As each implanted ion enters the target, it undergoes a series of collisions with the target atoms until it finally comes to rest at some depth Rp.
There are two different mechanisms of energy loss.
The total stopping power S of the target, defined as the energy (E) loss per unit path length of the ion(x), is the sum of these two terms:
The relative importance of the two processes depends on the energy E and the atomic number Z of the ion. Note that nuclear collisions dominate at low energies and electronic collisions at high energies. For light ions electronic collisions will dominate where as for heavier ions nuclear collisions will dominate. 
Fig. 2 Nuclear and electronic components of the ion stopping power as a function of ion velocity. The quantity vo is the Bohr velocity and Z1 is the ion atomic number.