As penetrating radiation moves from point to point in matter, it loses its energy
through various interactions with the atoms it encounters. The rate at which this
energy loss occurs depends upon the type and energy of the radiation and the density
and atomic composition of the matter through which it is passing.
The various types of penetrating radiation impart their energy to matter primarily
through excitation and ionization of orbital electrons. The term “excitation” is used to
describe an interaction where electrons acquire energy from a passing charged particle
but are not removed completely from their atom. Excited electrons may subsequently
emit energy in the form of X-rays during the process of returning to a lower energy
state. The term “ionization” refers to the complete removal of an electron from an
atom following the transfer of energy from a passing charged particle.