Experimental Studies of Neutron-Deficient Gadolinium Isotopes. III. The Strange Case ofGd145g

Abstract

The $\gamma$ rays emitted following the decay of 21.8-min ${\mathrm{Gd}}^{145g}$ have been studied using Ge(Li) and NaI(Tl) detectors in a variety of singles, anticoincidence, pair-coincidence, and two-dimensional ("megachannel") coincidence experiments. Of the $38 \gamma$ rays attributed to this decay, 27 (accounting for >97% of the intensity) have been placed in a consistent decay scheme that includes 20 states in ${\mathrm{Eu}}^{145}$. All of the single-proton states between $Z=50\mathrm{} \mathrm{and} 82$ are seen (including the $h_{\frac{11}{2}}$ state populated directly by the decay of ${\mathrm{Gd}}^{145m}$), and the associated $\beta$ and $\gamma$ transitions are accounted for quite well using simple shell-model arguments. In addition, we propose an explanation for the abrupt change in decay properties of the $N=81\mathrm{}$ isotones that occurs at ${\mathrm{Gd}}^{145g}$, viz., the lack of observable population directly to the ${\mathrm{Eu}}^{145}$ ground state but 72.6% of its decay going to states at 1757.8 and 1880.6 keV. With a ${\left(\nu s_{\frac{1}{2}}\right)}^{-1\mathrm{}}$ ground state for ${\mathrm{Gd}}^{145}$, these "fast" $\beta$ transitions can be represented as ${\left(\pi h_{\frac{11}{2}}\right)}^{2n}{\left(\nu s_{\frac{1}{2}}\right)}^{-1\mathrm{}}\to {\left(\pi h_{\frac{11}{2}}\right)}^{2n-1}\left(\nu h_{\frac{9}{2}}\right){\left(\nu s_{\frac{1}{2}}\right)}^{-1\mathrm{}},$ making the final states another example of three-quasiparticle states being populated by the $\frac{{\beta }^{+}}{\epsilon }$ decay of nuclei below $N=82\mathrm{}$.