Erhöhung der Ortsdosisleistung auf den Inseln Norddeutschlands durch Schwermineralanreicherungen

At the coast of the East- and North-Frisian Islands an enrichment of heavy sand grains with high heavy mineral concentrations results from water and wind effects. The development of heavy mineral placers is known since the beginning of the last century. In the heavy grains resulting from the boulder clay of the glacial period, the U 238- and Th 232-concentrations are partly rather high. Therefore, local dose rates due to gamma radiation result which are higher than those normally measured in northern Germany. They change from year to year because of sea and wind effects. The highest measured local dose rates in approx. 1 m height have been approx. 300 mrem/a on the islands of Norderney and Sylt. On the island of Norderney the local dose rate at the surface increased up to 500 mrem/a. If the heavy mineral placers were larger, local dose rates of up to approx. 1000 mrem/a could occur in 1 m height. The highest measured specific U 238-activity was approx. 18 pCi/g and the highest measured specific Th 232-activity was approx. 30 pCi/g. Using of magnetic separators fractions with essentially higher specific activities can be separated from the sand. The ratio of the specific activities of U 238 and Th 232 in the sand of the East- and North-Frisian Islands reflect the different basic material of the sand. At the south-west coast of India where the local dose rates are very high, the same enrichment effect exists. In the sand of South-India theU 238- and the Th 232-actitivity is, however, bound at monazite (Ce, La [PO4 ) contrary to the sand at the coast of northern Germany. At he coast of the East- and North-Frisian Islands no persons live on areas with enriched heavy minerals contrary to the south-west coast of India. The received dose of the tourists is small because the residence time on the areas with enriched heavy minerals is generally short and the local dose rate is not so much enhanced