Characterization of the acquisition modes implemented in Pixirad-1/Pixie-III X-ray Detector: Effects of charge sharing correction on spectral resolution and image quality

X-ray Photon Counting Detectors (XPCDs) with thick semiconductor sensors and small pixel sizes suffer from a charge-sharing effect which can induce multiple counts from a single interacting photon. This issue degrades not only the energy resolution, but it also worsens the spatial resolution. Multiple counts can be removed by acting on the discriminator threshold or they can be corrected by means of specific acquisition modalities implemented in the readout electronics. In this paper we present the results of the characterization of the Pixirad-1/Pixie-III device, a XPCD carrying a 650μm thick CdTe sensor with a small pixel (62×62μm2). The Pixie-III readout system includes programmable energy thresholds and it implements three acquisition modes: a pure photon counting mode and two modes specially designed to correct charge-sharing effects. The measured energy resolution of the three acquisition modes for different energies are here reported. Moreover, we characterize the imaging performance for different combinations of acquisition modes and thresholds by measuring the presampling Modulation Transfer Function (MTF), the Normalized Noise Power Spectrum (NNPS) and by assessing the Noise Equivalent number of Quanta (NEQ).