The $\beta$-decay half-life of ${}^{54}\mathrm{Mn}$ is needed to employ this isotope as a cosmic ray chronometer. We have determined the partial half-life of ${}^{54}\mathrm{Mn}$ for positron emission by counting a highly purified $35-\mu \mathrm{Ci}$ source of ${}^{54}\mathrm{Mn}$ 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 $24\pm 10$ back-to-back 511-511 keV coincident events. Based on this result, the branch for this decay mode is $(2.2\pm 0.9)\times {10}^{-7\mathrm{}}\%$. The implications of this result for the ${}^{54}\mathrm{Mn}$ cosmic-ray chronometer problem are discussed.

• Received 29 July 1997

DOI:https://doi.org/10.1103/PhysRevLett.79.4306