Early Onset of Ground State Deformation in Neutron Deficient Polonium Isotopes

In-source resonant ionization laser spectroscopy of the even-$A$ polonium isotopes ${}^{192–210,216,218}\mathrm{Po}$ has been performed using the $6p^{3}7s$ ${}^{5}S_2$ to $6p^{3}7p$ ${}^{5}P_2$ ($\lambda =843.38\mathrm{nm}$) transition in the polonium atom (Po-I) at the CERN ISOLDE facility. The comparison of the measured isotope shifts in ${}^{200–210}\mathrm{Po}$ with a previous data set allows us to test for the first time recent large-scale atomic calculations that are essential to extract the changes in the mean-square charge radius of the atomic nucleus. When going to lighter masses, a surprisingly large and early departure from sphericity is observed, which is only partly reproduced by beyond mean field calculations.

Figure 1

From top to bottom: example of a laser scan around the 843.38 nm transition between the $6p^{3}7s$ ${}^{5}S_2$ and $6p^{3}7p$ ${}^{5}P_2$ atomic excited states in even-$A$ Po-I for $A=218$ down to $A=192$. For $A=192$, several scans have been added together to obtain sufficient statistics. The line is the result from a fit of those spectra using an asymmetric Voigt profile; more information on this asymmetry can be found in the text.

Figure 2

King plot between the transitions at 255.8 nm ([17], $x$ axis) and at 843.38 nm (present work, $y$ axis) for ${}^{200–210}\mathrm{Po}$. The solid line is a linear fit through the data points; the dotted red line is the calculated relation from the large-scale atomic calculation and lies within $1\sigma$ of the fit.

Figure 3

(top) $\delta ⟨r^2⟩$ for the even-$Z$ isotopes from platinum ($Z=78$) to radium ($Z=88$) [5, 7, 24, 27, 29, 30, 31]. The solid black line represents the predictions from the spherical FRDM [18] using the second parametrization from Ref. 23. (bottom) Difference between the measured $\delta ⟨r^2⟩$ to the spherical FRDM. The dotted lines represent the beyond mean field calculations with the SLy4 and $\mathrm{SLy}{4}^{*}$ (with reduced pairing) interactions [25, 26].