Purpose: The modeling of radiographic detection can provide insight into the optimization strategies of the imaging system. This work aims to implement a detailed Monte Carlo (MC) simulation of DR digital detectors (semiconductor) considering pairs electron-hole creation and diffusion.
Methods: The modeling was divided into two steps: radiation interaction and charge dispersion based on the penEasy Imaging code. The transport of photons and electrons is simulated using the PENELOPE code, with 50 eV as their cut-off energies. In each electron interaction, the absorbed energy was converted into electron-hole pairs, considering the Poisson distribution, modulated by the Fano factor of the sensor material. The electron-hole pair creation energy (W) depends on the absorbed energy and the electric field (F). The point of charge detection was sampled considering a two-dimensional Gaussian distribution, where the standard deviation was obtained from the Einstein equation for electron diffusion. The pulse-height spectrum (PHS) was obtained from the simulations for monoenergetic pencil beams with energy between 10 and 100 keV, considering a detector composed of amorphous selenium, 150 μm thick, with 139 μm pixel, and (F) equals to 3, 10, and 30 V/μm. The Swank factor was calculated from the PHS moments.
Results: For energies lower than the K-edge the PHS consists of a single spectral peak. However, for energies above the K-edge, an additional peak is observed due to fluorescent photons. Moreover, the number of electron-hole pairs increases with F, resulting in a decrease in W. The Swank factor also increases with F, with the maximum difference of 2% between 3 and 30 V/μm cases. The Swank factor is up to 50% lower than the calculated without charge dispersion.
Conclusion: The simulation presented includes the electron-hole pair dispersion in the detector providing a more detailed detector modeling and, consequently, insight into the imaging system’s fundamental limitations.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by the Brazilian agencies: FAPESP [grant number 2015/21873-8] and CNPq [grant number 131963/2016-3 and 140629/2018-1]