Search for theβ+decay ofMn54

We have performed a search for the ${\mathrm{\beta }}^{+}$ decay of ${}_{}{}^{54}{}_{}{}^{}\mathrm{Mn}$ by looking for back-to-back 511-keV γ rays in two high-purity Ge detectors. No excess of events above background was observed, and a limit of 5.7×${10}^{\mathrm{-}7}$% has been established for the ${\mathrm{\beta }}^{+}$ branch. The significance of this result for the use of ${}_{}{}^{54}{}_{}{}^{}\mathrm{Mn}$ as a cosmic ray chronometer is discussed.