We derive the nuclear-spin-relaxation rate of gaseous He due to the magnetic-dipole interaction between the He nuclear spins. This dipolar relaxation rate is numerically evaluated for temperatures from 0.1 to 550 K. At room temperature, the relaxation time for a He density of 10 amagats is 74.4 h. We have made a series of high-density (4 -12 amagat) He samples for which nuclear relaxation is limited by the magnetic-dipole interaction. Both our theoretical and experimental results are particularly important for the growing use of He, polarized through spin exchange with optically pumped Rb vapor.PACS number(s): 34.50. Pi, 33.25.Bn, 32.80.Bx

We have studied the spin relaxation of Xe nuclei due to collisions with Rb atoms at Xe pressures of 245 -1817 Torr. Our results can be characterized by two parameters, the Rb-Xe velocityaveraged binary spin-exchange cross section (o v) and a rate pM that characterizes spin relaxation due to van der Waals molecules. Our results complement earlier studies performed at Xe pressures Qf about 1 Torr and N2 pressures of 10 -100 Torr. This work is useful for predicting spin-exchange rates between polarized Rb atoms and Xe nuclei.PACS number(s): 32.80.Bx, 33.25.Bn, 34.90. +q

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We have studied the &equency shift in the Rb electron-paramagnetic-resonance (EPR) line due to the presence of nuclear polarized He. By measuring the resonant frequency in a cylindrical cell oriented both parallel and perpendicular to an applied magnetic field, we determined the classical EPR shift due to long-range macroscopic fields. We thus calibrated the larger, geometry-independent shift, due to the Fermi-contact interaction between the Rb electron and the He nucleus, to 2.5' %%uo.The EPR frequency shift can be used to determine the He nuclear polarization.