Since they have no charge, gamma-rays and X-rays
proceeds through matter until there is a chance of
interaction with a particle. If the particle is an
electron, it may receive enough energy to be
ionized, whereupon it causes further ionization by
direct interactions with other electrons. As a result,
gamma-rays and X-rays can cause the liberation of
electrons deep inside a medium. As a result, a given
gamma or X-ray has a definite probability of
passing through any medium of any depth.