Nuclear Charge Radius of ${}^{12}\mathrm{Be}$

The nuclear charge radius of ${}^{12}\mathrm{Be}$ was precisely determined using the technique of collinear laser spectroscopy on the $2s_{1/2}\to 2p_{1/2,3/2}$ transition in the ${\mathrm{Be}}^{+}$ ion. The mean square charge radius increases from ${}^{10}\mathrm{Be}$ to ${}^{12}\mathrm{Be}$ by $\delta ⟨r_c^2{⟩}^{10,12}=0.69(5){\mathrm{fm}}^2$ compared to $\delta ⟨r_c^2{⟩}^{10,11}=0.49(5){\mathrm{fm}}^2$ for the one-neutron halo isotope ${}^{11}\mathrm{Be}$. Calculations in the fermionic molecular dynamics approach show a strong sensitivity of the charge radius to the structure of ${}^{12}\mathrm{Be}$. The experimental charge radius is consistent with a breakdown of the $N=8$ shell closure.

Figure 1

Fluorescence spectra of ${}^{12}\mathrm{Be}$ in the $D1$ transition as a function of the Doppler-tuned frequency in collinear (left) and anticollinear (right) direction. A Voigt profile is fitted to the data points. The lower trace shows the residua of the fitting.

Figure 2

Experimental charge radii of beryllium isotopes from isotope shift measurements in 2008 (large blue ◆) and in 2010 (this study, small red $•$) compared with the FMD results (this work ▪). Charge radii calculated for the pure $p^2$ configuration (▽) and pure $(sd{)}^2$ intruder configuration (▵) are also indicated.

Figure 3

Cuts through the intrinsic densities of the ${}^{12}\mathrm{Be}$ states in $p^2$ (left) and $(sd{)}^2$ configurations (right) obtained by variation after projection. The contour lines are in units of nuclear matter density.