Simulation Study of Energy Resolution with Changing Pixel Size for Radon Monitor Based on \textit{Topmetal-${II}^-$} TPC

TL;DR

This study investigates how pixel size affects energy resolution in a Topmetal-${II}^-$ TPC-based radon monitor, revealing a non-monotonic relationship and providing guidance for optimal pixel size selection.

Contribution

It introduces a simulation-based analysis of pixel size effects on energy resolution, considering noise and threshold effects, for the design of radon detectors using Topmetal sensors.

Findings

Energy resolution is influenced by pixel size and threshold settings.

Optimal pixel size balances signal-to-noise ratio and spatial resolution.

Estimated total energy resolution is 54 keV for 400 μm pixels.

Abstract

In this paper, we study how pixel size influences energy resolution for a proposed pixelated detector---a high sensitivity, low cost, and real-time radon monitor based on \textit{Topmetal-${II}^-$} time projection chamber (TPC). Using \textit{Topmetal-${II}^-$} sensors assembled by 0.35 $\mu$m CMOS Integrated Circuit process, this monitor is designed to improve the spatial resolution of detecting radon alpha particles. Concerning small pixel size might has a side effect of worsening energy resolution due to lower signal to noise ratio, a Great4-based simulation is used to figure out energy resolution dependence on pixel size ranging from 60 $\mu$m to 600 $\mu$m. A non-monotonic trend in this region shows a combination effect of pixel size with threshold on pixel, and is analyzed by introducing an empirical expression. Noise on pixel contributes 50 keV Full Width at Half Maximum (FWHM) energy resolution for 400 $\mu$m pixel size at 1 $\sim$ 4 $\sigma$ threshold, which is comparable to the energy resolution caused by energy fluctuation in ionization process of TPC ($\sim$ 20 keV). The total energy resolution after combining both factors is estimated to be 54 keV for 400 $\mu$m pixel size at 1 $\sim$ 4 $\sigma$ threshold. The analysis presented in this paper is helpful to choosing suitable pixel size for future pixelated detectors.