Rotation-aligned isomer and oblate collectivity in ${}^{196}\mathrm{Pt}$

An oblate rotational sequence, built on an aligned, two-quasineutron isomeric state has been established in ${}^{196}\mathrm{Pt}$. The isomer has a half-life of 7.7(7) ns and is associated with the $I^{\pi }={12}^{+},{\left(i_{13/2}\right)}^2$ neutron configuration. Excited states, with angular momentum generated primarily through successive nucleon alignments, have been populated through $1\mathit{p}$ transfer from ${}^{197}\mathrm{Au}$. The nucleus ${}^{196}\mathrm{Pt}$ is the most neutron-rich Pt isotope for which high-spin states, beyond the ${12}^{+}$ isomeric state, have been established thus far. Cranked shell model calculations have been performed to understand shape evolution with spin, and the role of nucleons occupying specific Nilsson orbitals in generating aligned angular momentum for both prolate and oblate deformations has been explored.

Figure 1

Partial decay scheme for ${}^{196}\mathrm{Pt}$ illustrating transitions observed in the present work. Transitions above the $8^{+}$ level in the positive parity band, including the isomeric ${12}^{+}$ state, are newly established, as are a number of negative-parity levels.

Figure 2

Triple-$\gamma$ coincidence spectra displaying transitions in the (a), (b) positive- and (c), (d) negative-parity sequences of ${}^{196}\mathrm{Pt}$. Asterisks indicate $\gamma$ rays observed through coincidence with complementary multinucleon transfer products, while contaminant transitions are labeled with hash marks.

Figure 3

Effective half-life, 5.4(4) ns, for the closely spaced $5^{-}$ and $7^{-}$ levels. The time difference between the 446.7-keV $\gamma$ ray above the $7^{-}$ state, and various transitions below the $5^{-}$ level, is indicated by the solid (red) line. The dotted line displays the time difference characteristic of prompt transitions with similar energies, for comparison.

Figure 4

Time difference spectra illustrating the determination of the lifetime of the ${12}^{+}$ state, in a manner similar to that in Fig. 3. A half-life of 7.7(7) ns is inferred from the observed centroid shift.

Figure 5

Systematic comparison of the excited level structures de-exciting isomeric states in even-Pt isotopes from ${}^{192\text{–}198}\mathrm{Pt}$ [5, 9, 39]. Details are given in the text.

Figure 6

Total energy surface plots illustrating energy minima in the positive-parity, yrast structure in ${}^{196}\mathrm{Pt}$, for different values of spin, from calculations performed using the UC code [32]. It is evident that the lowest energy minimum is oblate $\left(\gamma \approx -{75}^{\circ }\right)$ at high spin: $I=14\hslash$ and $I=22\hslash$. The spacing between adjacent energy contours is 250 keV.

Figure 7

Comparison of the experimental and calculated level energies for the positive-parity sequence in ${}^{196}\mathrm{Pt}$.

Figure 8

Neutron quasiparticle levels in ${}^{196}\mathrm{Pt}$ calculated using the universal parametrization of the Woods-Saxon potential for (a) prolate and (b) oblate deformations. The deformation parameters are indicated.

Figure 9

Proton quasiparticle levels in ${}^{196}\mathrm{Pt}$ calculated in a similar manner as in Fig. 8: for (a) prolate and (b) oblate deformations.

Figure 10

(a) Aligned angular momentum as a function of rotational frequency for $N=118$ isotones ${}^{196}\mathrm{Pt} \left(Z=78\right),{}^{194}\mathrm{Os} \left(Z=76\right)$ [38], and ${}^{198}\mathrm{Hg} \left(Z=80\right)$ [24]. A reference rotor with $J_0=8 {\hslash }^2{\mathrm{MeV}}^{-1}$ and $J_1=35 {\hslash }^4{\mathrm{MeV}}^{-3}$, appropriate for a comparison of the three isotones, is used. (b) Comparison of experimental and calculated aligned angular momentum for the yrast, positive-parity sequence in ${}^{196}\mathrm{Pt}$. Satisfactory agreement is evident between the crossing frequencies and magnitude of aligned angular momentum for the first two alignments.