Identification of the $J^{\pi }=1^{-}$ state in ${}^{218}\mathrm{Ra}$ populated via $\alpha$ decay of ${}^{222}\mathrm{Th}$

The $\alpha$ decay of ${}^{222}\mathrm{Th}$ populating the low-lying $J^{\pi }=3^{-}$ state, and also a proposed $1^{-}$ state, in ${}^{218}\mathrm{Ra}$ has been observed. The observations suggest an excitation energy of 853 keV for the $1^{-}$ state, which is 60 keV above the $3^{-}$ state. The hindrance factors of these $\alpha$ decays give a possible boundary to the region of ground-state octupole deformation in the light-actinide nuclei. The relative positions of the $J^{\pi }=1^{-}$ and $3^{-}$ states suggest they are produced by an octupole-vibrational mechanism, as opposed to $\alpha$ clustering or rotations of a reflection-asymmetric octupole-deformed shape.

Figure 1

Energies of coincident $\alpha$ particles and $\gamma$ rays following the decay of ${}^{222}\mathrm{Th}$. Panel (a) shows the coincidences when requiring a recoil-$\alpha$ correlation. The diagonal lines 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 are those between the ${}^{222}\mathrm{Th}$ ground state and the ground state (dashed) and $2^{+}$ state (dot dashed) of ${}^{218}\mathrm{Ra}$. Panels (b) and (c) show the $\gamma$ rays in coincidence with $\alpha$-particle energies of 7205 and 7143 keV, respectively. Panel (d) shows the $\alpha \gamma$ coincidences when requiring a recoil-$\alpha -\alpha$ correlation, as discussed in the text.

Figure 2

Level scheme of ${}^{218}\mathrm{Ra}$ populated following the $\alpha$ decay of ${}^{222}\mathrm{Th}$. The $\alpha$-particle energies $E_{\alpha }$ and hindrance factors are indicated. The $1^{-}$ level proposed in this work is shown at 853 keV.

Figure 3

Hindrance factors of the $\alpha$ decays populating the $1^{-}$ and $3^{-}$ states in (a) Th, (b) Ra, and (c) Rn isotopes as a function of neutron number $N$ [28, 29, 30, 31, 32, 33, 34]. The $1^{-}$ data point for Ra at $N=130$ has been shifted to the left for clarity; data points in brackets are tentative.

Figure 4

Relative positions of the $1^{-}$ and $3^{-}$ states in even-even ${}^{224–232}\mathrm{Th}$, ${}^{218–228}\mathrm{Ra}$, and ${}^{218–222}\mathrm{Rn}$ nuclei [28, 29, 30, 31, 32, 33, 34] compared with the ratio of their first $4^{+}$ and $2^{+}$ state energies, $E\left(4^{+}\right)/E\left(2^{+}\right)$. The new value for ${}^{218}\mathrm{Ra}$ is enlarged and the previously assigned value is shown as an open symbol; data points in brackets are tentative.