David Hanneke with Arnold Dahm

A New Technique for Measuring Isotopic Impurity Diffusion in Solid Helium

At low temperature and high pressure, ⁴He forms a solid with an hcp lattice. At temperatures near the melting point, ³He atoms diffuse by tunneling into delocalized vacancies about which little is known experimentally. Reported diffusion coefficients and their temperature dependencies are at variance with each other and may result from an anisotropic diffusion tensor. We propose to design and build a system that uses NMR techniques to observe this behavior. Using a gradient coil, we will observe NMR in a thin slice. We will saturate the spins in this slice with a large rf pulse, causing the NMR signal to vanish, and watch the signal recover as thermalized ³He atoms diffuse into the slice. The diffusion coefficient along the field gradient will thus be measured. Three orthogonal gradient coils will be used to measure the diffusion tensor in a single hcp helium crystal.