Approaching complete low-spin spectroscopy of ${}^{210}\mathrm{Bi}$ with a cold-neutron capture reaction

The low-spin structure of the ${}^{210}\mathrm{Bi}$ nucleus was investigated in the neutron capture experiment ${}^{209}\mathrm{Bi}\left(n,\gamma \right){}^{210}\mathrm{Bi}$ performed at ILL Grenoble at the PF1B cold-neutron facility. By using the EXILL multidetector array, consisting of 46 high-purity germanium crystals, and $\gamma \gamma$-coincidence technique, 64 primary $\gamma$ rays were observed (40 new) and a total number of 70 discrete states (33 new) were located below the neutron binding energy in ${}^{210}\mathrm{Bi}$. The analysis of the angular correlations of $\gamma$ rays provided information about transitions multipolarities, which made it possible to confirm most of the previously known spin-parity assignments and helped establish new ones. The obtained experimental results were compared to shell-model calculations involving one-valence-proton, one-valence-neutron excitations outside the ${}^{208}\mathrm{Pb}$ core. It has been found that while up to the energy of $\sim 2$ MeV each state observed in ${}^{210}\mathrm{Bi}$ has its calculated counterpart; at higher excitation energies some levels cannot be described by the valence particle couplings. These states may arise from couplings of valence particles to the $3^{-}$ octupole phonon of the doubly magic ${}^{208}\mathrm{Pb}$ core and may serve as a testing ground for models which describe single particle-phonon excitations.

Figure 1

Representative coincidence spectra for ${}^{210}\mathrm{Bi}$. (a) Spectrum single-gated on the 320-keV transition; (b) spectrum double-gated on the pair of transitions at 320 and 674 keV; (c) spectrum single-gated on the 162-keV transition; (d) spectrum double-gated on the 162- and 775-keV transitions.

Figure 2

Representative coincidence single-gated spectra for ${}^{210}\mathrm{Bi}$. The gates were set on the high-energy primary $\gamma$ rays specified in the labels of each panel, which significantly cleaned the spectra.

Figure 3

Primary $\gamma$ rays from the capture state at 4.6 MeV $(4^{-},5^{-})$ in ${}^{210}\mathrm{Bi}$. The dashed lines indicate tentatively identified levels.

Figure 4

Level scheme of ${}^{210}\mathrm{Bi}$ displaying the transitions which follow primary $\gamma$ rays from the capture state, shown in Fig. 3, established in the present work. The newly found levels and $\gamma$ rays are marked in red. The dashed lines indicate tentatively identified levels.

Figure 5

Level scheme of ${}^{210}\mathrm{Bi}$ displaying the transitions which follow primary $\gamma$ rays from the capture state, shown in Fig. 3, established in the present work. The newly found levels and $\gamma$ rays are marked in red. The dashed lines indicate tentatively identified levels.

Figure 6

Level scheme of ${}^{210}\mathrm{Bi}$ displaying the transitions which follow primary $\gamma$ rays from the capture state, shown in Fig. 3, established in the present work. The newly found levels and $\gamma$ rays are marked in red. The dashed lines indicate tentatively identified levels.

Figure 7

Corrected relative full-energy-peak $\gamma$ efficiency of the EXILL array for the neutron capture experiment on a 3-g-thick ${}^{209}\mathrm{Bi}$ target (in red) compared to the curve from Ref. [9], obtained for thinner targets. See text for further explanations.

Figure 8

Selected examples of angular correlations of $\gamma$ rays from ${}^{210}\mathrm{Bi}$. The $A_2$ and $A_4$ values are experimental coefficients of the fitted functions.

Figure 9

Comparison between the experimental results and shell-model calculations. (a) Level scheme between 0 and 2315 keV; (b) between 2525 and 4605 keV (capture state). In each panel: (i) left, levels observed in the present experiment (red, newly found; marked by closed star, observed, but without theoretical counterparts; marked by open star, new theoretical interpretation); (ii) middle, theoretical predictions from the shell-model calculations; (iii) right, experimental levels reported in previous experiments (NuDat database [3]), not observed in present data. See text for details.

Figure 10

The energy-spin distribution of states observed in the decay following cold-neutron capture on ${}^{209}\mathrm{Bi}$ and having their calculated counterparts.