This is similar to stopping a projectile in viscous medium:

k depends on ion and target mass and atomic number. The lighter ion, boron, has larger . The crossover point between electronic and nuclear stopping is an important parameter. The critical energy E_c when the nuclear and electronic stoppings are equal is

That is for B, S_e (E) dominates down to 10 keV and for As, nuclear stopping dominates up to 700 keV.