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-on-insulator (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 $\sim 10{\mathrm{ ~\mu m}}$ with a 4- ${\mathrm{ \mu m\phi }}$ 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 $\sigma _{\mathrm{ cloud}}=4.30\pm 0.07{\mathrm{ \,\,\mu m}}$ . 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.