Evolution of collectivity in Xe118

Abstract

A recoil-distance Doppler shift experiment has been performed using the $^{102}\mathrm{Pd}(^{19}\mathrm{F},p2n$) reaction at a beam energy of 73 MeV to measure the lifetime of excited states in $^{118}\mathrm{Xe}$. The differential decay-curve method using $\ensuremath{\gamma}\ensuremath{\gamma}$ coincidences and a gating procedure that allows to extract the lifetime without feeding assumptions has been employed. The lifetimes obtained for the yrast states up to spin-parity ${8}^{+}$ are compared with interacting boson model calculations and $^{118}\mathrm{Xe}$ can be classified as a transitional nucleus between the spherical and a deformed shape. Systematics of the $B(E2)$ values for the ${2}^{+}\ensuremath{\rightarrow}{0}^{+}$ and ${4}^{+}\ensuremath{\rightarrow}{2}^{+}$ transitions in the isotopic chains of tin, tellurium and xenon are presented. It is proposed that a ``critical point'' exists at which the $B(E2;{4}^{+}\ensuremath{\rightarrow}{2}^{+})/B(E2;{2}^{+}\ensuremath{\rightarrow}{0}^{+})$ ratio drops to unity for lower neutron numbers within the isotopic chain. The position of the ``critical point'' varies with proton number, i.e., it is presumed to be located at the same mass number $A=114$ in the Sn, Te, and Xe isotopes.