REINVESTIGATION OF NI-56 DECAY

In a series of experiments, we have reinvestigated the decay of the doubly magic nucleus Ni56, 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 5.8×10-7 for the branching ratio of the second forbidden unique + decay to the 158-keV level in Co56, leading to a lower limit of 2.9×104 yr for the half-life of fully ionized Ni56 nuclei in cosmic rays. We also establish an upper limit of 5.0×10-3 for the branching ratio of the isospin forbidden Fermi electron capture transition to the 1451-keV level in Co56, which in turn leads to an upper limit of 124 keV for the isospin mixing Coulomb matrix element of the Ni56 ground state. © 1990 The American Physical Society.