On-line high resolution laser spectroscopy has been performed on radioactive ${}_{}{}^{20-31}{}_{}{}^{}\mathrm{Na}$. New values for the isotope shift of the $D_1$ line of ${}_{}{}^{20-24,31}{}_{}{}^{}\mathrm{Na}$ relative to ${}_{}{}^{23}{}_{}{}^{}\mathrm{Na}$ have been obtained. The spectroscopic quadrupole moments of ${}_{}{}^{21,25-29}{}_{}{}^{}\mathrm{Na}$ have been determined using a double resonance method. The $D_2$ line hyperfine structure of ${}_{}{}^{21,25-27}{}_{}{}^{}\mathrm{Na}$ has also been studied by laser optical spectroscopy. Results are in agreement with those obtained by rf double resonance. No evidence of the sudden onset of a deformation expected at $N=20\mathrm{}$ has been found. A strong shell effect in the change in the mean-square charge radius is exhibited at $N=14\mathrm{}$.

NUCLEAR REACTIONS ${}_{}{}^{20-24,31}{}_{}{}^{}\mathrm{Na}$; measured isotope shift. ${}_{}{}^{20-22,24,31}{}_{}{}^{}\mathrm{Na}$; measured hyperfine constants $A\left(3s {}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}\right)$, $A\left(3p {}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}\right)$; deduced $\mu$. ${}_{}{}^{21,25-29}{}_{}{}^{}\mathrm{Na}$; measured hyperfine constants $A\left(3p {}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}\right)$, $B\left(3p {}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}\right)$; deduced $Q$. Atomic beam laser spectroscopy and laser rf double-resonance spectroscopy.

• Received 28 December 1981

DOI:https://doi.org/10.1103/PhysRevC.25.2756