Laboratory facility to create reference radon + thoron atmosphere under dynamic exposure conditions

Radon (²²²Rn) and thoron (²²⁰Rn) levels in the environment are typically subject to significant random and systematic variations. Creation in the laboratory of reproducible and controlled exposure conditions close to that in the real environment can be useful for testing ²²²Rn and ²²⁰Rn detectors and for research. In this report the design and performance of a novel laboratory facility with such functionality is presented. The facility allows the exposure of detectors under controlled dynamic as well as static activity concentrations of ²²²Rn and ²²⁰Rn (pure and mixed) and temperature. The temperature is measured and regulated within -15 °C ÷ +60 °C by a dedicated programmable thermostat. Different reference activity concentrations in the exposure vessel are made by regulating the flow-rate of the air that flushes ²²²Rn/²²⁰Rn activity from the sources towards the exposure vessel. Reference atmospheres that contain ²²²Rn, ²²⁰Rn or a specified ratio of the two can be created. Pilot experiments that demonstrate the feasibility of the approach are presented. They include follow-up of a pre-defined temperature profile (in the range -5 °C ÷ +35 °C), test of the correspondence between planned and measured ²²²Rn and ²²⁰Rn activity concentrations, follow-up of a pre-defined dynamic profile of ²²⁰Rn concentrations and test of the possibility to create mixed ²²⁰Rn/²²²Rn atmospheres (experimentally checked for ratio of the activity concentrations from 0.27 to 4.5). The results from the experimental tests are in agreement with the values obtained by the developed theoretical model. The proposed approach can be used to plan and create stationary and dynamic reference exposure conditions that are close to the real exposure regimes in the environment.