In a series of experiments, we have reinvestigated the decay of the doubly magic nucleus ${}_{}{}^{56}{}_{}{}^{}\mathrm{Ni}$, which is believed to be copiously produced in supernovae. We have confirmed its previously known decay scheme and half-life, and have searched for several rare decay modes. We establish an upper limit of 5.8×${10}^{\mathrm{-}7}$ for the branching ratio of the second forbidden unique ${\mathrm{\beta }}^{+}$ decay to the 158-keV level in ${}_{}{}^{56}{}_{}{}^{}\mathrm{Co}$, leading to a lower limit of 2.9×${10}^4$ yr for the half-life of fully ionized ${}_{}{}^{56}{}_{}{}^{}\mathrm{Ni}$ nuclei in cosmic rays. We also establish an upper limit of 5.0×${10}^{\mathrm{-}3}$ for the branching ratio of the isospin forbidden Fermi electron capture transition to the 1451-keV level in ${}_{}{}^{56}{}_{}{}^{}\mathrm{Co}$, which in turn leads to an upper limit of 124 keV for the isospin mixing Coulomb matrix element of the ${}_{}{}^{56}{}_{}{}^{}\mathrm{Ni}$ ground state.

• Received 12 February 1990

DOI:https://doi.org/10.1103/PhysRevC.42.573