An FPGA-based algorithm to correct the instability of high-resolution and high-flux X-ray spectroscopic imaging detectors

The emergence of CdTe Photon Counting Detectors (PCD) with energy discrimination capabilities opens up new perspectives in X-ray imaging. Medical and security applications require very high fluxes and consequently a very fast shaper in order to limit dead time losses due to pile-up. However, if the shaper is faster than the collection of the charges in the semiconductor, there is a loss of charge called ballistic deficit. Moreover, variations of the electrical field profile in the detector over time causes a change in the collection time of the charges. According to this, the response of the detector will be affected by these variations. The instability of the response is visible over time as a channel shift of the spectra, resulting in a false information of the photon energy. The aim of this work is to develop real-time digital algorithms implementable in an FPGA in order to correct these effects and to give a stable response of the detector at very high fluxes. The method has been tested with a 4×4 pixels detector (CdTe) of 3 mm thickness and 800 μm pitch which is able to measure transmitted X-ray spectra in the energy range of 20–160 kV on 256 energy bins. The detector is coupled with an innovative custom-designed digital readout electronics able to perform a fast digital signal processing for very high-fluxes measurements. The developed method was initially tested at low count rate with a 57Co and 241Am γ-ray sources. After this first validation, it was tested with an X-ray beam, with a counting rate up to ~2⋅106 c/s. With the characterization and the validation of this innovative algorithm we prove its ability in providing a stable response of the detector over time without losing on the energy resolution (~8% at 122 keV) and on the dead time (~70 ns).