Title: Measurement of Radon Concentration in Super-Kamiokande's Buffer Gas,
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.