Microscale X-ray mapping of CZT arrays: spatial dependence of amplitude, shape and multiplicity of detector pulses

In this work, we present the results of a microscale X-ray mapping of a 2 mm thick CZT pixel detector, with pixel pitches of 500 gm and 250 gm, using collimated synchrotron X-ray sources at the Diamond Light Source (U. K.). The detector is dc coupled to a fast and low noise ASIC (PIXIE ASIC), characterized only by the preamplifier stage. A custom 16-channel digital readout electronics was used, able to perform online fast pulse shape and height analysis (PSHA), with low dead time and reasonable energy resolution at both low and high fluxes. The detector allows high bias voltage operation (> 5000 V/cm) and good energy resolution at room temperature (5.3 %, 2.3 % and 2.1 %FWHM at 22.1, 59.5 and 122.1 keV, respectively) by using fast pulse shaping with low dead time (300 ns). Charge sharing investigations were performed by using uncollimated and collimated X-ray sources with energies above and below the K-shell absorption energy of the CZT material. Collimated 10 x 10 gm synchrotron X-ray beams were used to study the spatial dependence of the amplitude, the shape and the multiplicity of the detector pulses. Charge losses, after charge sharing addition (CSA), were observed for interactions occurring in the volume of the inter-pixel gap and an original method was proposed to recover these losses and improve the energy resolution. High rate measurements at 550 kcps/pixel were also performed, pointing out the key role of the pulse shape analysis in the detection of both charge sharing and pile-up events.