Natural gamma radiation in a Norwegian area rich in thorium

Results of indoor and outdoor gamma radiation measurements in a thorium-rich Norwegian area are reported, together with activity concentration measurements on rock samples from the area. The indoor to outdoor dose rate ratio in wooden houses decreased with increasing outdoor gamma dose rate, suggesting that the outdoor dose rate has less influence on the indoor dose rate than the activity concentrations in the concrete floor. Three houses were built of stone material from the area. Indoor gamma dose rates in these houses were measured as being between 500 and 1000 nGy.h-1. The annual effective dose equivalent from gamma radiation to the inhabitants in the area was estimated to be 2.1 mSv.y-1 on the basis of the measurements in and around wooden houses. (author)