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

Abstract

The radioactive noble gas radon (Rn) can be a serious background to underground particle physics experiments studying phenomena 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 Rn concentration in the detector's ultra-pure water. To this end, we have developed a measurement system consisting of a Rn extraction column, an activated charcoal trap, and a Rn detector. In this article, we discuss the design, calibration, and performance of the Rn extraction column. We also describe the design of the measurement system and evaluate its performance, including its background. Using this system we measured the Rn concentration in Super-Kamiokande's water during the period May 2014 to October 2015. The measured Rn concentration in the supply (return) lines of the water circulation system and found 1.62 ± 0.15 mBq/m 3 (9.06 ± 0.63 mBq/m 3 ). Water sampled from the center (bottom) region of the detector itself had a concentration of < 0.14 mBq/m 3 (95% C.L.) (2.44 ± 0.24 mBq/m 3 ).

Magnetic shielding simulation for particle detection

“…The distribution of this heat in the water can also create convection currents that distribute the radon emitted by the PMTs and other electronic devices throughout the detector. Radon is one of the main sources of background radiation [14,18]. The expression for the dispersed heat is as follows.…”

Improvement of radon detector performance by using a large-sized PIN-photodiode

“…However, the sensitivity of this method is limited by the inefficiency of the Rn detector in a humid gas after extracting Rn gas from the purified water. Another approach using both an extraction column and a cooled charcoal trap is adapted at the expense of real-time measurement instead of the membrane degasifier module [20]. By replacing the large PIN-photodiode and the newly developed membrane degasifier module with a low intrinsic background [39], the sensitivity of the Rn detector combined with the membrane degasifier module is expected to be improved.…”

“…The current design of this detector, whose inner volume is approximately 80 litter (hereinafter L), has been used to measure Rn concentrations less than a few mBq/m 3 in several gases [14,15]. Recently, we applied this detector to measure the Rn concentration in purified water in the Super-Kamiokande detector by combining it with a Rn extraction column and cooled charcoal [20] and to evaluate several absorbents for removing Rn from purified gases [21,22].…”