Lifetimes of low-lying excited states in ${}_{36}^{86}$Kr${}_{50}$

Abstract

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 ${}^{100}$Sn and ${}^{78}$Ni at the proton-rich and proton-deficient extremes, respectively. Experimental discrepancies exist in the data for less exotic systems. In ${}^{86}$Kr the $B(E2;2_1^{+}\to 0_1^{+})$ value - a key indicator of shell evolution - has been experimentally determined by two different methodologies, with the results deviating by $3\ddot{I}ƒ$. 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. The Doppler-shift attenuation method was implemented using the TIGRESS gamma-ray spectrometer and TIGRESS integrated plunger (TIP) device. High-statistics Monte-Carlo simulations were utilized to extract lifetimes in accordance with state-of-the-art methodologies. Lifetimes of $\ddot{I}„(2_1^{+})=336\pm 4\text{(stat.)}\pm 20\text{(sys.)}$ fs, $\ddot{I}„(2_2^{+})=263\pm 9\text{(stat.)}\pm 19\text{(sys.)}$ fs and $\ddot{I}„(3_{(2)}^{-})=73\pm 6\text{(stat.)}\pm 32\text{(sys.)}$ fs were extracted. This yields a transition strength for the first-excited state of $B(E2;2_1^{+}\to 0^{+})=259\pm 3\text{(stat.)}\pm 16\text{(sys.)}$ e${}^2$fm${}^4$. The measured lifetime disagrees with the previous Doppler-shift attenuation method measurement by more than $3\ddot{I}ƒ$, while agreeing well with a previous value extracted from Coulomb excitation. The newly extracted $B(E2;2_1^{+}\to 0_1^{+})$ value indicates a more sudden reduction in collectivity in the $N=50$ isotones approaching $Z=40$.

17 pages, 10 figures, to be submitted to Physical Review C