PROCEEDINGS ARTICLE | September 23, 2011

Proc. SPIE. 8143, Medical Applications of Radiation Detectors

KEYWORDS: Semiconductors, Sensors, Electrodes, Electrons, Amplifiers, Detector arrays, Collimation, Gamma radiation, Gamma ray imaging, Signal detection

Pixellated semiconductor detectors, such as CdZnTe, CdTe, or TlBr, are used for gamma-ray imaging in medicine and
astronomy. Data analysis for these detectors typically estimates the position (x, y, z) and energy (E) of each interacting
gamma ray from a set of detector signals {S<sub>i</sub>} corresponding to completed charge transport on the hit pixel and any of its
neighbors that take part in charge sharing, plus the cathode. However, it is clear from an analysis of signal induction, that
there are transient signal on all pixel electrodes during the charge transport and, when there is charge trapping, small
negative residual signals on all electrodes. If we wish to optimally obtain the event parameters, we should take all these
signals into account. We wish to estimate x,y,z and E from the set of all electrode signals, {S<sub>i</sub>(t)}, including time
dependence, using maximum-likelihood techniques[1]. To do this, we need to determine the probability of the electrode
signals, given the event parameters {x, y, z, E}, i.e. Pr( {S<sub>i</sub>(t)} | {x, y, z, E} ). Thus we need to map the detector response
of all pixels, {S<sub>i</sub>(t)}, for a large number of events with known x,y,z and E.In this paper we demonstrate the existence of
the transient signals and residual signals and determine their magnitudes. They are typically 50-100 times smaller than
the hit-pixel signals. We then describe development of an apparatus to measure the response of a 16-pixel semiconductor
detector and show some preliminary results. We also discuss techniques for measuring the event parameters for
individual gamma-ray interactions, a requirement for determining Pr( {S<sub>i</sub>(t)} | {x, y, z, E}).