Lifetime Measurements of Excited States in Pt172 and the Variation of Quadrupole Transition Strength with Angular Momentum

Abstract

Lifetimes of the first excited 2^{+} and 4^{+} states in the extremely neutron-deficient nuclide ^{172}Pt have been measured for the first time using the recoil-distance Doppler shift and recoil-decay tagging techniques. An unusually low value of the ratio B(E2:4_{1}^{+}→2_{1}^{+})/B(E2:2_{1}^{+}→0_{gs}^{+})=0.55(19) was found, similar to a handful of other such anomalous cases observed in the entire Segré chart. The observation adds to a cluster of a few extremely neutron-deficient nuclides of the heavy transition metals with neutron numbers N≈90-94 featuring the effect. No theoretical model calculations reported to date have been able to explain the anomalously low B(E2:4_{1}^{+}→2_{1}^{+})/B(E2:2_{1}^{+}→0_{gs}^{+}) ratios observed in these cases. Such low values cannot, e.g., be explained within the framework of the geometrical collective model or by algebraic approaches within the interacting boson model framework. It is proposed that the group of B(E2:4_{1}^{+}→2_{1}^{+})/B(E2:2_{1}^{+}→0_{gs}^{+}) ratios in the extremely neutron-deficient even-even W, Os, and Pt nuclei around neutron numbers N≈90-94 reveal a quantum phase transition from a seniority-conserving structure to a collective regime as a function of neutron number. Although a system governed by seniority symmetry is the only theoretical framework for which such an effect may naturally occur, the phenomenon is highly unexpected for these nuclei that are not situated near closed shells.

B(E2) anomalies in the yrast band of Os170

“…The B(E 2; 4 + 1 → 2 + 1 ) of 0.21(0.05) e 2 b 2 is, in contrast to the "normal" B(E 2; 2 + 1 → 0 + 1 ), surprisingly small, resulting in a B 4/2 ratio of 0.38 (11). Similar results were obtained in 166 W [28], 168 Os [23], and 172 Pt [29]. Based on those measurements, Cederwall et al [29] proposed a transition at N = 94 from a collective regime into a senioritylike scheme as a possible explanation for the B 4/2 < 1, despite the presence of a large number of valence nucleons.…”

“…Similar results were obtained in 166 W [28], 168 Os [23], and 172 Pt [29]. Based on those measurements, Cederwall et al [29] proposed a transition at N = 94 from a collective regime into a senioritylike scheme as a possible explanation for the B 4/2 < 1, despite the presence of a large number of valence nucleons. The position of such a phase transition at N = 94 is supported by the present data.…”

“…It is worth pointing out that for 168 Os Grahn et al [23] measured a reduced transition probability for the yrast 4 + resulting in B 4/2 = B(E 2; 4 + 1 → 2 + 1 )/(B(E 2; 2 + 1 → 0 + 1 ) = 0.34 (18). A B 4/2 < 1 is very rare throughout the nuclear chart, with only a handful of examples away from closed shell nuclei, e.g., 48,50 Cr [24], 72,74 Zn [25], 114 Te [26], 114 Xe [27], 166 W [28], 168 Os [23], and 172 Pt [29]. These observations have not been reproduced so far by any type of state-of-the-art nuclear structure calculations, neither large-scale shell models nor beyond-mean-field models.…”

New γ-soft rotation in the interacting boson model with SU(3) higher-order interactions *

“…Some experimental requirements remain to deepen our understanding on γ-soft triaxiality and its relationship with the higher-order interactions. In recent experimental investigations, a cluster of extremely neutron-deficient nuclei 168 Os [23], 166 W [24],…”

Quadrupole dominance in the light Sn and in the Cd isotopes

“…The B(E2 : 4 + 1 → 2 + 1 ) rates are seen to demand special scrutiny. To test how they are influenced by the single particle field, the energy of the s 1/2 orbit in 101 Sn was displaced by 0.0, 0.4 and 0.8 MeV with respect to the present GEMO choice [21], called DZ (Duflo Zuker) [32], in 114 Te [33] , and more recently in 172 Pt, Ref [34], where it is stressed that no theoretical explanation is available. Here, the sensitivity to the pairing strength provides a clue.…”