Lifetimes of low-lying excited states in Kr50 3686

Background: The evolution of nuclear magic numbers at extremes of isospin is a topic at the forefront of contemporary nuclear physics. N=50 is a prime example, with increasing experimental data coming to light on potentially doubly magic Sn100 and Ni78 at the proton-rich and proton-deficient extremes, respectively; however, experimental discrepancies exist in the data for less exotic systems. Purpose: In Kr86 the B(E2;21+→01+) value- A key indicator of shell evolution-has been experimentally determined by two different methodologies, with the results deviating by 3σ. Here, we report on a new high-precision measurement of this value, as well as the first measured lifetimes and hence transition strengths for the 22+ and 3(2)-states in the nucleus. Methods: The Doppler-shift attenuation method was implemented using the TRIUMF-ISAC γ-ray escape-suppressed spectrometer (TIGRESS) γ-ray spectrometer and the TIGRESS integrated plunger device. High-statistics Monte Carlo simulations were utilized to extract lifetimes in accordance with state-of-the-art methodologies. Results: Lifetimes of τ(21+)=336±4(stat.)±20(sys.) fs, τ(22+)=263±9(stat.)±19(sys.) fs, and τ(3(2)-)=73±6(stat.)±32(sys.) fs were extracted. This yields a transition strength for the first-excited state of B(E2;21+→01+)=259±3(stat.)±16(sys.) e2 fm4. Conclusions: The measured lifetime disagrees with the previous Doppler-shift attenuation method measurement by more than 3σ, while agreeing well with a previous value extracted from Coulomb excitation. The newly extracted B(E2;21+→01+) value indicates a more significant reduction in the N=50 isotones approaching Z=40.