${\beta }^{+}$ Decay Partial Half-Life of ${}^{54}\mathrm{Mn}$ and Cosmic Ray Chronometry

The weak ${\beta }^{+}$ decay of the astrophysically significant radioisotope ${}^{54}\mathrm{Mn}$ has been observed. The energies of positrons from a chemically purified ${}^{54}\mathrm{Mn}$ source were measured using the APEX spectrometer at Argonne National Laboratory. We deduce a ${\beta }^{+}$ decay branch of $(1.20\pm 0.26)\times {10}^{-9\mathrm{}}$, corresponding to a partial half-life of $(7.1\pm 1.5)\times {10}^8\mathrm{yr}$. The implications of this value for cosmic-ray confinement times are discussed in light of recent satellite measurements of the cosmic-ray abundance of ${}^{54}\mathrm{Mn}$.