New isotopeY80, and the decays ofSr79,Y81, andY82

The helium-jet recoil transfer technique has been used to study the decays of ${}_{}{}^{79}{}_{}{}^{}\mathrm{Sr}$, ${}_{}{}^{80}{}_{}{}^{}\mathrm{Y}$, ${}_{}{}^{81}{}_{}{}^{}\mathrm{Y}$, and ${}_{}{}^{82}{}_{}{}^{}\mathrm{Y}$, produced by ${}_{}{}^{58}{}_{}{}^{}\mathrm{Ni}({}_{}{}^{24}{}_{}{}^{}\mathrm{Mg},2pn)$, ${}_{}{}^{58}{}_{}{}^{}\mathrm{Ni}({}_{}{}^{24}{}_{}{}^{}\mathrm{Mg},pn)$, ${}_{}{}^{58}{}_{}{}^{}\mathrm{Ni}({}_{}{}^{25}{}_{}{}^{}\mathrm{Mg},pn)$, and ${}_{}{}^{60}{}_{}{}^{}\mathrm{Ni}({}_{}{}^{24}{}_{}{}^{}\mathrm{Mg},pn)$ reactions, respectively. Decay properties of these four radioactivities have been deduced from the analysis of extensive singles and multiparameter spectroscopic measurements. The low lying level structure of ${}_{}{}^{80}{}_{}{}^{}\mathrm{Sr}$, as revealed by the decay of ${}_{}{}^{80}{}_{}{}^{}\mathrm{Y}$, can be reproduced through calculations of the interacting-boson-approximation model which place ${}_{}{}^{80}{}_{}{}^{}\mathrm{Sr}$ at or near the O(6) limit. Positron endpoint measurements [deduced from ($\beta ,\gamma$) coincidence studies] have been used to determine decay energies for each of these nuclides. Nuclidic mass excesses inferred from these data show progressively larger deviations from the predictions of currently available mass models for the more neutron-deficient $Y$ isotopes as the $N=Z$ line is approached. It is suggested that this may reflect the influence of nuclear deformation of these isotopes on the mass surface in the vicinity of $N=Z=40\mathrm{}$.