Preliminary Experiments Using a Passive Detector for Measuring Indoor 220Rn Progeny Concentrations with an Aerosol Chamber

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This paper describes preliminary experiments using a passive detector for integrating measurements of indoor thoron (220Rn) progeny concentrations with an aerosol chamber. A solid state nuclear detector (CR‐39) covered with a thin aluminum-vaporized polyethylene plate (Mylar film) was used to detect only alpha particles emitted from 212Po due to 220Rn progeny deposited on the detector surfaces. The initial experiment showed that Mylar film with area density of more than 5 mg cm−2 was suitable to cut off completely alpha particles of 7.7 MeV from 214Po of 222Rn progeny decay. In the experiment using the passive detector, it was observed that the net track density increased linearly with an increase of time-integrating 220Rn progeny concentration. As a result of dividing deposition rates by atom concentrations, the deposition velocity was given as 0.023 cm s−1 for total 220Rn progeny. The model estimates of deposition velocities were 0.330 cm s−1 for unattached 220Rn progeny and 0.0011 cm s−1 for aerosol-attached 220Rn progeny using Lai-Nazaroff formulae. These deposition velocities were in the same range with the results reported in the literature. It was also found that the exposure experiments showed little influence of vertical profiles and surface orientations of the passive detector in the chamber on the detection responses, which was in good agreement with that in the model estimates. Furthermore, it was inferred that the main uncertainty of the passive detector was inhomogeneous deposition of 220Rn progeny onto its detection surfaces.

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