An experimental profile of  La implanted in a Ni crystal is theoretically described on the basis of a continuity equation for the La concentration, which includes a diffusive relocation of the implanted atoms and the matrix erosion velocity. The La sputtering process is explicitly considered by an appropriate boundary condition. The shape of the experimental La profile as well as the measured retained La dose has been well reproduced. It is shown that the erosion velocity of La atoms is greater than that of Ni atoms by a factor of 2.