Why does the relatively large mass and charge of an alpha particle limit its penetrating power?

Answer 1

Interactions between various types of radiation and the material they pass through are governed by two properties: how likely an interaction is and what the effect of an interaction will be.

The likely hood of an interaction is based on the "size" of the radiation (both mass and charge), the size of the atoms it is passing near (radius and Atomic Mass), and how close together those atoms are (density of the material). Alpha radiation is so large, combined with higher charge than other radiation, that the chance of an interaction is very great, even with small atoms spread thinly, like air.

Radiation with low momentum, like beta radiation or gamma radiation, can interact with atoms many times before losing energy. Picture a marble bouncing off a billiard ball. The billiard ball will only move a little, and the marble will go bouncing off at close to the same speed. The difference in "size" means the gamma radiation can hit many different atoms before losing it's energy. But an alpha particle will hit a single atom and lose a large portion of it's energy. Picture two billiard balls colliding. The quickly moving ball slows down a lot, and the other ball speeds off. So an alpha particle loses it's energy in a much smaller number of collisions.

Taken together, the high chance of a collision and the large loss of energy with each collision cause alpha particles to stop very quickly, even in lightweight materials like air, paper, cloth or plastic.