Level structure in the transitional nucleus ${}^{199}\mathrm{Tl}$

High-spin level structures of ${}^{199}\mathrm{Tl}$ have been studied by in-beam $\gamma$-ray spectroscopic methods using the ${}^{196}\mathrm{Pt}({}^7\mathrm{Li}, 4n){}^{199}\mathrm{Tl}$ reaction at 40 MeV of beam energy. A large number of new states have been found, resulting in a significant extension of the previously known level scheme with the observation of 44 new transitions and 33 new levels. By comparing with the level structures in the odd-mass Tl isotopes and the even-even core Hg nuclei, configurations are proposed for the newly observed rotational bands and states in ${}^{199}\mathrm{Tl}$. The properties of the strongly coupled positive- and negative-parity bands can be interpreted in terms of the cranked Nilsson-Strutinsky-Bogoliubov model calculations, which show that the observed bands are built on the oblate shapes.

Figure 1

Relative efficiencies of the CIAE detector array. Top: Whole array. Middle: ${42}^{\circ }$ and ${153}^{\circ }$ detectors. Bottom: ${90}^{\circ }$ detectors.

Figure 2

Partial level scheme of ${}^{199}\mathrm{Tl}$ obtained in the present work. The width of the arrows is proportional to the intensity of the transitions, while the white part of the arrows is the correction due to internal conversion [46]. The strongly populated states up to $I^{\pi }=15/2^{-}$ are in agreement with previous work.

Figure 3

$\gamma$-ray coincidence spectra for ${}^{199}\mathrm{Tl}$ gated (a) on the 795 keV ($19/2^{-}\to 15/2^{-}$), (b) on the 226 keV $[{(23/2)}^{-}\to (21/2^{-})]$, and (c) on the 535 keV $[{(13/2)}^{+}\to 13/2^{-}]$ transition. $\gamma$-ray lines denoted by $\#$ in the 226 keV gate and $\mathrm{\Delta }$ in the 535 keV gate are those belonging to ${}^{198}\mathrm{Hg}$ and ${}^{199}\mathrm{Hg}$ from incomplete fusion channels, respectively.

Figure 4

$\gamma$-ray coincidence spectra for ${}^{199}\mathrm{Tl}$ gated on the (a) 629 keV ($15/2^{+}\to 13/2^{-}$), (b) 237 keV ($21/2^{+}\to 19/2^{+}$), and (c) 301 keV $[(21/2^{+})\to 19/2^{+}]$ transition. The insets in (a) and (b) show an expanded region around 800 keV.

Figure 5

$\gamma$-ray coincidence spectra for ${}^{199}\mathrm{Tl}$ gated (a) on the 382 keV ($9/2^{-}\to 3/2^{+}$), (b) on the 702 keV ($13/2^{-}\to 9/2^{-}$) transition. The peak positions of the 365.5 keV and 366.7 keV $\gamma$-ray transitions are indicated with blue dash line.

Figure 6

Experimental alignments for the negative-parity oblate band structures in ${}^{197}\mathrm{Tl}, {}^{199}\mathrm{Tl}$, and ${}^{201}\mathrm{Tl}$ (based on $9/2^{-}\left[505\right]$ configuration) and the yrast bands in their respective Hg core nuclei. The Harris reference parameters are chosen to be $J_0=8.0{\hslash }^2 {\mathrm{MeV}}^{-1}$ and $J_1=40.0{\hslash }^4 {\mathrm{MeV}}^{-3}$.

Figure 7

Level energy systematics of the $13/2^{+}$ states in odd Tl and Bi isotopes as a function of neutron number. The encircled one corresponds to that in ${}^{199}\mathrm{Tl}$ from the present work. The trigonal one corresponds to the suggested $13/2^{+}$ state in ${}^{197}\mathrm{Tl}$ by H. Pai et al. [44, 59].

Figure 8

Systematics of the excitation energy in the negative-parity yrast states and the positive-parity three-qp yrast states in odd-mass ${}^{195-199}\mathrm{Tl}$.

Figure 9

The experimental and CNSB rotating energies in ${}^{199}\mathrm{T}1$. The theoretical bands are labeled by the combination of parity and signature of protons and neutrons $({\pi }_p,{\alpha }_p)({\pi }_n,{\alpha }_n)$.

Figure 10

Calculated potential energy surfaces for (a) $I^{\pi }=17/2^{-}$ state of the $(-,1/2)(+,0)$ configuration; (b) $I^{\pi }=15/2^{+}$ state of the $(-,1/2)(-,1)$ configuration. The contour line separation is 0.2 MeV.