Evolution of Collectivity in ${}^{72}\mathrm{Kr}$: Evidence for Rapid Shape Transition

The transition rates from the yrast $2^{+}$ and $4^{+}$ states in the self-conjugate ${}^{72}\mathrm{Kr}$ nucleus were studied via lifetime measurements employing the GRETINA array with a novel application of the recoil-distance method. The large collectivity observed for the $4^{+}\to 2^{+}$ transition suggests a prolate character of the excited states. The reduced collectivity previously reported for the $2^{+}\to 0^{+}$ transition was confirmed. The irregular behavior of collectivity points to the occurrence of a rapid oblate-prolate shape transition in ${}^{72}\mathrm{Kr}$, providing stringent tests for advanced theories to describe the shape coexistence and its evolution.

Figure 1

Doppler-shift corrected $\gamma$-ray spectra for (a) ${}^{74}\mathrm{Kr}$ and (b) ${}^{72}\mathrm{Kr}$ compared to the simulated ones (solid curves). The arrows indicate the different components ($f$ is fast, $r$ is reduced, and $s$ is slow) in lifetime measurements. In (b), a reference spectrum simulated with $\tau (2^{+})=5.6\mathrm{ps}$ and $\tau (4^{+})=0\mathrm{ps}$ is also shown (the dotted curve) for ${}^{72}\mathrm{Kr}$.

Figure 2

The results of the line-shape analysis are shown for the $4^{+}\to 2^{+}$ transitions in (a) ${}^{74}\mathrm{Kr}$ and (b) ${}^{72}\mathrm{Kr}$. Data are compared to simulated spectra where the $4^{+}$ lifetime of ${}^{74}\mathrm{Kr}$ (${}^{72}\mathrm{Kr}$) is set as 4.1, 5.9, and 7.7 ps (1.4, 3.5, and 5.6 ps).

Figure 3

Experimental values of (a) excitation energies of the yrast $2^{+}$ and $4^{+}$ states in ${}^{72–78}\mathrm{Kr}$ with $N=36–42$ and (b) $B(E2)$ for their decays. The filled (open) squares denote the present results obtained for the $2^{+}$ ($4^{+}$) states in ${}^{72,74}\mathrm{Kr}$, while previous data (circles) are taken from Refs. [5, 8, 21]. The level scheme for the Se isotopes [21] is shown in (a) for comparison.

Figure 4

The experimental and theoretical values of $B(E2;2^{+}\to 0^{+})$ (filled symbols) and $B(E2;4^{+}\to 2^{+})$ (open symbols) are compared for ${}^{72}\mathrm{Kr}$ (see text). The gray bar for the $2^{+}\to 0^{+}$ transition rate indicates the average of the present and previous [5] data.