The 54Mn Clock and Its Implications for Cosmic-Ray Propagation and Fe Isotope Studies

Radioactive Mn-54 suggested as a 'clock' for measuring the lifetime of heavy cosmic rays, has a poorly known beta-decay half-life estimated to be in the range from 10 exp 5 to 10 exp 7 yr. Some years ago Koch et al. (1981) concluded from measurements of the Mn/Fe ratio that a significant fraction of low-energy Mn-54 produced by Fe fragmentation had decayed. Using a propagation code that includes improved fragmentation cross-sections, and recent data from HEAO 3 and a number of other spacecraft, the evidence for Mn-54 decay in cosmic rays has been reexamined. It is concluded that present cosmic-ray data cannot establish the degree of Mn-54 decay, but point out that this question has important implications for studies of the Fe-54 abundance in cosmic-ray source material, as well as for cosmic-ray propagation studies.