Paramagnetic relaxation of spin polarized 3He at bare glass surfaces – Part I

In this first in a series of three papers on wall relaxation of spin polarized, gaseous He-3 we investigate both by theory and by experiment surface-induced spin relaxation due to paramagnetic sites in the containing glass. We present experimental and theoretical evidence that - contrary to the traditional opinion - distant dipolar coupling to paramagnetic impurities in the glass, in particular iron ions, cannot be the dominant relaxation mechanism of He-3-spins, although iron dominates the bulk static permeability. Instead dangling-bond type defects in the glass matrix are found to interact much stronger via the isotropic Fermi contact interaction. A model of paramagnetic site controlled He-3 relaxation including the Fermi contact interaction is presented. With reasonable semi-empirical assumptions our model allows to describe satisfactorily the measured relaxivities, both in the dissolution-dominated regime of fused silica or borosilicate glasses of the Pyrex type as well as in the surface dominated situation of aluminosilicate glasses which have only a low permeability for He atoms. In a large sample of 1.1 litre cells, built from various aluminosilicate glasses, an average relaxation time of 150 h is reached in case contaminant ferromagnetic particles have been demagnetized beforehand. From the maximum observed value of 250 h we derive after subtraction of dipolar relaxation in the gas phase a paramagnetic surface relaxivity of rho < 0.005 cm/h at room temperature