We have discovered an arithmetic error in our Letter affecting the calculation of the positron branching ratio and partial half-life of the 54 Mn decay. The net signal of 24 6 10 events deduced from our experiments leads to the corrected values of ͑1.8 6 0.8͒ 3 10 27 % for the branching ratio of the b 1 decay, 14.6 for the log ft value, and 4.7 3 10 8 yr for the partial half-life. Using the same assumptions stated in our paper, we deduce the b 2 branching ratio to be 0.9 3 10 24 % and the cosmic-ray half-life of 54 Mn to be 9.3 3 10 5 yr. Although these values differ slightly from the corresponding ones quoted in our Letter, our conclusions regarding the applicability of 54 Mn as a cosmic-ray chronometer remain unaffected by these changes.

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The positron decay partial half-life of 56 Ni is needed to employ this isotope as a cosmic-ray chronometer. We conducted an experiment by counting a purified 2.8-Ci source of 56 Ni in GAMMASPHERE in order to search for the ␤ ϩ -decay branch of this isotope. A plastic scintillator was used to measure the energy of positrons in coincidence with the positron-annihilation ␥ rays and the characteristic 158-keV ␥ ray line. A careful analysis of 96 h of source counting shows no net signal and results in an upper limit of 77 counts of 511-511-158 keV plus scintillator coincident events. From this result we establish a 1 upper limit on the branch for this decay mode to be (6.3ϫ10 Ϫ5 )%. The discrepancy between the outcome of this experiment and previous measurements of this branch and the implications of this result for the 56 Ni cosmic-ray chronometer problem are discussed. ͓S0556-2813͑99͒01306-0͔

The b-decay half-life of 54 Mn is needed to employ this isotope as a cosmic ray chronometer. We have determined the partial half-life of 54 Mn for positron emission by counting a highly purified 35-mCi source of 54 Mn in GAMMASPHERE to search for the astrophysically interesting b 1 decay branch through the observation of coincident positron-annihilation g rays. A careful analysis of 97 hours of source counting and 61 hours of background shows a net signal of 24 6 10 back-to-back 511-511 keV coincident events. Based on this result, the branch for this decay mode is ͑2.2 6 0.9͒ 3 10 27 %. The implications of this result for the 54 Mn cosmic-ray chronometer problem are discussed.[S0031-9007 (97)04630-9] PACS numbers: 98.70.Sa, 26.40. + r, 27.40. + zIn the leaky-box model, cosmic rays (CR) propagate through our galaxy along complex paths as a result of scattering by random magnetic fields. Eventually this random walk leads the CR out or the galaxy. The mean confinement time of CR within our Galaxy can be determined by comparing the CR abundances of suitably long-lived radioactive isotopes with those of their stable neighbors. Radioisotopes that have been used for determinations of the CR confinement time include 10 Be (t 1͞2 1.6 Myr [1,2,3]), 26 Al (t 1͞2 0.87 Myr [4,5]), and 36 Cl (t 1͞2 0.3 Myr [6]). Measurements of the abundances of these isotopes lead to CR confinement times in the range 10-20 Myr and imply a mean density of interstellar matter traversed by the CR of approximately 0.3 atoms͞cm 3 . This is substantially lower than the mean density found in the galactic disk, suggesting that the CR spend a substantial amount of time in the halo of our galaxy.In addition to these light elements, it is of critical interest to understand the confinement time of the nuclei of the iron group, which are produced in explosive nuclear burning. Cassé [7] suggested that 54 Mn, which is a product of spallation of iron nuclei on interstellar hydrogen, might serve as a CR clock. Grove et al. [8] have emphasized the importance of knowledge of the 54 Mn half-life for understanding CR propagation. The 53,54,55 Mn spallation production cross sections from 56 Fe at an energy of 600 MeV͞nucleon were measured by Webber et al.[9] to be 37.5, 42.3, and 40.0 mb ͑66 mb͒, respectively. Thus, one might expect their cosmic-ray abundances to be nearly equal. However, recent measurements of cosmic-ray manganese by Leske [10] and DuVernois [11] show that the abundance of 54 Mn is much smaller than that of its neighboring isotopes. The decay of CR 54 Mn can possibly account for this discrepancy and thus provide the crucial datum point for the confinement time of iron-group nuclei.

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