Laser Spectroscopic Determination of the Nuclear Charge Radius of $^6$He and $^8$He*

Laser spectroscopic measurements of atomic isotope shifts provide unique access to the nuclear charge distribution of short-lived isotopes. The isotopes of interest for this study are $^6$He (t$_{1/2}$ = 807 ms) and $^8$He (t$_{1/2}$ = 119 ms), which exhibit a loosely bound neutron halo around an $\alpha $-like core. Charge radii measurements of both isotopes provide corroboration for their halo structure and test nuclear structure theories of light nuclei. We have performed high-resolution laser spectroscopy on individual radioactive helium atoms captured in a magneto-optical trap. This technique enabled us to accurately measure the isotope shift between $^6$He and $^4$He in a selected atomic transition. Based on this result and precision atomic theory calculation of helium, the root-mean-square charge radius of the $^6$He nucleus could be determined to be 2.054(14) fm [1]. Currently, we are working to expand this technique to also measure the nuclear charge radius of $^8$He, and we will report on first results from this experiment. [1] L.-B. Wang {\it et al.}, PRL 93, 142501 (2004)