Investigation of GaAs:Cr Timepix assemblies under high flux irradiation

Abstract

High resistivity, chromium compensated gallium arsenide (HR-GaAs:Cr) has recently shown to be a promising sensor material for X-ray detectors due to its high resistivity, its fully active volume, the good electron transport properties and good absorption properties for photon energies up to around 40 keV. These properties make this material a favorable candidate for producing sensors for photon counting X-ray imaging detectors. Such detector systems have also gained increasing attention to be used for high flux applications as found at synchrotrons or in medical applications like computed tomography. Whereas other interesting high-Z materials such as CdTe have already been studied under high flux irradiation, the behavior of HR-GaAs:Cr under such conditions has yet to be investigated.In this work, we study the performance of a 500 µm thick GaAs:Cr sensor, bump-bonded to Timepix readout electronics, with a main emphasis laid on the behavior under the irradiation of high X-ray fluxes using a monochromatic synchrotron beam. We find that, at high X-ray fluxes, the detector performance degrades, and that the dislocation network typically present in melt-grown GaAs influences the detection properties due to local variations of the charge transport properties. However, we also demonstrate that the high flux behavior can be substantially improved by optimized chip settings, namely by tuning the Timepix' preamplifier output pulse shape. In this way, the photon counting capabilities of the detector assembly are restored to a certain extent and it is possible to operate the detector assembly at impinging fluxes as high as 10 10 s −1 mm −2 .

Characterization of chromium compensated GaAs as an X-ray sensor material for charge-integrating pixel array detectors

“…We found this point was best to balance all the positive and negative effects of temperature and voltage. Please note than many results presented by others, especially those obtained with photon counters from the Medipix family are obtained at room temperature [6][7][8][9][10].…”

Properties of GaAs:Cr-based Timepix detectors

“…It has been shown in [7] that α particles cannot be used for measuring the electron µτ -product of the GaAs:Cr material due to incomplete charge collection efficiency (CCE) because of a short lifetime of charge carriers (electrons) in comparison with the lifetime of plasma created by α particle [8]. To overcome this problem we selected characteristic X-ray radiation from zirconium foil (K α1 = 15.7 keV ) as particle source.…”

“…To avoid blurring due to geometrical unsharpness the mask was placed on the top of the sensor. In order to study the influence of charge sharing on the spatial resolution the MTF was calculated 9 for detectors of different thickness and for two threshold 8 Hamamatsu L10951 9 The ImageJ plugin "SE MTF" [1] was used for MTF calculations levels. Figure 16 shows the MTF calculation results.…”

Characterization of Chromium Compensated GaAs Sensors with the Charge-Integrating JUNGFRAU Readout Chip by Means of a Highly Collimated Pencil Beam

“…Further details on the compensation process are explained in Reference [ 9 ]. Chromium compensated GaAs has been successfully evaluated as sensor material by several groups as it exhibits good spectral performance and stable detector operation, even under high photon fluxes which makes it a promising sensor material for X-ray detectors at synchrotrons or FELs [ 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 ].…”