The effects of charge diffusion on soft x-ray response for future high-resolution imagers

Future solid state imagers for high-spatial-resolution X-ray missions will require an unprecedented combination of small pixel size and large detector thickness. This presents challenges for the accurate detection of soft X-rays, since the cloud of charge produced by these photons near the entrance window will laterally diffuse to multiple pixels by the time it is collected by the rear surface electrodes, complicating photon energy reconstruction. Using realistic models for the electric field distribution in a silicon-based detector, we have performed simulations of soft X-ray detection over a range of depletion depth, pixel size, and back bias voltage. These simulations start at the generation of photoelectrons by the incoming X-ray, and include diffusion to surrounding pixels as the charge cloud is quickly gathered by the electrode gate structure. We then perform standard X-ray event identification in the presence of a range of simulated pixel-based noise, and compare the spectral response to predicted requirements for future missions at energies down to 0.2 keV. The results show that while increasing the backside bias voltage can decrease the charge collection time and thus also the lateral diffusion, charge splitting among pixels is still significant. The soft X-ray response of future high-resolution missions will greatly benefit from few-electron readout noise or better.