Observation of a1+ (πh112, νh92) state inTb148populated in the decay of a new isotope,Dy148

Abstract

A new activity with a half-life of 3.1±0.1 min was observed in bombardments of ${}_{}{}^{142}{}_{}{}^{}\mathrm{Nd}$ with ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ ions accelerated in the Oak Ridge isochronous cyclotron. A gas-jet-capillary system was used to transport product nuclei to a shielded area so that singles and coincidence $\gamma$-ray measurements could be made. Only one transition, 620.2±0.1 keV, was found to be associated with this new activity. It was assigned to ${}_{}{}^{148}{}_{}{}^{}\mathrm{Dy}$ because (1) the variation of its yield as a function of bombarding energy was the same as for ${}_{}{}^{148}{}_{}{}^{}\mathrm{Tb}$ $\gamma$ rays, indicating that an equal number of nucleons were evaporated from the compound system, and (2) it was found to be in coincidence with terbium $K$ x rays. No $\gamma$ rays were observed in its coincidence spectrum, and no initial growth period was seen for transitions that follow the decay of the ${}_{}{}^{148}{}_{}{}^{}\mathrm{Tb}$ high-spin isomer. For these reasons the transition very probably proceeds directly to the low-spin isomeric state in ${}_{}{}^{148}{}_{}{}^{}\mathrm{Tb}$ and establishes an intermediate level at ∼620 keV. The spin and parity of this level has to be $1^{+}$ because the transition from the $0^{+}$ ${}_{}{}^{148}{}_{}{}^{}\mathrm{Dy}$ ground state has a $log\mathrm{ft}$ value of 3.9. Our suggestion is that this allowed $\beta$ decay connects states with the following characters: $0^{+} (\pi h_{\frac{11}{2}}, \pi h_{\frac{11}{2}})\to 1^{+} (\pi h_{\frac{11}{2}}, \nu h_{\frac{9}{2}})$. $\gamma$-ray data were also obtained for ${}_{}{}^{149}{}_{}{}^{}\mathrm{Dy}$ and ${}_{}{}^{149}{}_{}{}^{}\mathrm{Tb}_{}^m{}_{}{}^{}$. From this information the half-life of ${}_{}{}^{149}{}_{}{}^{}\mathrm{Dy}$ was determined to be 4.1±0.2 min, a value which agrees with recent half-lives reported for this isotope.