Determination of muon attenuation lengths in depth profiles from in situ produced cosmogenic nuclides

Cosmogenic nuclides are important tools to understand and quantify the processes that control the development and evolution of landscapes during the quaternary. Among all published studies, few are related to the accurate and precise determination of the physical parameters governing their production in the Earth's crust surface (in situ produced cosmogenic nuclides) and its evolution as a function of depth below the Earth's surface. Currently, it is nearly impossible to advocate global parameters that could be used worldwide. Indeed, at each sampling site, not only the geometry and the mineralogy will differ but also their evolution as a function of depth. In this paper, a new approach based on the measurement of the evolution of cosmogenic nuclide concentrations along depth profiles to determine the muon attenuation lengths is proposed. Contrarily to previous studies that used to describe both slow and fast muons, only one type of muons will be considered in this paper and nuclide accumulation at depth will be described by a single exponential. The determined attenuation length integrates the potential effect of the chemical composition of the overlying matrix and takes into account the entire energy range of the incident particles. Additionally, when denudational steady state is reached, muon contributions can be determined. When scaled to sea level, these contributions appear to be comparable for a given nuclide whatever the site where they have been determined. The average weighted muon contributions are 0.028 +/- 0.004 atoms g(-1) a(-1) for Be-10, 0.233 +/- 0.045 atoms g(-1) a(-1) for Al-26 and 1.063 +/- 0.329 atoms g(-1) a(-1) for Cl-36 and are valid within the depth range 0-6500 g cm(-2). (C) 2012 Elsevier B.V. All rights reserved.