Measurement of Charge Cloud Size in X-ray SOI Pixel Sensors

We report on a measurement of the size of charge clouds produced by X-ray photons in X-ray Silicon-oninsulator (SOI) pixel sensor named XRPIX. We carry out a beam scanning experiment of XRPIX using a monochromatic X-ray beam at 5.0 keV collimated to ~10 μm with a 4-μmφ pinhole and obtain the spatial distribution of single-pixel events at a subpixel scale. The standard deviation of charge clouds of 5.0-keV X-ray is estimated to be σ cloud = 4.30 ± 0.07 μm. Compared to the detector response simulation, the estimated charge cloud size is well explained by a combination of photoelectron range, thermal diffusion, and Coulomb repulsion. Moreover, by analyzing the fraction of multi-pixel events in various energies, we find that the energy dependence of the charge cloud size is also consistent with the simulation.