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

TL;DR

This paper presents high-precision measurements of lifetimes and transition strengths of low-lying excited states in $^{86}$Kr, providing new insights into shell evolution and nuclear structure at the $N=50$ magic number.

Contribution

The study introduces the first measured lifetimes for certain excited states in $^{86}$Kr and a new high-precision value for the $B(E2)$ transition strength, resolving previous experimental discrepancies.

Findings

Measured lifetimes: $ au(2^+_1)=336 ext{ fs}$, $ au(2^+_2)=263 ext{ fs}$, $ au(3^-_{(2)})=73 ext{ fs}$.

New $B(E2;2^+_1 ightarrow0^+)=259 ext{ e}^2 ext{fm}^4$ value.

Discrepancy with previous Doppler-shift measurement resolved, aligning with Coulomb excitation results.

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 $B(E2;2^+_1\rightarrow0^+_1)$ 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. 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 $\tau(2^+_1)=336\pm4\text{(stat.)}\pm20\text{(sys.)}$ fs, $\tau(2^+_2)=263\pm9\text{(stat.)}\pm19\text{(sys.)}$ fs and $\tau(3^-_{(2)})=73\pm6\text{(stat.)}\pm32\text{(sys.)}$ fs were extracted. This yields a transition strength for the first-excited state of $B(E2;2^+_1\rightarrow0^+)=259\pm3\text{(stat.)}\pm16\text{(sys.)}$ e$^2$fm$^4$. 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 $B(E2;2^+_1\rightarrow0^+_1)$ value indicates a more sudden reduction in collectivity in the $N=50$ isotones approaching $Z=40$.