High thoron exp. in loam houses, thorium-containing building material

In opposition to radon no statutory regulations are applied for the exposure to thoron daughter products. It should be noticed that, assuming the same source strength, the biological impact of the thoron daughters exceeds the one of the radon daughters significantly. Various kinds of loam but also other building materials contain, either by their nature or by additives, large amounts of thorium.

Within the thorium decay chain appears the radon isotope Rn-220 which is frequently called thoron with respect to its origin. Due to its rare gas nature it can emanate easily from the large surfaces of the building structures. In most cases it will be present only in the direct surrounding of the surfaces because of the short half-life of about one minute. However, the daughter nuclide Pb-212 has a half-life of more than ten hours which allows its distribution and accumulation in the room. Unlike radon, high thoron daughter product concentration can occur without the presence of thoron. That means, it is not possible to estimate the thoron daughter product concentration by the measurement of thoron gas. Therefore, the daughter products must be measured directly to achieve realistic results.

The measurement of the thoron daughter products is based on the collection of the daughters on a filter using a sampling pump. To achieve a fast response in case of concentration changes, the change of the filter activity between two sequential intervals needs to be determined. The duration of the interval should be chosen as long as acceptable to minimize the detection limit. A good approach could be a two hours interval which is related to the response time of the radon (Rn-222) daughter product measurement.

For measurements of the thoron daughter product concentration, the monitors EQF 3200 and EQF 3220 are a good choice. These instruments combine gas and daughter product measurement in one unit. For exclusive daughter product measurements the usage of the poCAMon or the Aer 5000 is recommended. The Aer 5000 offers a very low detection limit even at short sampling intervals due to its high flow rate and the automatic filter replacement.