Abstract
A simple encounter probability model is constructed for approximating the mean L-shell ionisation probability at zero impact parameter, pL(0), as used in the multiple ionisation theories of heavy-ion collisions. The model establishes a simple scaling rule for pL(0) as a function of Z1/vi which behaves asymptotically as pL(0) approximately=(Z1/vi)2 for low projectile charge Z1 and tends to unity for high Z1 values, in the high velocity limit vi>>va.