Recoil distance method lifetime measurement of the 21+ state in Sr94 and implications for the structure of neutron-rich Sr isotopes

Abstract

A high precision lifetime measurement of the ${2}_{1}^{+}$ state in $^{94}\mathrm{Sr}$ was performed at TRIUMF's ISAC-II facility by coupling the recoil distance method implemented via the TIGRESS integrated plunger with unsafe Coulomb excitation in inverse kinematics. Due to limited statistics imposed by the use of a radioactive $^{94}\mathrm{Sr}$ beam, a likelihood ratio ${\ensuremath{\chi}}^{2}$ method was derived and used to compare experimental data to Geant4 simulations. The $B(E2;{2}_{1}^{+}\ensuremath{\rightarrow}{0}_{1}^{+})$ value extracted from the lifetime measurement of $7.{80}_{\ensuremath{-}0.40}^{+0.50}\text{(stat.)}\ifmmode\pm\else\textpm\fi{}0.07\text{(sys.)}$ ps is approximately 25% larger than previously reported while the relative error has been reduced by a factor of approximately 8. A baseline deformation has been established for Sr isotopes with $N\ensuremath{\le}58$ which is a necessary condition for the quantum phase transition interpretation of the onset of deformation in this region. A comparison to existing theoretical models is presented.