Fine structure in the α decay of ${}^{218}\mathrm{At}$

An $\alpha$-decay study of ${}^{218}\mathrm{At}$ was performed at the CERN-ISOLDE facility. Laser-ionized beams of ${}^{218}\mathrm{At}$ were mass separated and implanted into an $\alpha -\gamma$ detection setup. Coincidence $\alpha -\gamma$ data were collected for the first time and a more precise half-life value of $T_{1/2}=1.27\left(6\right)$ s was measured. A new $\alpha$-decay scheme was deduced based on the extracted reduced $\alpha$-decay widths for fine-structure decays. The results from this work lead to a preferred spin and parity assignment of $J^{\pi }=\left(3^{-}\right)$; however, $J^{\pi }={\left(2\right)}^{-}$ cannot be fully excluded.

Figure 1

(a) Sum of singles $\alpha$-decay spectra recorded in $\text{Si}1+\text{Si}2$ detectors, for $A=218$. The inset of panel (a) shows the singles $\alpha$-decay spectrum (black histogram), zoomed in on the energy region of ${}^{218}\mathrm{At}$ and overlapped with results from geant4 simulations (blue histogram, see Sec. 4a for more details). (b) $\alpha -\gamma$ coincidences for $\alpha$-decay data shown in panel (a), measured within a $\mathrm{\Delta }T(\alpha -\gamma )⩽300$ ns time interval. (c) Projection on the $E_{\gamma }$ axis of panel (b), for the gating condition $6600<E_{\alpha }<6800$ keV (indicated by vertical, red dashed lines). The inset of panel (c) shows the decay curve for ${}^{218}\mathrm{At}$ extracted from the $\text{Si}3+\text{Si}4$ detectors, fit with an exponential plus a constant background.

Figure 2

The preferred decay scheme for ${}^{218}\mathrm{At}$ deduced from the present work, with $b_{\alpha }\left({}^{218}\mathrm{At}\right)=99.9\%$ taken from Ref. [7]. The $\alpha$ decays and $\gamma$-ray transition observed in the present data are represented by the solid straight and curly arrows, respectively, and the dashed lines represent the possible, yet currently unobserved transitions. The hindrance factors were calculated relative to the unhindered, ${\delta }_{\alpha }^2=124\left(1\right)$ keV, $9/2^{-}\to 9/2^{-}\alpha$ decay of the neighboring odd-mass isotope, ${}^{217}\mathrm{At}$ [4].

Figure 3

Excitation energies of the low-spin states that are presumed to have a configuration with a dominant $\pi 1h_{9/2}\otimes \nu 2g_{9/2}$ component, relative to the $J^{\pi }=1^{-}$ ground states of (a) ${}^{210,212,214}\mathrm{Bi}$ and (b) their respective isotones, ${}^{212,214,216}\mathrm{At}$. States with the same spin and parity are connected by dotted lines. States with tentative spin, parity, or spin and parity assignments are indicated by a dashed line. The energy for the $J^{\pi }=\left(4^{-}\right)$ state in ${}^{214}\mathrm{Bi}$ is taken from the tentative assignment proposed in the current work, whereas other data are taken from the Evaluated Nuclear Structure Data File [4].