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

## TL;DR

This study measures the size of charge clouds generated by 5.0 keV X-ray photons in SOI pixel sensors, using beam scanning and simulations to understand the charge cloud dynamics at a sub-pixel level.

## Contribution

It provides the first detailed measurement of charge cloud size in XRPIX sensors and validates the simulation models for charge cloud formation and energy dependence.

## Key findings

- Charge cloud size is approximately 4.3 μm for 5.0 keV X-rays.
- Measured charge cloud size agrees with simulation models.
- Charge cloud size varies with energy as predicted by simulations.

## Abstract

We report on a measurement of the size of charge clouds produced by X-ray photons in X-ray SOI (Silicon-On-Insulator) 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 $\sim 10$ $\mu$m with a 4-$\mu$m$\phi$ pinhole, and obtain the spatial distribution of single-pixel events at a sub-pixel scale. The standard deviation of charge clouds of 5.0 keV X-ray is estimated to be $\sigma_{\rm cloud} = 4.30 \pm 0.07$ $\mu$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.