An helicopter-borne system for radiological assessment

Industrial sites having nuclear activities have to be monitored to detect any contamination. Natural radioactivity of every site has to be known in order to be able to detect any extra radioactivity. CEA DAM has an helicopter-borne gamma spectrometer (named helinuc) that can make a radiological mapping of an area from tens to hundreds of square kilometers in a few hours. The helicopter flies at a height of 50 m with a speed of 70 km/h. It has been shown that some temporal variations of the mapping were not due to radioactivity variations but to variations in ground moisture or in air concentration of radon. A 14 month long campaign combining the measurement of the radioactivity at a top of a 50 m high pole and the measurement of other parameters like ground moisture, atmospheric concentration of radon, has shown, for instance, that an increase of 25% of ground moisture implies a decrease of 25% of the gamma signal. This study has enabled us to draw a corrective protocol in order that the radiological mapping can be consistent whatever time of the year. (A.C.)