Evidence for octupole collectivity in $^{172}{\mathrm {Pt}}$
Résumé
Excited states in the extremely neutron-deficient nucleus $^{172}{\mathrm {Pt}}$ were populated via $^{96}\mathrm {Ru}(^{78}\mathrm {Kr},2p)$ and $^{92}\mathrm {Mo}(^{83}\mathrm {Kr},3n)$ reactions. The level scheme has been extended up to an excitation energy of $\approx 5~\hbox {MeV}$ and tentative spin-parity assignments up to $I^\pi = 18^+$. Linear polarization and angular distribution measurements were used to determine the electromagnetic E1 character of the dipole transitions connecting the positive-parity ground-state band with an excited side-band, firmly establishing it as a negative-parity band. The lowest member of this negative-parity structure was firmly assigned spin-parity $3^-$. In addition, we observed an E3 transition from this $3^-$ state to the ground state, providing direct evidence for octupole collectivity in $^{172}{\mathrm {Pt}}$. Large-scale shell model (LSSM) and total Routhian surface (TRS) calculations have been performed, supporting the interpretation of the $3^-$ state as a collective octupole-vibrational state.
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