Shape Evolution in Neutron-Rich Krypton Isotopes Beyond $N=60$: First Spectroscopy of ${}^{98,100}\mathrm{Kr}$

We report on the first $\gamma$-ray spectroscopy of low-lying states in neutron-rich ${}^{98,100}\mathrm{Kr}$ isotopes obtained from ${}^{99,101}\mathrm{Rb}(p,2p)$ reactions at $\sim 220\mathrm{MeV}/\mathrm{nucleon}$. A reduction of the $2_1^{+}$ state energies beyond $N=60$ demonstrates a significant increase of deformation, shifted in neutron number compared to the sharper transition observed in strontium and zirconium isotopes. State-of-the-art beyond-mean-field calculations using the Gogny D1S interaction predict level energies in good agreement with experimental results. The identification of a low-lying ($0_2^{+}$, $2_2^{+}$) state in ${}^{98}\mathrm{Kr}$ provides the first experimental evidence of a competing configuration at low energy in neutron-rich krypton isotopes consistent with the oblate-prolate shape coexistence picture suggested by theory.

Figure 1

Doppler-corrected $\gamma$ spectrum of ${}^{98}\mathrm{Kr}$ (top) and ${}^{100}\mathrm{Kr}$ (bottom) produced from ${}^{99,101}\mathrm{Rb}(p,2p)$ reactions. For each spectrum, the global fit (solid black line) includes individual simulated responses (red dashed lines) and a two-component exponential background (red dotted line). Add-back was applied and $\gamma$ multiplicities below 6 were considered. The inset of the top panel shows background-subtracted $\gamma \text{−}\gamma$ coincidence spectra for ${}^{98}\mathrm{Kr}$.

Figure 2

(a) Systematics of experimental data for $E(2_1^{+})$. Data points from this Letter are highlighted in red. Error bars are smaller than the marker size. (b) Experimental level schemes for ${}^{98,100}\mathrm{Kr}$.

Figure 3

Comparison between experimental and theoretical low-lying states in ${}^{98,100}\mathrm{Kr}$ isotopes from this work.

Figure 4

Probability densities in the $\beta \text{−}\gamma$ deformation space for low-lying $0_1^{+}$, $2_1^{+}$, $4_1^{+}$, $0_2^{+}$, and $2_2^{+}$ collective states in ${}^{98,100}\mathrm{Kr}$ from 5DCH calculations (see text for details).

Figure 5

Comparison between experimental and 5DCH+D1S theoretical energies of the low-lying $2_1^{+}$, $4_1^{+}$, $0_2^{+}$, and $2_2^{+}$ states in krypton isotopes. Open symbols are for states seen experimentally for which spin-parity assignment is uncertain. Up to $N=60$, energies are taken from [26, 40, 49].