Cosmic ray produced Mg²⁸, Si³¹, S³⁸, C^l38, Cl^34m and other short-lived radioisotopes in wet precipitation

Abstract

The concentrations of seven radioisotopes, expected to be produced in the troposphere by interactions of secondary cosmic rays with atmospheric nuclei, have been measured in "fresh" rain collections. The half-lives of these isotopes range from about half an hour to a day. The procedures developed for rapid, specific and sensitive analyses of these nuclides are discussed. Detection of two of the isotopes studied, Cl³⁹ (half-life: 55 mins.) and Na²⁴ (15 hrs.), has been reported earlier by Winsberg and Rodel respectively. Amongst the remaining nuclides, two: S³⁸ (2.9 hrs.) and Cl³⁸ (37.3 mins.) were independently and almost simultaneously detected by us and Perkins and his collaborators. Three other isotopes, Cl^34m (32 mins.), Si³¹ (2.6 hrs.) and Mg²⁸ (21.2 hrs.), detected in the present work have not yet been reported elsewhere. The nature of cosmic ray secondary particles responsible for the production of these short-lived radionuclides in the troposphere is discussed. Isotope production is found to vary strongly with altitude in the troposphere; it increases by a factor of two every 1.5-2 km depending on the radioisotope under question. This fact combined with the availability of several isotopes of half-lives ranging from about half an hour to a day leads to the possibility of using them as tracers for studying short-term tropospheric processes, e.g. those occurring prior to and during condensation in a precipitating cloud. The implications of the present measurements are discussed.