Fine structure in the $\alpha$ decay of the $8^{+}$ isomer in ${}^{216,218}\mathrm{U}$

The extremely neutron-deficient even-even uranium isotopes ${}^{216,218}\mathrm{U}$ were produced in the complete-fusion reactions induced by impinging ${}^{40}\mathrm{Ar}$ and ${}^{40}\mathrm{Ca}$ ions on ${}^{180,182,184}\mathrm{W}$ targets. Fusion evaporation residues were separated in flight by the gas-filled recoil separator SHANS (Spectrometer for Heavy Atoms and Nuclear Structure) and subsequently identified using the recoil-$\alpha$-correlation method. The improved ground-state to ground-state $\alpha$-decay properties of ${}^{216,218}\mathrm{U}$ were reported in [Z. Y. Zhang et al., Phys. Rev. Lett. 126, 152502 (2021)]. In this paper, we report on new $\alpha$-decay activities with $E_{\alpha }=10163\left(27\right)$ keV for ${}^{216}\mathrm{U}$ and $E_{\alpha }=10073\left(16\right)$ keV for ${}^{218}\mathrm{U}$, which decay from the $8^{+}$ isomeric states of ${}^{216,218}\mathrm{U}$ into the $2^{+}$ states of their daughter nuclei ${}^{212,214}\mathrm{Th}$, respectively. The new results extend the systematics of the $\alpha$-decay fine structure for the $N=124$ and 126 even-even isotones.

Figure 1

(a) Energy spectrum for $\alpha$ particles following implanted residues within a time window of 6 ms measured in the ${}^{40}\mathrm{Ar}+{}^{182}\mathrm{W}$ reaction. (b) Two-dimensional plot of parent and daughter $\alpha$-particle energies for correlated ER-$\alpha 1\text{−}\alpha 2$ events measured in the PSSDs. The searching time windows were 6 ms for the ER-$\alpha 1$ pair and 37 s for the $\alpha 1\text{−}\alpha 2$ pair. The newly observed $8^{+}\to 2^{+}\alpha$ decays of ${}^{218\mathrm{m}}\mathrm{U}$ are marked with red arrows.

Figure 2

The $\alpha$-particle energy and the decay-time distributions of the correlated $\alpha$-decay chains stemming from ${}^{218\mathrm{g}}\mathrm{U}$ (blue) and ${}^{218\mathrm{m}}\mathrm{U}$ (green) observed in the present work. The newly observed $\alpha$-decay chains of ${}^{218\mathrm{m}}\mathrm{U}$ are marked in red. Note that only the $\alpha$ events with their full energy deposited in the PSSDs are shown in the energy spectra. The italic values in brackets are taken from Refs. [8, 9, 18, 19] as comparison. The solid curves in the decay-time distributions are drawn by using the deduced mean lifetimes for ${}^{218\mathrm{g},\mathrm{m}}\mathrm{U}$ and the literature values for ${}^{214}\mathrm{Th}$ and ${}^{210}\mathrm{Ra}$.

Figure 3

$\alpha$-decay schemes of the $N=126$ even-even isotones. Values are taken from Refs. [10, 11] and the present work. The $\alpha$-decay hindrance factors (HF) were calculated according to Ref. [20]. Newly observed $\alpha$ decay from ${}^{218\mathrm{m}}\mathrm{U}$ to the $2^{+}$ state of ${}^{214}\mathrm{Th}$ is marked with a red arrow.

Figure 4

(a) Energy spectrum for $\alpha$ particles following implanted residues within a time window of 50 ms measured in the ${}^{40}\mathrm{Ar}+{}^{180}\mathrm{W}$ reaction. (b) Two-dimensional plot of parent and daughter $\alpha$-particle energies for correlated ER-$\alpha 1\text{−}\alpha 2$ events measured in the PSSDs. The searching time windows were 50 ms for the ER-$\alpha 1$ pair and 10 s for the $\alpha 1\text{−}\alpha 2$ pair. The newly observed $8^{+}\to 2^{+}\alpha$ decay of ${}^{216\mathrm{m}}\mathrm{U}$ is labeled by a red ellipse.

Figure 5

Measured $\alpha$-particle energy and decay-time distributions of the correlated $\alpha$-decay events stemming from ${}^{216\mathrm{g}}\mathrm{U}$ (blue) and ${}^{216\mathrm{m}}\mathrm{U}$ (green). The newly observed $\alpha$-decay chain of ${}^{216\mathrm{m}}\mathrm{U}$ is marked in red. Note that only the $\alpha$ events with their full energy deposited in the PSSDs are shown in the energy spectra. The half-lives of ${}^{216\mathrm{g}}\mathrm{U}$ and ${}^{216\mathrm{m}}\mathrm{U}$ are deduced by combining all the decay-time data from the present work and from Refs. [4, 5, 6]. The italic values in brackets are taken from Refs. [5, 19, 26, 27]. The solid curves in the decay-time distributions are drawn by using the deduced mean lifetimes for ${}^{216\mathrm{g},\mathrm{m}}\mathrm{U}$ and the literature values for ${}^{212}\mathrm{Th}, {}^{208}\mathrm{Ra}$, and ${}^{204}\mathrm{Rn}$.

Figure 6

Proposed $\alpha$-decay scheme of ${}^{216}\mathrm{U}$. The newly observed $\alpha$ decay from ${}^{216\mathrm{m}}\mathrm{U}$ to the $2^{+}$ state of ${}^{212}\mathrm{Th}$ is marked with red.