Three-Nucleon Charge Radius: A Precise Laser Determination Using3He

The isotope shift of the $2^3{S}_1-2^3{P}_0$ transition in helium has been measured using laser excitation of an atomic beam, yielding 33 668.074(5) MHz. This value, combined with atomic theory and the ${}^4$He nuclear charge radius, gives a ${}^3$He radius of 1.9506(14) fm. This result is over an order of magnitude more precise than previous determinations. It agrees with two recent theoretical values 1.958(6) and 1.954(7) fm from realistic nucleon-nucleon potentials and the ${}^3$He binding energy. Our result provides both a confirmation and a sensitive test of the nuclear theory of few-nucleon systems.