Speaker
Description
The realization of large-area photon-counting X-ray detectors remains challenging due to the stringent requirements on charge transport and spectral performance. Although CdTe and CZT exhibit excellent energy-resolving capabilities, their implementation in large-area imaging systems is constrained by fabrication cost, scalability limitations, and spectral distortions associated with charge sharing and K-fluorescence escape 1. Amorphous selenium (a-Se) is particularly attractive for large-area applications because it combines direct X-ray conversion with mature deposition techniques. However, limited electron transport in a-Se results in significant charge trapping, leading to pulse-height degradation and reduced energy resolution 2. To address this limitation, a unipolar charge sensing detector (UCSD) architecture was evaluated as a means of minimizing electron-induced signal degradation. Experimental measurements were compared with GEANT4 simulations and analytical calculations to assess detector performance and establish consistency among the three approaches [3].
For the UCSD experimental setup, 135 V and 680 V are applied to the octagonal grid and top electrode, respectively, corresponding to an average electric field of approximately 8 V/μm. A 30 × 30 pixel binning scheme using 20 μm pixels is employed to reduce the input capacitance while preserving an adequate signal amplitude under low-signal conditions arising from the limited X-ray absorption efficiency of the 70 μm-thick a-Se sensor and the low activity (10 μCi) of the ²⁴¹Am source. For comparison, the same average electric field is applied to the conventional detector. The shaping times are set to 2 μs for the UCSD and 12 μs for the conventional detector. The same experimental configuration is implemented in both the simulation framework and the analytical model.
Figure 1 shows that the UCSD exhibits a significantly narrower photopeak and improved spectral response compared with the conventional detector in both the simulated and experimental spectra. The conventional detector shows substantial peak broadening and low-energy tailing due to incomplete charge collection associated with deep electron trapping, with FWHM values of approximately 24.2 keV and ~21.11 keV for the experimental and simulated spectra, respectively. In contrast, the UCSD reduces depth-dependent signal formation by decreasing the electron contribution to the induced signal, thereby preserving the spectral information more effectively. The FWHM energy resolution of the UCSD is approximately 8.3 keV and ~7.92 keV for the experimental and simulated spectra, respectively. The close agreement between the simulated and measured spectra indicates that the simulation framework successfully reproduces the dominant hole-transport and signal-formation mechanisms responsible for the improved detector performance.
Figure 2 presents the analytically calculated detector response under deep-trapping conditions. The analytical model is developed using charge induction and carrier-trapping formalisms adapted from previously reported charge collection efficiency models for photoconductive detectors [4]. A Gaussian
fitting is applied to estimate the FWHM values. The conventional detector exhibits a broad photopeak with an FWHM of approximately 22.4 keV, while the Gaussian-fitted photopeak width is 15.57 keV due to the strong low-energy tailing caused by deep electron trapping. In contrast, the UCSD demonstrates a significantly narrower photopeak with an FWHM of 7.88 keV. These analytical results predict a substantial improvement in energy resolution for the UCSD and further support the superior performance of the proposed detector architecture.
Overall, the UCSD structure improved the spectral performance of the detector by suppressing the electron contribution to signal formation. This improvement is supported by the good agreement between the experimental, Monte Carlo simulation, and analytical results. The remaining discrepancies between simulation, analytic and experimental results are likely associated with noise contributions, parameter uncertainties, and physical non-idealities that are not fully incorporated into the model, such as recombination, uncertainties in a-Se transport and trapping parameters, charge-cloud spreading, trap-filling effects, the assumed Fano factor, and Coulombic interactions between charge carriers.
1 S. O. Kasap, Photoconductivity and Photoconductive Materials. Wiley, 2022.
2 S. O. Kasap et al., J. Appl. Phys. 128 (2020), 124501.
[3] Y. Fang et al., Med. Phys. 39 (2012), 308–319.
[4] J. Stavro et al., J. Med. Imag. 5 (2018), 043502.
[5] A. Camlica, PhD thesis, University of Waterloo, 2019.