Why DoesNi56Decay so Slowly?

Abstract

The decay of the doubly magic nucleus ${}_{}{}^{56}{}_{}{}^{}\mathrm{Ni}$ to the lowest $1^{+}$ state of ${}_{}{}^{56}{}_{}{}^{}\mathrm{Co}$ proceeds about a hundred times slower than the shell model predicts. The explanation is that the lowest $1^{+}$ state of ${}_{}{}^{56}{}_{}{}^{}\mathrm{Co}$ is mostly a two-particle, two-hole state. The giant spin-isospin resonance lies at a higher energy.