LARGE, LOW-BACKGROUND WELL-TYPE DETECTORS FOR MEASUREMENTS OF ENVIRONMENTAL-RADIOACTIVITY

During the two last decades, the development of gamma counting techniq ues has led to the application of natural radioactivity to the study o f numerous processes occurring in the environment. Such work flourishe d particularly when large HP germanium detectors with low background b ecame available. For some studies, the sample to be measured is small (e.g. sediment, BaSO4 precipitate). In such cases, the use of high eff iciency well-type detectors, when compared with similar detectors of c onventional shape, provides the best choice. This paper describes thre e large, well-type detectors used for environmental studies. The backg round of the crystals has been lowered by: i) placing the detectors in an underground laboratory (Laboratoire Souterrain de Modane, LSM), sh ielded from cosmic radiation by 1700 m of rocks; ii) using only very l ow activity materials for the detector assembly and copper-lead shield . Results from various oceanic samples such as particles, sediments, w ater and hydrothermal sulphides are presented to illustrate the useful ness of this kind of detector for the measurement of environmental rad ioactivity. For sample volumes of about 10 cm(3), these installations are shown to be the most sensitive way to measure nuclides as Ra-226, Ra-228, Th-228 and Cs-137.