T-Rex: the transportable radon-222 explorer for environmental or public health studies

We describe the development and testing of a "suitcase sized" 100 L two-filter dual flow-loop radon monitor, weighing ∼18 kg, flow rate ∼5 L min^-1, Rn-222 sensitivity ∼0.02 s^-1∙(Bq∙m^-3)^-1, temporal resolution 30-min, and operational range ∼0.3 - 30,000 Bq∙m^-3. Its portability and sensitivity make it an ideal "crossover instrument" between the needs of the Public Health and Environmental research communities. Pending traceable calibration in a controlled climate chamber, here its performance was compared with a 200 L two-filter monitor (1.65 m tall, ∼80 kg), sampling outdoor air over 3 weeks during which radon varied between 0.14 Bq∙m^-3 and 9.1 Bq∙m^-3. The distribution (10th/50th/90th percentile) of absolute hourly bias was 0.03/0.19/0.54 Bq∙m^-3. A separate comparison was made with an AlphaGUARD PQ2000 Pro and DF2000 over 8 days during which radon ranged from 0.6 Bq·m^-3 to 9.9 Bq·m^-3. The reported 8-day average radon was 3.5 ± 1.97 Bq·m^-3 (100 L monitor), 4.01 ± 2.92 Bq·m^-3 (PQ2000 Pro) and 4.24 ± 2.65 Bq·m^-3 (DF 2000). Daily mean absolute bias reached 1.7 Bq·m^-3, and absolute bias of 6-hourly means reached 3.2 Bq·m^-3. For the PQ2000 and DF 2000, hourly counting uncertainty reached 30 % at 6 Bq·m^-3 and 4.5 Bq·m^-3, respectively (c.f. manufacturers reported detection limit of 2 Bq·m^-3). The 100 L monitor reached 30 % counting uncertainty at 0.3 Bq·m^-3. While this does not meet the measurement criteria for Global Atmosphere Watch "baseline" monitoring (detection limit ≤0.05 Bq∙m^-3), the monitor would be well suited for many environmental monitoring purposes and portable enough for indoor use.