Competition between allowed and first-forbidden $\beta$ decays of ${}^{208}\mathrm{At}$ and expansion of the ${}^{208}\mathrm{Po}$ level scheme

The structure of ${}^{208}\mathrm{Po}$ populated through the EC/${\beta }^{+}$ decay of ${}^{208}\mathrm{At}$ is investigated using $\gamma$-ray spectroscopy at the ISOLDE Decay Station. The presented level scheme contains 27 new excited states and 43 new transitions, as well as a further 50 previously observed $\gamma$ rays which have been (re)assigned a position. The level scheme is compared to shell model calculations. Through this analysis approximately half of the $\beta$-decay strength of ${}^{208}\mathrm{At}$ is found to proceed via allowed decay and half via first-forbidden decay. The first-forbidden transitions predominantly populate core excited states at high excitation energies, which is qualitatively understood using shell model considerations. This mass region provides an excellent testing ground for the competition between allowed and first-forbidden $\beta$-decay calculations, important for the detailed understanding of the nucleosynthesis of heavy elements.

Figure 1

Full $\gamma -\gamma$ matrix projection (1-$\mu \mathrm{s}$ coincidence window) for all $A=208$ data collected. Peak positions are given for previously identified ${}^{208}\mathrm{Po}\gamma$ rays (indicated by filled red circles), known ${}^{208}\mathrm{Po}$ but previously unplaced $\gamma$ rays (indicated by open red circles), and known contaminant peaks (indicated by filled blue squares). The contaminant peaks feature results from decays in ${}^{207}\mathrm{Bi}$, as well as strong background peaks (${}^{40}\mathrm{K}$ at 1460.8 keV and ${}^{208}\mathrm{Tl}$ at 2614.5 keV).

Figure 2

Level scheme for ${}^{208}\mathrm{Po}$ observed in this work. Energies of levels and transitions are given in keV. Relative intensities of the most intense transitions are indicated by the thicknesses of arrows.

Figure 3

Comparison of shell model and experimental excited states in ${}^{208}\mathrm{Po}$. Left: Dominant configurations are given, taken from shell model calculations, with # denoting $\pi h_{9/2}^2$. Right: Spin-parity assignments, with asterisks denoting states where assignments have been made using branching ratio comparisons.

Figure 4

$\beta$-population intensity as a function of the ${}^{208}\mathrm{Po}$ excitation energy. The present results are indicated by filled circles. These are compared with result obtained from experiments performed in the 1960s [8] (x's) and the 1980s [14] (filled triangles). The parity of the excited states is indicated by the color of the symbol.

Figure 5

Intensity of first-forbidden decays (${\mathrm{I}}_{{\beta }_{\mathrm{ff}}}\%$) for proton-rich $\mathrm{A}\approx 208$ nuclei [14, 20, 32, 33, 34, 35, 36, 37, 38]. $I_{{\beta }_{\mathrm{ff}}}\%=I_{{\beta }_{\mathrm{ff}}}/I_{{\beta }_{\mathrm{tot}}}$. The parent nuclei are indicated. $Q_{\mathrm{EC}}$ values, in MeV, are also given. In the majority of nuclei, competition between first-forbidden and allowed $\beta$ decay is expected. For details, see the text.