Importance of Considering the Response Function of Photon Counting Detectors with the Goal of Precise Material Identification

Energy Resolving Photon Counting Detectors (ERPCD) have been developed as a medical X-ray imaging application. The ERPCD can analyze energy information of measured X-rays. There is the possibility that quantitative information of an object such as the effective atomic number (Z _eff ) can be derived by analyzing the energy information of the X-rays. In order to realize accurate Z _eff identification, it is necessary to consider the effect of secondary-produced radiations which are produced by the interaction between incident X-rays and the detector. In addition, these secondary-produced radiations can be transferred to adjacent pixels. In this study, we evaluated the influence of the response function on the measured X-ray spectrum in terms of accuracy of derived Z _eff . We focused on CZT, Si, and GaAs detectors, and the response functions of these detectors were reproduced by Monte-Carlo simulations. A detector with an area of 10 mm × 10 mm, a thickness of 1.5 mm, and a pixel size of 200 μm × 200 μm was used for the simulation. Aluminum samples having mass thicknesses of 1, 5, and 10 g/cm ² were used as an object. 50.0 kV polychromatic X-rays were irradiated to the detector area of 1 mm × 1 mm. The X-ray spectra measured within one center pixel were analyzed. The mass attenuation factor "μ/ρ" was calculated from the simulated X-ray spectra of being incident and penetrating the object. The Z _eff was derived from the theoretical relationship between μ/ρ and Z _eff , and the accuracy was evaluated. It was found that the effect of the response function decreases the accuracy of the derived Z _eff . We clarified the energy regions which can derive Z _eff accurately. These results are valuable to optimize analytical energy regions for accomplishing accurate Z _eff analysis using ERPCD.