Nuclear Magnetic Susceptibility ofHe3Vapor

Measurements of relative values of the nuclear magnetic susceptibility of ${\mathrm{He}}^3$ vapor have been made by a pulsed resonance technique for several values of vapor density at 2 and 3.1°K. The results of these measurements show that deviations from Curie's law are considerably smaller than those expected for a perfect Fermi gas. A theoretical expression is derived for the susceptibility at vapor densities sufficiently low that only binary collisions are important; a modification of the calculation of the second virial coefficient is used to evaluate the effect of direct interactions. The susceptibility is related to the difference between two virial coefficients, that of ${\mathrm{He}}^3$ and that calculated for a hypothetical ${\mathrm{He}}^3$ with zero spin. The theoretical results are in good quantitative agreement with the experimental data. They indicate that the perfect-Fermi-gas theory is not applicable to the calculation of the susceptibility of ${\mathrm{He}}^3$ vapor at low density except at temperatures lower than 0.002°K. Moreover, from the theoretical results it is concluded that deviations from Curie's law in the vapor are expected to be less than about 1% whenever higher order collisions do not contribute significantly to the interaction between ${\mathrm{He}}^3$ atoms.