[en] The normal muonium signal is studied in Silicon single crystals at low temperature (from 60 mK to 80 K) and low external fields (from 2 Oe to 100 Oe) for different crystal orientations. The inverse damping increases below 5 K and above 80 K; in the intermediate region a constant value of 2 μsec^-1 is found. The angular dependence of the damping and of the doublet's splitting at 70 Oe confirm the dominant isotropic character of the center. Anomalies in the doublets appear in the low temperature region, in particular, and are compatible with perturbations of magnetic origin. (orig.)