  Hyperfine intervals in light hydrogenic atoms and ions are among the most
accurately measured quantities in physics. The theory of QED corrections has
recently advanced to the point that uncalculated terms for hydrogenic atoms and
ions are probably smaller than 0.1 parts per million (ppm), and the experiments
are even more accurate. The difference of the experiments and QED theory is
interpreted as the effect on the hyperfine interaction of the (finite) nuclear
charge and magnetization distributions, and this difference varies from tens to
hundreds of ppm. We have calculated the dominant component of the 1s hyperfine
interval for deuterium, tritium and singly ionized helium, using modern
second-generation potentials to compute the nuclear component of the hyperfine
splitting for the deuteron and the trinucleon systems. The calculated nuclear
corrections are within 3% of the experimental values for deuterium and tritium,
but are about 20% discrepant for singly ionized helium. The nuclear corrections
for the trinucleon systems can be qualitatively understood by invoking SU(4)
symmetry.
