Zero-Spin Assignment for the 2.05-MeV State ofNi62from a (p,p′γ−γ) Angular Correlation Measurement

Abstract

The $\gamma -\gamma$ angular correlation of the 0.88-1.17-MeV cascade, following excitation of the 2.05-MeV state of ${\mathrm{Ni}}^{62}$ by the ($p, p^{\prime }$) reaction, has been measured for angular separations of 90° to 180° at an average bombarding energy of 4.71 MeV. Convincing agreement was found with the correlation expected for a $0\left(Q\right)2\left(Q\right)0$ cascade, thereby providing a unique zero-spin assignment for the second level of ${\mathrm{Ni}}^{62}$. This assignment accounts for the measured isotropy (within ±3%) of the 0.88-MeV cascade transition with respect to the incident beam, as well as the lack of an observed ground-state transition. While a 2+ spin would have been expected for the second level according to level systematics for neighboring even-even nuclei, a spin of zero can be accommodated by the recent theoretical predictions of Kerman and Shakin. Other evidence relating to the spin assignments and possible two-phonon character of the well-separated triplet of states in ${\mathrm{Ni}}^{62}$ is discussed. It is concluded that the measurement of a $0\left(Q\right)2\left(Q\right)0$ $\gamma -\gamma$ angular correlation following ($p, p^{\prime }$) excitation provides a promising method for the unique identification of zero-spin states in medium-weight even-even nuclei.