CURRENT RESEARCH ON NUCLEAR TRACKS IN SOLIDS AT J-STEFAN-INSTITUTE

A brief review of the current basic and applied research in the field of nuclear tracks at the J. Stefan Institute is summarized and illustr ated by some new selected results. To study the interactions of radon and its daughters with solid surfaces a new method was developed and a pplied for the measurement of the deposition rate of radon daughters o n different materials. The method is based on a combination of a high- sensitivity (10 kBq m-3) autoradiographic technique, which enables loc alization of a single atom adsorbed on a solid surface and a high reso lution digital spectroscopic technique, which enables discernment amon g background, radon alpha particle, and polonium alpha particle tracks . To increase the sensitivity of passive methods for radon measurement s a new dosimeter based on a combination of charcoal (which collects r adon from the air) and the CR-39 detector (which simultaneously regist ers alpha particles) was designed. Its sensitivity (20 Bq m-3) for sho rt exposure time (3 h) was found to be two orders of magnitude higher in comparison with theoretically attainable standard etched track dosi meters. An etched track radon dosimeter is widely used for indoor and outdoor radon survey monitoring. An average radon concentration in Slo venian homes was found to be approximately 130 Bq m-3. In some materia ls, such as gelatine, an irradiated zone at high fluences of charged p articles (10(12)-10(15) cm-2) can be transformed to a stable relief st ructure by a non-etching process, by soaking in pure water at room tem perature. A search for the mechanism which could explain this phenomen on and an analysis of new applicability to radiation measurements and ion beam microtechnology may lead to the development of new dosimeters for high fluences of charged particles (10(12)-10(15) cm-2) and neutr ons (> 10(16) cm-2) as well as development of a new lithographic techn ique with the depth/width ratio of the structure > 10(5). In the old a rsenic mine in Allchar (Macedonia), which contains the mineral lorandi te (TlAsS2), experiments started a few years ago to estimate the flux of solar neutrinos from the amounts of Pb-205 isotope in the ore induc ed by Tl-205 (nu, e-) Pb-205 reaction. A good knowledge of the thermal neutron flux is needed to evaluate the rate of the contribution to ba ckground events through the neutron capture reaction Pb-204 (n, gamma) Pb-205. In the frame of this project thermal neutron flux and radon c oncentration are measured by a CR-39 detector in combination with a bo ron radiator and an etched track radon dosimeter. Our preliminary resu lts showed that the thermal neutron flux and radon concentration in th e mine are (2-5) x 10(-3) cm-2 s-1 and approximately 1.5 x 10(4) Bq m- 3, respectively. The further improvements of the CR-39/B-10 neutron do simeter and its application to the safeguard verification in neutron m easurements of spent fuel assemblies and neutron personal dosimetry sh owed that thermal neutron fluence as small as 2 x 10(3) cm-2 can be me asured. A new promising technique for charged particle spectroscopy ba sed on the simultaneous measurements of surface track size parameters (area, minor axis) and grey level value averaged over a single track u sing a microcomputer controlled system is developed.