Lifetimes of low-lying excited states in 8636Kr50

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 Sn-100 and Ni-78 at the proton-rich and proton-deficient extremes, respectively; however, experimental discrepancies exist in the data for less exotic systems. Purpose: In Kr-86 the B(E2; 2(1)(+) -> 0(1)(+)) value-a key indicator of shell evolution-has been experimentally determined by two different methodologies, with the results deviating by 3 sigma. 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 2(2)(+) and 3((2))(-) states in the nucleus. Methods: The Doppler-shift attenuation method was implemented using the TRIUMF-ISAC gamma-ray escape-suppressed spectrometer (TIGRESS) gamma-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 tau(2(1)(+)) = 336 +/- 4(stat.) +/- 20(sys.) fs, tau(2(2)(+)) = 263 +/- 9(stat.) +/- 19(sys.) fs, and tau (3((2))(-)) = 73 +/- 6(stat.) +/- 32(sys.) fs were extracted. This yields a transition strength for the first-excited state of B(E2; 2(1)(+) -> 0(1)(+)) = 259 +/- 3(stat.) +/- 16(sys.) e(2) fm(4). Conclusions: The measured lifetime disagrees with the previous Doppler-shift attenuation method measurement by more than 3 sigma, while agreeing well with a previous value extracted from Coulomb excitation. The newly extracted B(E2; 2(1)(+) -> 0(1)(+)) value indicates a more significant reduction in the N = 50 isotones approaching Z = 40.