Nuclear deformation in the $A\approx 100$ region: Comparison between new masses and mean-field predictions

An extension of the atomic mass surface in the region $A\approx 100$ was performed via measurements of the ${}^{100-102}\mathrm{Sr}$ and ${}^{100-102}\mathrm{Rb}$ masses with the ion-trap spectrometer ISOLTRAP at CERN-ISOLDE, including the first direct mass determination of ${}^{102}\mathrm{Sr}$ and ${}^{101,102}\mathrm{Rb}$. These measurements confirm the continuation of the region of deformation with the increase of neutron number, at least as far as $N=65$. To interpret the deformation in the strontium isotopic chain and to determine whether an onset of deformation is present in heavier krypton isotopes, a comparison is made between the experimental values and mean-field and beyond mean-field results available in the literature. To complete this comparison Hartree-Fock-Bogoliubov calculations for even and odd isotopes were performed, illustrating the competition of nuclear shapes in the region.

Figure 1

Scheme of the ISOLTRAP mass spectrometer. For details, see text.

Figure 2

Time-of-flight spectrum of ${}^{101}\mathrm{Sr}$ and ${}^{101}\mathrm{Rb}$ ions after 1000 revolutions inside the MR-ToF MS.

Figure 3

Ramsey-type time-of-flight ion cyclotron resonance of ${}^{102}\mathrm{Sr}$ ions with the fit curve for determining the cyclotron frequency [46].

Figure 4

(a) Experimental two-neutron separation energies in the $A\approx 100$ region. The experimental values from the AME2012 [22] are shown with solid points in different colors, the newly reported values of strontium and rubidium are shown with red solid points and dotted lines. (b) Evolution of experimental mean-square charge radii $\delta {\langle r^2\rangle }_{N,50}$, relative to the corresponding $N=50$ isotopes, in the same region [16, 17, 18, 19]. To avoid an overlap of the $\delta {\langle r^2\rangle }_{N,50}$ curves, they are plotted with an arbitrary offset with respect to the krypton chain.

Figure 5

Experimental two-neutron separation energies (a), and mean-square charge radii (b), for the strontium and krypton isotopic chains. The data are compared to three self-consistent HFB calculations and a beyond-mean-field calculation. For details see text. To avoid an overlap of the $\delta {\langle r^2\rangle }_{N,50}$ curves, the Sr isotopic chains are plotted with an arbitrary offset of $2\mathrm{f}{\mathrm{m}}^2$ with respect to the krypton chain.

Figure 6

Comparison of the experimental odd-even staggering in the region $A\approx 100$ with hfodd calculations. The experimental values from the AME2012 [22] are shown with black solid points and the calculated ground state, prolate-deformed and oblate-deformed solutions with line, dashed line, and dot-dashed line, respectively. To avoid an overlap between the curves, the Sr isotopic chains are plotted with an arbitrary offset of $1\mathrm{M}\mathrm{e}\mathrm{V}$ with respect to the Kr chains.

Figure 7

Comparison of the experimental two-neutron separation energy (a) and mean-square charge radii (b) in the region $A\approx 100$ with HFODD calculations. The experimental values from the AME2012 [22] are shown with black solid triangles, the newly reported values of strontium and rubidium with a dotted line and red solid triangles, and the calculated ground state, prolate-deformed and oblate-deformed solutions with line, dashed line, and dot-dashed line, respectively. The odd nuclei are computed only between $N=57$ and $N=65$. To avoid an overlap of the $\delta {\langle r^2\rangle }_{N,50}$ curves, the Sr isotopic chains are plotted with an arbitrary offset of $2\mathrm{f}{\mathrm{m}}^2$ with respect to the Kr chains.