Penetrating Power
The depth to which a given type of radiation can travel into a material before being absorbed.
Unit
mm
In depth
Penetration is inversely related to ionising power: heavy, highly charged alpha particles lose their energy within a few centimetres of air, beta particles pass through millimetres of aluminium, and gamma rays require centimetres of lead to attenuate significantly.
Examples in the real world
Paper stopping alpha, thin aluminium stopping beta, and thick lead or concrete needed for gamma.