconductor Detectors for Ionizing Radiation
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FUNDAMENTALS OF SEMICONDUCTOR DETECTORS FOR IONIZING RADIATION
G.F. KNOLL AND D.S. McGREGOR The Department of Nuclear Engineering, University of Michigan, Ann Arbor, MI 48109-2104
ABSTRACT The modes of operation of semiconductor detectors are reviewed, together with the influence of charge carrier collection in developing a signal pulse for spectroscopic applications. Because of the importance of charge trapping in many semiconductors of interest in the fabrication of room temperature radiation detectors, the effects of incomplete charge collection are quantified. Calculated results are presented for the expected pulse height and energy resolution under a variety of charge collection conditions. INTRODUCTION The direct consequence of the interaction of ionizing radiation in materials often involves the generation of free electrical charges. The motion of these charges under the influence of an electric field serves as the fundamental basis of the signal observed from several categories of detection devices. Some of the common forms of material in which these charges can be made to move over significant distances include gases, cryogenic liquids, and semiconductor materials. Ionizing radiation interactions in a semiconductor excite bound electrons that create a high density of electron-hole pairs. It is the subsequent motion of these electrons and holes under the influence of an electric field that is the origin of the basic electrical signal from a semiconductor detector. For every electron excited into the conduction band, a hole remains in the valence band. It is the motion of both charge carriers that contributes to the observed output signal. The number of electron-hole pairs that are created reflects the magnitude of the kinetic energy deposited in the detector volume by the interacting quantum of radiation. The total number of pairs, hereinafter referred to as No, demonstrates a closely linear relationship with the initial kinetic energy deposited. For instance, the average value of No produced by an energetic charged particle can be predicted from E/E where E is the particle energy and E is the average energy necessary to create an electron-hole pair. The specific value of E in a given material will depend primarily on the band structure of the semiconducting material and to some extent on the nature of the ionizing radiation (especially in the case of heavy ions); however the value of c is remarkably independent of the kinetic energy of the ionizing radiation. Since N0 represents the number of carrier pairs, the initial charge produced can be represented by qNo where q is the unit electronic charge. This charge represents either the negative charge on the conduction electrons or the positive charge on the holes, the two quantities being of equal but opposite sign. A measurement of qN 0 is often taken as proportional to the initial energy of the interacting quantum of radiation, whether it is from incident photons or charged particles. MODES OF DETECTOR OPERATION In the application of semiconductor detecto
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