Galactic confinement time of iron-group cosmic rays derived from the Mn-54 chronometer

The beta-decay half-life of Mn-54 is needed to employ this isotope as a cosmic ray chronometer. We have determined the partial half-life of Mn-54 for positron emission by counting a highly purified 35-mu Ci source of Mn-54 in GAMMASPHERE to search for the astrophysically interesting beta(+) decay branch through the observation of coincident positron annihilation gamma rays. A careful analysis of 97 hours of source counting and 61 hours of background shows a net signal of 24 +/- 10 back-to-back 511-511 keV coincident events. Based on this result, the branch for this decay mode is (2.2 +/- 0.9) x 10(-7)%. The implications of this result for the Mn-54 cosmic-ray chronometer problem are discussed.