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}hbox {Bi}, and {}^{210}hbox {Po}, easily plate out onto surfaces exposed to radon-loaded air. The alpha emitter {}^{210}hbox {Po} is particularly dangerous for detectors searching for weakly-interacting dark matter particles. Neutrons produced via (upalpha , 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} 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} specific activity in air achieved by the device is 0.05,hbox {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.
Highlights
IntroductionSurface alpha background, such as 210Po, can be a dominant contributor to the background for direct dark matter experiments
After calibration, the detector background was measured for two different operating modes
Radon concentration has been measured sequentially from the air immediately at the output of the Ateko radon abatement system (Ateko air ) and the air from both clean rooms following the installation of the radon monitor
Summary
Surface alpha background, such as 210Po, can be a dominant contributor to the background for direct dark matter experiments. A radon detector based on this technique was built for continuous monitoring of the air of the two clean rooms in which the DarkSide-50 TPC was assembled and commissioned. In addition to the usual clean room procedures, staff members entering CR1 and CRH are required to station in air-shower vestibules via software-controlled interlocks for long enough to allow sufficient lung volume air exchange and flushing of the air volumes mixed with outside air with low-radon air before access to the main rooms. Both DARKSIDE-50 clean rooms, supplied with 222Rn (and 220Rn) depleted air, were instrumental in meeting the background requirements of DARKSIDE-50. The system was calibrated for sampling argon and nitrogen used by the DARKSIDE-50 experiment
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