Isomers and oblate rotation in Pt isotopes: Delineating the limit for collectivity at high spins

Abstract

Rotation-aligned isomeric states and associated oblate collective sequences are established in even Pt isotopes. Reduced E2 transition probabilities for the deexcitation of the 12 + isomers indicate an abrupt and unexpected quenching of oblate collectivity around neutron number N = 120. Structure and shape evolution at high spin in the heaviest stable isotopes is found to be markedly different from observations in the lighter ones.

Isomers and oblate collectivity at high spin in neutron-rich Pt isotopes

“…Total Energy Surface plots for 192,194 Pt indicate oblate energy minima near and beyond the 12 + states as illustrated in Fig. 2 agreement with the ones inferred from experiment; prolate minima are predicted to become progressively more non-yrast with increasing neutron number [11]. The magnitude of quadrupole deformation is predicted to decrease for higher neutron numbers consistent with the reduction in B(E2; 2 + → 0 + ) values described earlier.…”

“…Shape evolution for lighter isotopes (up to 192 Pt) follows a similar pattern with triaxial shapes being favored at intermediate spin (I ≤ 10 ), and oblate ones lowest in energy at high spin. In 194,196 Pt though, an oblate shape is realized at lower spin and persists to high spin [11]. The excitation energy of the 12 + states gradually decreases with increase in neutron number up to 194 Pt, and is associated with the lowering of the oblate energy minimum evident from calculations [11].…”

“…The beam was delivered by the ATLAS accelerator at Argonne National Laboratory and γ rays emitted in the deexcitation of the reaction products were detected by the Gammasphere array consisting of 100 high-purity, Compton-suppressed Ge detectors [10]. The data were sorted into both symmetric and asymmetric histograms ranging from two-to four-fold, depending on the energy and time parameters involved; details may be found in [11]. Lifetimes of isomeric states were determined, and both prompt and delayed (from isomeric decays) transitions were established from the data 198 Pt) have been determined using the centroid shift method.…”

“…1, wherein the centroid of the time distribution of a delayed transition is shifted with respect to that of a prompt one by an amount equal to the mean life of the state. For 198 Pt, the half-life has been determined by observing the time variation in cumulative intensity of delayed transitions [11]. Isomer half-lives and B(E2) values are listed in Table 1.…”

“…In 194,196 Pt though, an oblate shape is realized at lower spin and persists to high spin [11]. The excitation energy of the 12 + states gradually decreases with increase in neutron number up to 194 Pt, and is associated with the lowering of the oblate energy minimum evident from calculations [11]. Beyond 194 Pt, the 12 + energy increases since the complete filling of the i 13/2 subshell at N=120 ( 198 Pt) approaches, as a result of which more energy is required to excite a pair of neutrons from this state.…”