The change of the basic cell parameters, due to hydrogen passivation with a Kaufmann source, has been measured as a function of passivation temperature, ion-beam energy, and dose rate. The inverse relationship between the observed improvements and the initial values of the parameters is evidentiated. The contributions of annealing and molecular hydrogen passivation are independently evaluated. The results are explained by a straight model of thermally activated hydrogen diffusion, with an amplitude factor which contains the depth of implantation.