THE MN-54 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 approximately 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 (< 1 GeV/nucleon) 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, we have reexamined the evidence for Mn-54 decay in cosmic rays. We conclude 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.