Subnanosecond lifetime measurements in Y and Sr nuclei and the saturation of deformation nearA=100

Half-lives of 47(6) ps in ${}_{}{}^{99}{}_{}{}^{}\mathrm{Y}$ and 72(7) ps in ${}_{}{}^{100}{}_{}{}^{}\mathrm{Y}$, measured by β-γ-γ coincidences, and intraband M1/E2 ratios give M1 strengths and g factors of the bands π(5/2[422] in ${}_{}{}^{99}{}_{}{}^{}\mathrm{Y}$, ν(3/2[411] in ${}_{}{}^{99}{}_{}{}^{}\mathrm{Sr}$, and π(5/2[422]ν 3) / 2 [411] in ${}_{}{}^{100}{}_{}{}^{}\mathrm{Y}$. The results are in remarkable agreement with the simple picture of these nuclei as a deformed ${}_{}{}^{98}{}_{}{}^{}\mathrm{Sr}$ core plus one or two valence nucleons, thus indicating that the deformation saturates for A≃100 Sr and Y nuclei. This ‘‘early’’ saturation can be explained in terms of valence Nilsson orbitals.