Gaseous 3He-3He magnetic dipolar spin relaxation.

We derive the nuclear-spin relaxation rate of gaseous [sup 3]He due to the magnetic-dipole interaction between the [sup 3]He nuclear spins. This dipolar relaxation rate is numerically evaluated for temperatures from 0.1 K to 550 K. At room temperature, the relaxation time for a [sup 3]He density of 10 amagats is 74.4 h. We have made a series of high-density (4--12 amagat) [sup 3]He samples for which nulcear relaxation is limited by the magnetic-dipole interaction. Both our theoretical and experimental results are particularly important for the growing use of [sup 3]He, polarized through spin exchange with optically pumped Rb vapor.