Spin diffusion measurements in spin-polarised liquid He-3-He-4 mixtures

We report on investigations of spin diffusion in isotopic helium liquid mix tures using a multi-echo technique in applied magnetic field gradients. Sam ples (up to 1 cm(3) in volume) are obtained by liquefaction of optically po larised gas. NMR measurements are performed at saturated vapour pressure in a low magnetic field (2.2 mT). Signal-to-noise ratios stay high down to He -3 concentrations of order a few parts per thousand at 1K for instance. We demonstrate that the experimental accuracy of spin diffusion measurements i s substantially improved using optically polarised mixtures. We have observ ed that the dynamics of the precession of the transverse magnetisation can be altered due to large dipolar fields at high magnetisation densities. Eff ects on spin-echo signals and implications for the extension of spin diffus ion measurements with our technique are discussed.