Complete spectroscopy of ${}^{211}\mathbf{Po}$ below 2.0 MeV via the ($\alpha , n$) reaction

We report the results of a $\gamma$-ray spectroscopic study of ${}^{211}\mathrm{Po}$ via the ${}^{208}\mathrm{Pb}(\alpha ,n$) reaction at 24-MeV incident energy using a thick target. We observe 26 new $\gamma$ rays, allowing us to identify 18 states that were not observed in previous $\gamma$-ray studies. In total, we observe 45 states below 2.0 MeV. A shell model calculation using the modified Kuo-Herling interaction developed by Warburton and Brown predicts 46 states below 2.0 MeV having spins of 21/2 and below, demonstrating the power of this calculation to provide detailed nuclear structure information on nuclei in the vicinity of ${}^{208}\mathrm{Pb}$.

Figure 1

Spectrum gated on the 1051-keV $\gamma$ ray. The peaks labeled in red were not observed in previous studies.

Figure 2

Spectrum gated on the 1064-keV $\gamma$ ray.

Figure 3

Spectrum gated on the 1122-keV $\gamma$ ray.

Figure 4

Spectrum gated on the 1160-keV $\gamma$ ray.

Figure 5

Low-spin section of the level scheme observed here. The states and transitions drawn in red were not observed in previous studies.

Figure 6

High-spin section of the level scheme observed here.

Figure 7

States observed below 1.3 MeV compared to those calculated in the same energy range in the shell-model calculations described here and using a weak coupling scheme.

Figure 8

Number of accumulated states observed in the experiment and for shell model and weak coupling calculations. The upper panel, which shows states up to 2.0 MeV, includes two shell-model lines: one for all states calculated using the shell model and the other excluding states having $J=23/2$ or higher. The weak coupling line excludes states having $J=23/2$ or higher. The lower panel shows all shell model states up to 3.0 MeV.