The ^(54)Mn Clock and Its Implications for Cosmic Ray Propagation and Fe Isotope Studies

Abstract

Radioactive ^(54)Mn, suggested as a clock for measuring the lifetime of heavy cosmic rays, has a poorly known B-decay halflife estimated to be in the range from ~10^(5) to 10^(7) yr. Some years ago Koch et al. concluded from measurements of the Mn/Fe ratio that a significant fraction of low-energy (<l GeV/nuc) ^(54)Mn produced by Fe fragmentation had decayed. Using a propagation code that includes improved fragmentation cross-sections, and data from HEA0-3 and other spacecraft, we have re-examined the evidence for ^(54)Mn decay. We conclude that present cosmic ray data cannot distinguish the possibility of ^(54)Mn decay in cosmic rays, but point out that this question has important implications for studies of the ^(54)Fe abundance in cosmic ray source material, as well as for cosmic ray propagation studies.