Fine structure in the $\alpha$ decay of high-spin isomers in ${}^{155}\mathrm{Lu}$ and ${}^{156}\mathrm{Hf}$

Fine structure in the $\alpha$ decay of high-spin isomers in ${}^{155}\mathrm{Lu}(25/2^{-})$ and ${}^{156}\mathrm{Hf}\left(8^{+}\right)$ has been studied for the first time using $\alpha \gamma$-coincidence analysis. Three new $\alpha$ decays from ${}^{155}\mathrm{Lu}(25/2^{-})$ and two from ${}^{156}\mathrm{Hf}\left(8^{+}\right)$ have been identified, populating seniority $s>1$ states in the $N=82$ nuclei ${}^{151}\mathrm{Tm}$ and ${}^{152}\mathrm{Yb}$, respectively. The reduced hindrance factors of the $\alpha$ decays support the previous configuration assignments of the populated states. This is the first observation of states with excitation energy greater than 1.5 MeV being populated following $\alpha$ decay in nuclei outside of the ${}^{208}\mathrm{Pb}$ region.

Figure 1

Level schemes of ${}^{151}\mathrm{Tm}$ and ${}^{152}\mathrm{Yb}$ populated following the $\alpha$ decays of the ${}^{155}\mathrm{Lu}{J}^{\pi }=25/2^{-}$ isomer and ground state and the ${}^{156}\mathrm{Hf}{J}^{\pi }=8^{+}$ isomer and ground state, respectively. The spins, parities, and energies of each level are given along with the energies of the transitions. For each $\alpha$ decay the $\alpha$-particle energy and reduced hindrance factors are given from the results of the present work and the state populated is also indicated. The configurations that have previously been assigned to each state (see text for details) are shown.

Figure 2

DSSD $\alpha$-particle energy spectra recorded up to 8.2 ms after the identification of a recoil implantation in the same DSSD pixel. (a) shows all $\alpha$-particle energies. The other panels show $\alpha$-particle energies in coincidence with (b) 415-, (c) 1490-, and (d) 1478-keV $\gamma$ rays identified from ${}^{151}\mathrm{Tm}$.

Figure 3

Decay time for the $\alpha$ decays identified from ${}^{155}\mathrm{Lu}(25/2^{-})$ to the (a) ($19/2^{+}$), (b) ($15/2^{+}$), and (c) $15/2^{-}$ states in ${}^{151}\mathrm{Tm}$ and from ${}^{156}\mathrm{Hf}\left(8^{+}\right)$ to the (d) $2^{+}$ and (e) $3^{-}$ states in ${}^{152}\mathrm{Yb}$. Also shown as a dashed line on each panel is the distribution of the decay times from the respective isomer to the ground state. These have been scaled for comparison with the weaker branches.

Figure 4

Energies of coincident $\alpha$ particles and $\gamma$ rays measured following the decay of ${}^{155}\mathrm{Lu}(25/2^{-})$. The diagonal lines on (a) represent a constant energy for the sum of the $\alpha$-decay $Q$ value, calculated from the $\alpha$-particle energy, and the $\gamma$-ray energy; the energies represented are those between the ${}^{155}\mathrm{Lu}(25/2^{-})$ isomeric state and both the ground state (dashed line) and excited state at 1490 keV (dot-dashed line) in ${}^{151}\mathrm{Tm}$. The $\alpha \gamma$ coincidences identified are circled and the $\gamma$-ray projections in coincidence with (b) the 5521(8)-keV and (c) the 5928(5)- or 5937(15)-keV $\alpha$ particles are shown.

Figure 5

Energies of coincident $\alpha$ particles and $\gamma$ rays measured following the decay of ${}^{156}\mathrm{Hf}\left(8^{+}\right)$. The diagonal line on (a) represents a constant energy for the sum of the $\alpha$-decay $Q$ value, calculated from the $\alpha$-particle energy, and the $\gamma$-ray energy; the energy represents that between the ${}^{156}\mathrm{Hf}\left(8^{+}\right)$ isomeric state and the ground state of ${}^{152}\mathrm{Yb}$. The $\alpha \gamma$ coincidences identified from ${}^{156}\mathrm{Hf}\left(8^{+}\right)$ are circled with contaminant coincidences from ${}^{155}\mathrm{Lu}(25/2^{-})$ also labeled. Also shown are the $\gamma$-ray energies in coincidence with (b) the 5942(15)- or 6274(15)-keV $\alpha$ particles and (c) the $\alpha$-particle energies in coincidence with the 1531-keV $\gamma$ rays.

Figure 6

Reduced hindrance factors of the $\alpha$ decays from the ${}^{155}\mathrm{Lu}(25/2^{-})$ and ${}^{156}\mathrm{Hf}\left(8^{+}\right)$ isomers (except where labeled) to states in ${}^{151}\mathrm{Tm}$ and ${}^{152}\mathrm{Yb}$, respectively, with $J^{\pi }$ shown on the $x$ axis. The configurations assigned to each of the states populated is indicated and analogous states in ${}^{151}\mathrm{Tm}$ and ${}^{152}\mathrm{Yb}$ have the same symbols.