Cosmogenic nuclide production rate systematics in terrestrial materials: Present knowledge, needs and future actions for improvement

Abstract

The growing applications of cosmogenic nuclides produced in the Earth’s atmosphere, and in situ in a variety of terrestrial materials, as tracers in a wide ranging Earth science problems, has put a greater demand on accurate determination of the nuclide production rates in a variety of targets exposed in different settings on the Earth in the troposphere. The present state of our knowledge of cosmogenic production rates is reviewed briefly, in conjunction with the phenomenological and theoretical framework for: (i) the cosmic ray flux incident in the near Earth environment, (ii) the nucleonic cascade set off in the Earth’s atmosphere by the primary cosmic radiation, and (iii) the rate for production of nuclides in terrestrial materials, in widely different settings. These considerations set the stage for the diverse questions, which must be taken into account for determining the source functions of isotopic changes in terrestrial materials. We discuss the different approaches which have been adopted earlier to obtain the source strengths of nuclear interacting particles of the cosmic radiation, and direct measurements of nuclide production rates made by exposing targets to cosmic radiation at sea level and at mountain altitudes. We show that rapid progress in determining nuclide source functions with sufficient information on temporal variability is indeed expected in the near future as a result of: (i) dramatic improvements in the past 2–3 decades in our understanding of the character of propagation of cosmic radiation within the heliosphere, (ii) experiments now being conducted by a few groups to determine the source strengths of cosmic ray slow neutrons, and nuclide production rates in cosmic ray exposed targets, and finally (iii) the emergence of better nuclear codes which deal with the development of nucleonic cascades in the Earth’s atmosphere.