Recoil distance method lifetime measurement of the $2^{+}_{1}$ state in $^{94}$Sr and implications for the structure of neutron-rich Sr isotopes

The TIGRESS Integrated Plunger (TIP) device [1] has been constructed to enable Doppler-shift lifetime measurements at TRIUMF's ISAC-II facility. TIP was commissioned using a stable $^{84}$Kr beam by coupling the recoil distance method with unsafe Coulomb excitation in inverse kinematics [2]. A high-precision lifetime measurement of the $2^{+}_{1}$ state in $^{94}$Sr was performed using the same Coulex-RDM technique. The data set had low statistics due to the luminosity of the radioactive $^{94}$Sr beam. A lifetime of $\tau = 7.80^{+0.50}_{-0.40}~\text{(stat.)} \pm 0.04~\text{(sys.)}$ ps was determined by comparing experimental data to Geant4 simulations using a likelihood ratio $\chi^{2}$ method. The corresponding $B(E2;2^{+}_{1} \rightarrow 0^{+}_{1})$ value 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$\leq$58 [3]. The experimental results, data analysis methods, and a comparison to existing theoretical models are presented. [1] Voss et al., NIM A 746 87--97 (2014). [2] Chester et al., NIM A 882 69--83 (2018). [3] Chester et al., PRC 96 011302(R) (2017).