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High magnetic fields (> 1 T) are measured by NMR magnetometers withun-rivaled precision if the precessing spin sample provides long coherencetimes. The longest coherence times are found in diluted ${}^{3}$He samples,which can be hyperpolarized for sufficient signal strength. In order to haveminimal influence on the homogeneity and value of the measured magnetic fieldthe optimal container for the ${}^{3}$He should be a perfect sphere. A fusedsilica sphere with an inner diameter of 8 mm and an outer diameter of 12 mm wasmade from two hemispheres by diffusion bonding leaving only a small hole forcleaning and evacuation. This hole was closed in vacuum by a CO${}_{2}$ laserand the inner volume was filled with a few mbars of ${}^3$He via wallpermeation. NMR-measurements on such a sample had coherence times of 5 min.While the hemispheres were produced with < 1 $\mu$m deviation from sphericity,the bonding left a step of ca. 50 $\mu$m at maximum. The influence of such amismatch, its orientation and materials in the direct vicinity of the sampleare analyzed by FEM-simulations and discussed in view of coherence times andabsolute fields.

Spherical Fused Silica Cells Filled with Pure Helium for NMR-Magnetometry