Development of new radon monitoring systems in the Kamioka mine

Abstract

Radioactivity from radon is a major threat for high-precision low-energy physics experiments like the ones in the Kamioka mine. We developed a new high-sensitivity radon monitoring system and conducted systematic radon concentration measurements for the first time in Kamioka. The system consists of portable radon detectors with a capacity of 1 L and new electronics based on a Raspberry Pi [Raspberry Pi documentation. (Available at: https://www.raspberrypi.org/documentation/)]. These radon detectors measure the radon in the air with electrostatic collection and a PIN photo-diode. We measured the absolute humidity dependence of the 1-L radon detector for air as CF(AH) = (12.86 ± 0.40)− (1.66 ± 0.19)√AH (counts/day)/(Bq/m3). The background level of the 1-L radon detector is 0.65 ± 0.15 (stat.) counts/day. This corresponds to a detection limit of ∼ 0.4 Bq/m3 in a one-day measurement. Data were collected for a period of more than one year with twenty-one 1-L radon detectors in the Kamioka mine. They indicate seasonal and day-night variations in radon concentration within the mine. These results also allow us to confirm the stability of the new Raspberry Pi electronics.