  We present the results of experiments on dynamic nuclear polarization and
relaxation of 75As in silicon crystals. Experiments are performed in strong
magnetic fields of 4.6 T and temperatures below 1 K. At these conditions donor
electron spins are fully polarized, and the allowed and forbidden ESR
transitions are well resolved. We demonstrate effective nuclear polarization of
75As nuclei via the Overhauser effect on the time scale of several hundred
seconds. Excitation of the forbidden transitions leads to a polarization
through the solid effect. The relaxation rate of donor nuclei has strong
temperature dependence characteristic of Orbach process.
