Decay ofCo56andMn56

Abstract

Scintillation spectroscopy and coincidence techniques have been used to measure the decay schemes of ${\mathrm{Mn}}^{56}$ and ${\mathrm{Co}}^{56}$, both of which decay to ${\mathrm{Fe}}^{56}$. Gamma-ray energies and intensities were measured with the aid of a large NaI(Tl) crystal and a three-crystal pair spectrometer. Combining these data with $\gamma -\gamma$ and $\beta -\gamma$ coincidence measurements, decay schemes are constructed for both nuclei. The $log\mathrm{ft}$ values for the various $\beta$ branches are examined and all of the $\beta$ branches from ${\mathrm{Co}}^{56}$ show higher than usual $log\mathrm{ft}$ values for allowed transitions. The ${\mathrm{Co}}^{56}$ decay to the second excited state in ${\mathrm{Fe}}^{56}$ is inhibited by a factor of ∼${10}^3$ while the ${\mathrm{Mn}}^{56}$ decay to the same state is inhibited even further. The allowed ${\mathrm{Mn}}^{56}$ decay to the first excited state in ${\mathrm{Fe}}^{56}$ is also slowed, while ${\mathrm{Mn}}^{56}$ decays to higher states proceed at more normal speeds. A discussion of the $\beta$-transition rates in terms of shell model assignments is given, from which some explanation of the $\beta$ rates results. The high inhibitions of the decays to the second excited state in ${\mathrm{Fe}}^{56}$ do not seem susceptible to a simple shell-model explanation.