# Measurement of Radon Concentration in Super-Kamiokande's Buffer Gas

## TL;DR

This paper presents a highly sensitive radon detector used to measure and monitor radon levels in the buffer gas of the Super-Kamiokande detector, revealing the primary radon source and demonstrating significant reduction after system upgrades.

## Contribution

Development and calibration of a low-background radon detector for precise radon measurement in large-scale neutrino detector environments.

## Key findings

- Radon levels in supply gas reduced to 0.08 mBq/m³ after upgrade.
- In-situ buffer gas radon concentration measured at 28.8 mBq/m³.
- Main radon source identified as contamination from the detector tank.

## Abstract

To precisely measure radon concentrations in purified air supplied to the Super-Kamiokande detector as a buffer gas, we have developed a highly sensitive radon detector with an intrinsic background as low as 0.33$\pm$0.07 mBq/m$^{3}$. In this article, we discuss the construction and calibration of this detector as well as results of its application to the measurement and monitoring of the buffer gas layer above Super-Kamiokande. In March 2013, the chilled activated charcoal system used to remove radon in the input buffer gas was upgraded. After this improvement, a dramatic reduction in the radon concentration of the supply gas down to 0.08 $\pm$ 0.07 mBq/m$^{3}$. Additionally, the Rn concentration of the in-situ buffer gas has been measured 28.8$\pm$1.7 mBq/m$^{3}$ using the new radon detector. Based on these measurements we have determined that the dominant source of Rn in the buffer gas arises from contamination from the Super-Kamiokande tank itself.