The alpha particle is a helium nucleus produced from the radioactive decay of heavy metals and some nuclear reactions. Alpha decay often occurs among nuclei that have a favorable neutron/proton ratio, but contain too many nucleons for stability. The alpha particle is a massive particle consisting of an assembly of two protons and two neutrons and a resultant charge of +2.
Alpha particles are the least penetrating radiation. The major energy loss for alpha particles is due to electrical excitation and ionization. As an alpha particle passes through air or soft tissue, it loses, on the average, 35 eV per ion pair created. Due to its highly charged state, large mass, and low velocity, the specific ionization of an alpha particle is very high.
Figure 1 illustrates the specific ionization of an alpha particle, on the order of tens of thousands of ion pairs per centimeter in air. An alpha particle travels a relatively straight path over a short distance.
Beta Particle
The beta particle is an ordinary electron or positron ejected from the nucleus of a beta-unstable radioactive atom. The beta has a single negative or positive electrical charge and a very small mass.
The interaction of a beta particle and an orbital electron leads to electrical excitation and ionization of the orbital electron. These interactions cause the beta particle to lose energy in overcoming the electrical forces of the orbital electron. The electrical forces act over long distances; therefore, the two particles do not have to come into direct contact for ionization to occur.
The amount of energy lost by the beta particle depends upon both its distance of approach to the electron and its kinetic energy. Beta particles and orbital electrons have the same mass; therefore, they are easily deflected by collision. Because of this fact, the beta particle follows a tortuous path as it passes through absorbing material. The specific ionization of a beta particle is low due to its small mass, small charge, and relatively high speed of travel.
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