Heavy ion radioactivity studies using the fossil tracks in minerals

Due to the difficulties to produce a large area source of heavy ion emitter radionuclides, the authors tried to obtain information about natural heavy ion radioactivity using the fossil tracks registered during geological time in minerals with crystalline layer. The U and Th impurities in minerals represent the heavy ion source (especially U and Th inclusions are considered). Three minerals - apatite, biotite mica and muscovite mica - were studied from the following points of view: - the U and Th contents; - the easiness to distinguish the tracks; - the possibility to discriminate the tracks of different particles such as: spallation recoils, heavy ions, fission fragments, interactions with mineral constituents and crystalline layer dislocations; the registration of heavy ions predicted to be emitted by U and Th, i.e., 24,26 Ne, 28,30 Mg and 32,34 Si nuclides; - the possibility to study a large mineral area at low magnifications by optical microscopy. From the three minerals, the muscovite mica was chosen. The muscovite mineral track detector, etched for the visualisation of the fossil tracks, has been studied in order to determine the branching ratio λCl/λSF. Using the optical microscopy, the fossil tracks of heavy ions and spontaneous fission fragments were counted. These tracks were distinguished from each other by their specific track patterns. In order to determine this ratio, the following problems have to be solved: - calibration of muscovite mineral detector for the heavy ions of theoretically predicted mass, atomic number and energy (A, Z, E). A HV-FN Van de Graaff tandem accelerator was used to obtain the beams of nearly heavy ions; - discrimination of tracks which belong to different registered particles by using the track patterns; - establishing of possible track origins in muscovite mica; - preserving the track patterns at high etching durations. For good statistics, a large muscovite detector area must be studied. Up to present an area of 4680 cm2 of muscovite detector was studied. (authors)