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Chromium-Compensated GaAs X-ray Sensors

Abstract: In this work we present the results of experimental study of the current-voltage characteristics, the electric field distribution, the charge collection efficiency and X-ray sensitivity of X-ray sensors based on chromium compensated gallium arsenide (GaAs:Cr). The experimental samples were 0.1-0.25 cm2 pad sensors with the sensitive layer thickness in the range of 250-1000 μm. The electric field distribution was investigated using the Pockels effect at a wavelength of 920 nm. It has been found experimentally that in HR GaAs sensors the electric field distribution is much more homogeneous compared to the sensors based on SI GaAs: EL2. It has been shown that the temporal fluctuations of the electric field are absent in HR GaAs sensors. Analysis of the charge collection efficiency as a function of bias has demonstrated, that in the HR GaAs material the values of the mobility-lifetime product of the nonequilibrium charge carriers are in the order of 10-4 cm2/V and 310-7 cm2/V for electrons and holes, respectively. Investigations on the sensor sensitivity to X-ray energies in the range between 40 and 150 keV were performed. Prototype pixel sensors measuring 256256 and 512768 pixels with a 55Ám pitch and a 500 Ám thick sensitive layer were produced. The dependence of the photocurrent and count rate on the X-ray radiation intensity and bias voltage was examined. In the 40−80 keV energy range, the maximum count rate amounted to 800 kHz/pixel for a negative sensor bias voltage of 800 V. The sensors are demonstrated to provide spatial resolution varying with the pixel pitch and to enable high-quality X-ray images to be obtained.
Speaker: Anton Tyazhev - Tomsk State University
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