Decay of a New 5.8-minTb150Activity

Abstract

A new short-lived isomer of ${}_{}{}^{150}{}_{}{}^{}\mathrm{Tb}$ has been identified. The mass assignment was confirmed by excitation functions and cross bombardments. The 5.8 ± 0.2-min activity is probably a $9^{+}$ state ($\pi h_{\frac{11}{2}}\nu f_{\frac{7}{2}}$) and decays chiefly to an ($8^{+}$) state at 2554.4 keV in ${}_{}{}^{150}{}_{}{}^{}\mathrm{Gd}$ with a $log\mathrm{ft}$ value of 4.1. No isomeric transition to the 3.1-h ${}_{}{}^{150}{}_{}{}^{}\mathrm{Tb}$ was observed. The $\gamma$-ray spectrum was investigated with Ge(Li) detectors. The 19 $\gamma$ rays observed were assigned to 10 excited levels (in keV) at 638.05, $2^{+}$; 1134.35, $3^{-}$; 1288.4, ($4^{+}$); 1700.9, ($5^{-}$); 1936.8, ($6^{+}$); 2116.1, ($6^{+}$); 2211.2, ($7^{-}$); 2392.5; 2554.4, ($8^{+}$); and 2906.0. The energy spacing of the positive-parity states in ${}_{}{}^{150}{}_{}{}^{}\mathrm{Gd}$ suggests a close correspondence to a vibrator model although low-spin members of multiplets corresponding to multiphonon states are not observed. The quasiband representation of Sakai appears also to be a useful way of interpreting these states. Three negative-parity states form a sequence which may correspond to a combination of octupole and quadrupole phonons or an octupole quasiband.