Characterisation of GaAs:Cr pixel sensors coupled to Timepix chips in view of synchrotron applications

In order to meet the needs of some ESRF beamlines for highly efficient 2D X-ray detectors in the 20–50 keV range, GaAs:Cr pixel sensors coupled to TIMEPIX readout chips were implemented into a MAXIPIX detector. Use of GaAs:Cr sensor material is intended to overcome the limitations of Si (low absorption) and of CdTe (fluorescence) in this energy range The GaAs:Cr sensor assemblies were characterised with both laboratory X-ray sources and monochromatic synchrotron X-ray beams. The sensor response as a function of bias voltage was compared to a theoretical model, leading to an estimation of the μτ product of electrons in GaAs:Cr sensor material of 1.6×10⁻⁴ cm²/V. The spatial homogeneity of X-ray images obtained with the sensors was measured in different irradiation conditions, showing a particular sensitivity to small variations in the incident beam spectrum. 2D-resolved elemental mapping of the sensor surface was carried out to investigate a possible relation between the noise pattern observed in X-ray images and local fluctuations in chemical composition. A scanning of the sensor response at subpixel scale revealed that these irregularities can be correlated with a distortion of the effective pixel shapes.