NUCLEAR-MAGNETIC-RESONANCE OF HE-3 DISSOLVED IN VITREOUS SILICA

For the first time He-3 ps dissolved in solids (pyrex and suprasil glasses) has been studied by NMR. The chemical shifts are mainly determined by interaction with diamagnetic atoms of the glass structure and not by magnetic interaction with Fe3+ ions. In pyrex the relaxation times at 25-degrees-C are: T1(He-3) = 65 +/- 6.5 ms, T2(He-3) = 1.1 +/- 0.1 ms and the activation energy for relaxation is E(a)(T1) = E(a)(T2) = 3.4 +/- 0.3 kJ/mol (-70 to 100-degrees-C). In suprasil T2(He-3) = 1.1 +/- 0.1 s (25-degrees-C), E(a) = 18 +/- 2 kJ/mol (-40 to 20-degrees-C) and T1 (He-3) = 370 +/- 40 s (0-degrees-C). The relaxation times in both glasses are determined by magnetic dipole-dipole interaction, in pyrex with Fe3+ ions modulated by spin-flip and in suprasil with unpaired electrons of defects in the glass structure modulated by the fast diffusion of helium atoms. The estimated values of distances of closest approach of diffusing He-3 atoms to Fe3+ ions in pyrex and to defects in suprasil are approximately 5 and approximately 3 angstrom, respectively.