National survey of indoor thoron concentration in FYR of Macedonia (continental Europe – Balkan region)

As a part of the national survey of natural radioactivity in The Former Yugoslav Republic of Macedonia, indoor thoron gas concentration was measured in 300 dwellings during one year, from December 2008 till December 2009 using passive discriminative radon–thoron detectors. Detectors were deployed at a distance of >50 cm from walls in order to be less sensitive to distance from walls. Altogether 532 measurements were performed: 53 in winter, 57 in spring, 122 in summer and 300 in autumn. The frequency distribution is well described by a log-normal function. The geometric means of indoor thoron concentration (with geometric standard deviations in brackets) in winter, spring, summer and autumn were obtained to be: 39 Bq m−3 (3.4), 32 Bq m−3 (2.8), 18 Bq m−3 (2.8), 31 Bq m−3 (2.9), respectively. Seasonal variations of thoron appear lower than those of radon. The seasonal corrected annual mean concentration ranges between 3 and 272 Bq m−3 with a geometric mean of 28 Bq m−3 (2.12). A detailed statistical analysis of the geogenic and building factors which influence the indoor thoron concentration is also reported. This survey represents the first national survey on indoor thoron in continental Europe.