Half-life determinations of Rb and Cs fission products available at an on-line mass separator have been made for several neutron-rich Rb, Sr, Cs, and Ba isotopes using both $\beta$-multiscale and $\gamma$-multispectra measurements. The half-lives and rms uncertainties (in sec) are ${}_{}{}^{96}{}_{}{}^{}\mathrm{Rb}$, 0.203 ± 0.004; ${}_{}{}^{96}{}_{}{}^{}\mathrm{Sr}$, 1.015 ± 0.019; ${}_{}{}^{97}{}_{}{}^{}\mathrm{Rb}$, 0.170 ± 0.002; ${}_{}{}^{97}{}_{}{}^{}\mathrm{Sr}$, 0.441 ± 0.015; ${}_{}{}^{98}{}_{}{}^{}\mathrm{Rb}$, 0.108 ± 0.005; ${}_{}{}^{98}{}_{}{}^{}\mathrm{Sr}$, 0.66 ± 0.07; ${}_{}{}^{144}{}_{}{}^{}\mathrm{Cs}$, 1.02 ± 0.04; ${}_{}{}^{144}{}_{}{}^{}\mathrm{Ba}$, 12.3 ± 0.4; ${}_{}{}^{145}{}_{}{}^{}\mathrm{Cs}$, 0.590 ± 0.020; ${}_{}{}^{145}{}_{}{}^{}\mathrm{Ba}$, 4.31 ± 0.16; ${}_{}{}^{146}{}_{}{}^{}\mathrm{Cs}$, 0.305 ± 0.010; ${}_{}{}^{146}{}_{}{}^{}\mathrm{Ba}$, 2.18 ± 0.11; ${}_{}{}^{147}{}_{}{}^{}\mathrm{Cs}$, 0.218 ± 0.009; ${}_{}{}^{147}{}_{}{}^{}\mathrm{Ba}$, 0.70 ± 0.06. This work provides an unambiguous identification of the new neutron-rich isotope ${}_{}{}^{147}{}_{}{}^{}\mathrm{Cs}$. The half-life values are compared with predictions of the gross theory of $\beta$ decay and an apparent shell effect at $N=58\mathrm{}$ on the half-lives of neutron-rich isotopes of Kr, Rb, Sr, Y, and Zr is discussed.

RADIOACTIVITY ${}_{}{}^{96,97,98}{}_{}{}^{}\mathrm{Rb}$, ${}_{}{}^{96,97,98}{}_{}{}^{}\mathrm{Sr}$, ${}_{}{}^{144,145,146,147}{}_{}{}^{}\mathrm{Cs}$, ${}_{}{}^{144,145,146,147}{}_{}{}^{}\mathrm{Ba}$ [from ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}(n,\mathrm{fission})$]; measured $T_{\frac{1}{2}}$. Ge(Li) and plastic scintillation detectors. Mass-separated Rb and Cs activities.

• Received 3 January 1978

DOI:https://doi.org/10.1103/PhysRevC.17.2185