Measurement of the radon concentration in purified water in the Super-Kamiokande IV detector

TL;DR

This paper presents a new measurement system for radon in ultra-pure water at Super-Kamiokande IV, providing precise radon concentration data crucial for reducing background in solar neutrino detection.

Contribution

The paper introduces a novel radon measurement system with detailed design, calibration, and performance evaluation, enabling accurate radon quantification in the detector's water.

Findings

Radon concentrations in supply and return water lines were quantified.

Water from the detector center had very low radon levels (<0.23 mBq/m^3).

Water from the bottom region showed higher radon levels (~2.63 mBq/m^3).

Abstract

The radioactive noble gas radon can be a serious background source in the underground particle physics experiments studying processes that deposit energy comparable to its decay products. Low energy solar neutrino measurements at Super-Kamiokande suffer from these backgrounds and therefore require precise characterization of the radon concentration in the detector's ultra-pure water. For this purpose, we have developed a measurement system consisting of a radon extraction column, a charcoal trap, and a radon detector. In this article we discuss the design, calibration, and performance of the radon extraction column. We also describe the design of the measurement system and evaluate its performance, including its background. Using this system we measured the radon concentration in Super-Kamiokande's water between May 2014 and October 2015. The measured radon concentration in the supply lines of the water circulation system was $1.74\pm0.14~\mathrm{mBq/m^{3}}$ and in the return line was $9.06\pm0.58~\mathrm{mBq/m^{3}}$. Water sampled from the center region of the detector itself had a concentration of $<0.23~\mathrm{mBq/m^{3}}$ ($95\%$ C.L.) and water sampled from the bottom region of the detector had a concentration of $2.63\pm0.22~\mathrm{mBq/m^{3}}$.