Tellurium donors in GaP have been ionized by phonon-assisted tunneling in the electric field of pulsed far-infrared laser radiation. In response to the laser pulse a photoconductive signal has been detected with a fast component that follows in time the laser pulse and a slow component that rises after the irradiation has ceased and finally exponentially decays with a strongly temperature-dependent time constant varying from several microseconds to milliseconds. It is shown that this temporal structure of the signal is due to a storage of carriers in the valley-orbit split 1s(E) shallow donor state. Observation of far-infrared to mid-infrared up-conversion demonstrates that the final step of cascade recombination is achieved by radiative transitions.