Decay ofSm141m- A Three-Quasiparticle Multiplet inPm141

Abstract

We have studied the $\gamma$ rays emitted following the decay of 22.1-min ${}_{}{}^{141m}{}_{}{}^{}\mathrm{Sm}$ with Ge(Li) and NaI(Tl) detectors in various singles, coincidence, and anticoincidence configurations, including Ge(Li)-Ge(Li) two-dimensional "megachannel" coincidence experiments. Of the 47 $\gamma$ rays definitely established as belonging to ${}_{}{}^{141m}{}_{}{}^{}\mathrm{Sm}$ decay, all but six very weak ones have been placed in a consistent decay scheme. The energies in keV [and $J^{\pi }$ assignments] of the states in ${}_{}{}^{141}{}_{}{}^{}\mathrm{Pm}$ populated by the decay of ${}_{}{}^{141m}{}_{}{}^{}\mathrm{Sm}$ are 0 [${\frac{5}{2}}^{+}$], 196.6 [${\frac{7}{2}}^{+}$], 628.6 [${\frac{11}{2}}^{-}$], 804.5 [${\frac{11}{2}}^{+},{\frac{9}{2}}^{+}$], (837.1) [${\frac{9}{2}}^{+}$], 974.0 [${\frac{9}{2}}^{+}$], 1108.1 [${\frac{7}{2}}^{+},{\frac{9}{2}}^{+},\left({\frac{5}{2}}^{-}\right)$], 1167.2 [${\frac{13}{2}}^{-(+)\mathrm{}},{\frac{11}{2}}^{-(+)\mathrm{}},{\frac{9}{2}}^{-(+)\mathrm{}}$], 1313.2 [${\frac{13}{2}}^{-(+)\mathrm{}},{\frac{11}{2}}^{-(+)\mathrm{}},{\frac{9}{2}}^{-(+)\mathrm{}}$], 1414.8 [${\frac{11}{2}}^{-},{\frac{9}{2}}^{-}$], (1834.0) [${\frac{11}{2}}^{-},{\frac{9}{2}}^{-}$], 1983.1 [${\frac{9}{2}}^{-}$], 2063.5 [${\frac{11}{2}}^{-},{\frac{9}{2}}^{-}$], 2091.6 [${\frac{11}{2}}^{-},{\frac{9}{2}}^{-}$], 2119.0 [${\frac{11}{2}}^{-},{\frac{9}{2}}^{-}$], and 2702.4 [${\frac{13}{2}}^{-},{\frac{11}{2}}^{-},{\frac{9}{2}}^{-}$]. Less than 0.2% of the decay of ${\frac{11}{2}}^{-}$ ${}_{}{}^{141m}{}_{}{}^{}\mathrm{Sm}$ proceeds via an $M4\mathrm{}$ isomeric transition to 11.3-min ${\frac{3}{2}}^{+}$ ${}_{}{}^{141g}{}_{}{}^{}\mathrm{Sm}$. Two-thirds of the ${}_{}{}^{141m}{}_{}{}^{}\mathrm{Sm}$ electron-capture decay goes to the six (possibly seven) highest-lying states in ${}_{}{}^{141}{}_{}{}^{}\mathrm{Pm}$, another example analogous to ${}_{}{}^{139m}{}_{}{}^{}\mathrm{Nd}$ decay of the population of a well-defined three-quasiparticle multiplet by an $N=79\mathrm{}$ nuclide. In simple shell-model terms this can be written as ${\left(\pi d_{\frac{5}{2}}\right)}^2{\left(\nu d_{\frac{3}{2}}\right)}^{-2\mathrm{}}{\left(\nu h_{\frac{11}{2}}\right)}^{-1\mathrm{}}\to \left(\pi d_{\frac{5}{2}}\right){\left(\nu d_{\frac{3}{2}}\right)}^{-1\mathrm{}}{\left(\nu h_{\frac{11}{2}}\right)}^{-1\mathrm{}}$. Most of the remaining states can be characterized quite satisfactorily as specific single-particle and core-coupled states. The behavior of these states allows us to add considerably to the systematics of shell-model orbits and their occupations in this region below $N=82\mathrm{}$.