Semiconductor materials for x-ray detectors
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roduction X-ray detectors are a crucial component in any x-ray experiment. In synchrotron experiments, detector performance has often been a limiting factor, particularly as the brilliance of these sources has increased. For example, when performing time-resolved experiments, the sensitivity and readout speed of the detector are crucial. Likewise, when studying radiationsensitive samples, improving the detector may benefit the experiment, but increasing the flux may not. A wide range of detectors are currently in use at synchrotron facilities, and the overwhelming majority of these are semiconductor detectors. The main strength of semiconductor detectors is that they can provide an outstanding combination of high speed, spatial resolution, and sensitivity, as compared to other types of detectors such as gas detectors or image plates. While semiconductor detectors vary in design,1 they all rely on the incoming radiation generating electron–hole pairs in the semiconductor, which can then be measured by readout electronics. This process relies on two key features of semiconductors. First, in the absence of ionizing radiation under appropriate conditions (e.g., a reverse-biased p–n junction), they contain few mobile electrons and holes. Second, electrons and holes generated by ionizing radiation are highly
mobile and long-lived, making it possible to separate them with an electric field and measure them before they recombine. These features are highly dependent on crystal quality. Broadly speaking, semiconductor detectors used in synchrotrons can be divided into two types—spectroscopic detectors, whose key role is energy measurement, and imaging detectors, used to obtain an x-ray image or diffraction pattern.2 Spectroscopic detectors typically consist of a single sensing element, or a small number of elements, each connected to a high-performance readout channel for optimal energy measurement. Imaging detectors, on the other hand, are finely segmented, in some cases into millions of pixels, and different schemes exist for reading out these pixels efficiently. One conceptually simple, but powerful, imaging detector design is the “hybrid pixel” structure, shown in Figure 1. A pixelated semiconductor sensor is connected face to face with a custom-designed silicon readout chip by a fine array of solder balls. Each pixel in the sensor is connected to a channel of readout electronics on the chip. This design makes it possible to achieve direct detection in a relatively thick sensor layer with small pixels. The readout chip can provide sophisticated signal processing in each individual pixel and fast readout. X-ray diffraction experiments have greatly benefited from “photon counting” readout chip designs such as Pilatus3 and
David Pennicard, X-Spectrum GmbH, Germany; and Deutsches Elektronen-Synchrotron, Germany; [email protected] Benoît Pirard, Mirion Technologies, France; [email protected] Oleg Tolbanov, Tomsk State University, Russia; [email protected] Krzysztof Iniewski, Redlen Technologies Inc., Canada; kris.iniewski
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