Nuclear Shape Transition, Triaxiality and Energy Staggering of gamma Band States for Even-Even Xenon Isotopic Chain

TL;DR

This paper investigates shape phase transitions, triaxiality, and energy staggering in even-even Xenon isotopes using the interacting boson model and a collective nuclear softness model, revealing critical points and triaxial features.

Contribution

It introduces a modified IBM1 approach with a three-body interaction to describe triaxiality and analyzes energy staggering using a combined model, providing new insights into shape evolution.

Findings

Xenon isotopes evolve from spherical to gamma-soft shapes with increasing boson number.

The critical nucleus is identified at 126Xe, with 128Xe exhibiting triaxiality.

Strong odd-even spin energy staggering is observed in gamma bands.

Abstract

The positive-parity states of even-even Xe nuclei are investigated within the framework of modified O(6) limit of the interacting boson model (IBM1). The effective three-body interaction [QQQ] where Q is the IBM O(6) quadrupole operator is introduced to exhibit the triaxiality nature. The shape of nuclear surface is described by the deformation parameters beta, gamma by using the intrinsic coherent state. The potential energy surfaces (PES) of the transition U(5)-Triaxiality-O(6) are calculated and analyzed and the critical phase transition points are identified. For each nucleus a fitting procedure is adopted to get the best model parameters by fitting some selected calculated energy levels and B(E2)transition rates ratios with experimental ones. These ratios are analyzed because they serve as effective order parameters in the shape phase transition. The nuclei in Xe isotopic chain evolve from spherical vibrator U(5) to gamma-soft rotor O(6) by increasing the boson number from N=3 (heavy isotope 132Xe) to N=10 (light isotope 120Xe) and the isotope 126Xe represent the critical nucleus. The nucleus 128Xe has triaxial nature. To deal with high spin states in gamma band in 118- 128Xe isotopic chain to investigate and exhibit the odd-even spin energy staggering, we introduce the two parameters collective nuclear softness rotor model (CNS2). Three different staggering indices depending on the dipole transitions linking the two families of spins and the quadrupole transitions within each spin family are considered. Strong odd-even spin energy staggering has been seen. As a link between the IBM and CNS2 models we observed that the energy difference between the gamma-band and ground state band normalized to decreases with increasing the mass number