An online radon monitor for low-background detector assembly facilities

Abstract

Abstract Backgrounds from long-lived radon decay products are often problematic for low-energy neutrino and rare-event experiments. These isotopes, specifically $${}^{210}\hbox {Pb}$$ 210 Pb , $${}^{210}\hbox {Bi}$$ 210 Bi , and $${}^{210}\hbox {Po}$$ 210 Po , easily plate out onto surfaces exposed to radon-loaded air. The alpha emitter $${}^{210}\hbox {Po}$$ 210 Po is particularly dangerous for detectors searching for weakly-interacting dark matter particles. Neutrons produced via ( $$\upalpha $$ α , n) reactions in detector materials are, in some cases, a residual background that can limit the sensitivity of the experiment. An effective solution is to reduce the $${}^{222}\hbox {Rn}$$ 222 Rn activity in the air in contact with detector components during fabrication, assembly, commissioning, and operation. We present the design, construction, calibration procedures and performance of an electrostatic radon detector made to monitor two radon-suppressed clean rooms built for the DARKSIDE-50 experiment. A dedicated data acquisition system immune to harsh operating conditions of the radon monitor is also described. A record detection limit for $${}^{222}\hbox {Rn}$$ 222 Rn specific activity in air achieved by the device is $$0.05\,\hbox {mBqm}^{-3}$$ 0.05 mBqm - 3 (STP). The radon concentration of different air samples collected from the two DARKSIDE-50 clean rooms measured with the electrostatic detector is presented.