A Wide Energy Range Calibration Algorithm for X-ray Photon Counting Pixel Detectors Using High-Z Sensor Material

We developed a novel, accurate and robust energy calibration algorithm for single X-ray photon counting detectors suitable for the use of high-Z sensor materials. The calibratable energy range was demonstrated to extend over a wide interval from 12 keV up to 150 keV and the same method can also be applied to lower or higher X-ray energies. Fluorescence lines of several elemental targets are used as reference up to 75 keV. For higher energies, the reference is given by direct X-ray tube spectra end-points - extracted with a semi-empirical model independently of the broadening by electronic noise. The pixel-wise threshold trimming relies on a target curve fitting iterative procedure which is able to cope with offset, gain and flat-field variations. The algorithm is highly parallelizable and can profit from modern mull-core machines. The proposed method is able to deal with the spectral complexity introduced by atomic fluorescence effects arising in high-Z sensor materials such as CdTe, CdZnTe and GaAs for energies above the corresponding K-edges and enhanced by a decreasing pixel size. We report the qualification results for the case study of a 1 mm-thick CdTe sensor with 150 mu m pixel size, bonded to an IBEX ASIC and calibrated in the range 12-150 keV. The validity of the method was assessed by analyzing calibrated pulse height spectra recorded with Sn and W fluorescence samples and with a 99 mTc radioactive source. The global deviation was found to be smaller than 0.5% at 140.5 keV. The residual threshold dispersion was 0.54 keV rms at the Sn K-alpha peak and 1.71 keV rms at the Tc-99(m) gamma-decay peak, still with a limited impact on the overall energy resolution of the system.