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 ± 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 ± 0.07 mBq/m 3 . Additionally, the Rn concentration of the in-situ buffer gas has been measured 28.8±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.

Development of the database for low-background studies in Kamioka

“…In addition, in order to reduce the intrinsic background, the inner surface of the stainless vessel has been electropolished. Because of these improvements, the intrinsic background becomes 0.33 ± 0.07 mBq/m 3 , which is lower than that of the previous 70 litter Rn detectors by a factor of three [13,10]. Finally, this low-background detector enables to measure ∼0.5 mBq/m 3 level of Rn concentration for a single day measurement.…”

“…We have developed an 80-litter high sensitivity Rn detector to measure Rn concentrations in either the air [13] or the novel gases [12]. Figure 3 shows the design of the 80-litter Rn detector and its energy spectrum of the background measurement [13].…”

“…Figure 3 shows the design of the 80-litter Rn detector and its energy spectrum of the background measurement [13]. Since most of the radon daughter nuclei are positively charged, they are easily collected on the Surface α-ray measurement w/ µ-PIC Evaluate the emanation rate by using the old NEWAGE detector [25] High sensitivity α-ray counter ∼10 mBq/m 3 level of 210 Pb/ 210 Po [26] Low-level Rn measurement (∼80 litter Rn detector) Measurement of the Rn concentration in the in-situ buffer gas in SK [13] Rn emanation measurement system Rn emanation measurement for material screening [27] Emanation/permeation test system Evaluate the Rn permeation through a plastic sheet [28] Absorption test system Evaluate the capture efficiency of Rn in gas with a cold trap [28] Small Rn detectors (∼1 litter Rn detector) Monitoring the Rn concentration in the experimental site -Radiopurity assay system for scintillation crystals Evaluate the impurities in the crystal powder -PIN photodiode by the electric field made in the inside of the Rn detector. In order to reduce the lower limit of the vessel's vacuum, knife-edge flanges with metal gaskets (CF flanges) is used.…”

“…In order to develop low-radioactivity R&D studies, several apparatuses are available in the experimental site to measure and to evaluate impurities in detector materials, such as Ge detector, ICP-MS, API-MS and Rn detector [10,11,12,13] as shown in Figure 1. In addition, a new experimental site, called LAB-A, has been prepared near the Super-Kamiokande detector since 2015.…”

The development of $$^{222}\hbox {Rn}$$222Rn detectors for JUNO prototype

“…According to the MC simulation for JUNO experiment requirements, the Rn concentration in the water should be less than 0.2 Bq/m 3 [1]. The high-sensitivity Rn assay is one of the essential techniques for JUNO as well as other underground experiments [2,3].…”