Measurement of radon concentration in super-Kamiokande's buffer gas

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 0.07 mBq/m. 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 0.07 mBq/m. Additionally, the Rn concentration of the in-situ buffer gas has been measured ± 28.8 1.7 mBq/m 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 SuperKamiokande tank itself. & 2017 Elsevier B.V. All rights reserved.