Relativistic contribution to the hyperfine interval of the metastable triplet state of 3He

The relativistic correction to the hyperfine-structure splitting of the metastable $2^{3}S_{1}$ state of He³, first calculated by Sessler and Foley to be 315 ± 3 ppm, is reevaluated as ${\Delta }_{rel}=327\pm 3$ ppm. Using this new value, the theoretical ratio of the hyperfine splitting in the $2^{3}S_{1}$ state of He³ to the hyperfine splitting in the 2²S_1/2 state of He³⁺ becomes ${\rho }_{th}=6.2211911\pm 0.0000187$ The experimental value for the ratio is 6.221 1384±0.000 0012, and the 8±3 ppm discrepancy is presently unexplained.