Experimental and theoretical studies of hyperfine structures in ${}^{21}$Na

We measured the hyperfine structure constants, $A(3s^2{S}_{1\mathrm{/}2})$ and $A(3p^2{P}_{1\mathrm{/}2})$, of the neutron-deficient isotope ${}^{21}\text{Na}$ using CLaSsy, a setup dedicated to collinear laser spectroscopy at RAON. The hyperfine structure constants of ${}^{21}\text{Na}$ were measured to be $103.6(10{)}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}}(9{)}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}}$ MHz for $A(3p^2{P}_{1\mathrm{/}2})$ and $954.9(11{)}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}}(25{)}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}}$ MHz for $A(3s^2{S}_{1\mathrm{/}2})$. A systematic comparison with the state-of-the-art ab-initio relativistic coupled cluster calculations shows the role of higher-order correlation effects such as triple excitations in ${}^{21}$Na. Furthermore, the measurement demonstrates a capability of the CLaSsy setup to conduct collinear laser spectroscopy experiments with a radioactive beam.