The positron decay partial half-lives of ${}_{}{}^{143}{}_{}{}^{}\mathrm{Pm}$ and ${}_{}{}^{144}{}_{}{}^{}\mathrm{Pm}$ are needed to assess the viability of elemental Pm as a cosmic-ray clock. We have conducted experiments to measure the ${\mathrm{\beta }}^{+}$ branches of these isotopes; we find ${\mathrm{\beta }}^{+}$ branches of <5.7×${10}^{\mathrm{-}6}$% for ${}_{}{}^{143}{}_{}{}^{}\mathrm{Pm}$ and <8×${10}^{\mathrm{-}5}$% for ${}_{}{}^{144}{}_{}{}^{}\mathrm{Pm}$. Although these branches are a factor of 20 lower than the previous experimental limits, the resulting partial half-lives are still too uncertain to permit any firm conclusions.

• Received 14 March 1994

DOI:https://doi.org/10.1103/PhysRevC.50.728