Jochen Tschiersch

Experiments have been carried out using the deposition-based Direct Thoron Progeny Sensors (DTPS) in a thoron experimental house. The objective was to study the thoron decay product characteristics such as the deposition velocities, spatial variability and dependence on aerosol particle concentrations. Since the deposition velocity is an important characteristic in the calibration of the DTPS, it is very important to study its dependence on aerosol concentration in a controlled environment. At low aerosol concentration (1500 particles/cm ³ ) the mean effective deposition velocity was measured to be 0.159 ± 0.045 m h ⁻¹ ; at high aerosol concentration (30 000 particles/cm ³ ) it decreased to 0.079 ± 0.009 m h ⁻¹ . The deposition velocity for the attached fraction of the thoron decay products did not change with increasing aerosol concentration, showing measurement results of 0.048 ± 0.005 m h ⁻¹ and 0.043 ± 0.014 m h ⁻¹ , respectively. At low particle concentration, the effective deposition velocity showed large scattering within the room at different distances from center. The attached fraction deposition velocity remained uniform at different distances from the wall. The measurements in the thoron experimental house can be used as a sensitivity test of the DTPS in an indoor environment with changing aerosol concentration. The uniform spatial distribution of thoron decay products was confirmed within the experimental house. This indicates that direct measurement of thoron decay product concentration should be carried out instead of inferring it from thoron gas concentration, which is very inhomogeneous within the experimental house.

Increasing attention has been paid in recent years to the radioactive gas thoron (²²⁰Rn), which can cause a significant exposure and increase of lung cancer risk in some regions worldwide. Some experiments were designed to examine whether different types of wall decoration in the room, from ordinary newsprint to commercial wallpaper, can mitigate the concentrations of indoor thoron decay products. Decoration with coated paper was very effective in decreasing the thoron decay products concentration, thus reducing the effective dose by 90 %, while newsprint decoration, which is common in many rural parts of the world, was found to have a smaller but still significant effect in reducing the thoron decay products concentration when applied to the same house.

The importance of ²²⁰Rn (thoron) progeny for human exposure has been widely recognised in the past decades. Since no stable equilibrium factor was found between indoor thoron and its progeny, and the concentration of thoron progeny varies with time, it is necessary to develop detectors for long-term measurement that directly sample and detect thoron progeny. However, power supply of this kind of detectors has always been a problem. In this study, a set of device that is suitable for long-term measurement is introduced. A high-voltage electric field was formed for the collection of charged aerosols attached by ²²²Rn (radon) and thoron progenies on solid-state nuclear track detector. Impact from radon progeny could be eliminated with a shield of Al foil of appropriate thickness. Tests were made both in an experimental house and in a thoron chamber in Helmholtz Zentrum München to determine the parameters and to verify the universality under different conditions.

A thoron experimental house was constructed in a laboratory room of Helmholtz Zentrum München to perform exposure studies of thoron and its decay products under controlled conditions. The single room house (7.1 m³) was built from unfired clay stones and clay plaster. For the plaster of the inner side, the clay was mixed with granite powder enriched with ²³²Th. The thoron inventory increased by this means to about 1700 Bq and the progeny potential alpha energy to 130 µJ inside the room. The instrumentation of the experimental house includes active and passive devices for thoron and thoron decay product measurement including attached and unattached progeny, for aerosol particle number and size measurement and characterisation of the climatic conditions. Various parameters as ventilation rate and aerosol concentration can be adjusted. Experiments performed in the experimental house demonstrate the experimental power of this new tool for indoor thoron exposure assessment.