Alpha particles

The alpha particle, comprised of two protons and two neutrons, is very massive, has high kinetic energy, and a charge of +2. Due to its relatively low velocity, it leaves a dense track of ionizations caused by coulumbic interactions. An alpha particle can penetrate about 3 cm of air, but only a few microns of tissue.

Beta particles

The beta particle is a high speed electron, with a charge of ‐1, ejected from a nucleus. The beta particles from a given isotope have a continuous spectrum of energy that is characterized only by the maximum energy associated with the isotope. Depending on the maximum energy, beta particles can penetrate a few microns to a few centimeters of tissue. They also leave a moderately dense track of ionizations caused by coulumbic interactions. Like the electronic devices described above, beta particles will produce x‐rays when absorbed by a target. The fraction of beta energy converted to x‐rays is approximately:

F=3.3×10^‐4 ZE_max

Z is the atomic number of the target, and E_max is the maximum beta energy in MeV. This relationship explains why we use low Z materials to shield beta sources. There is less bremsstrahlung production.

Neutrons

Depending on their source, neutrons can range in energy from as high as tens of MeV to 0.015 eV. Because they are uncharged, they interact primarily by physical collision with absorber nuclei. The collisions are characterized by conservation of momentum and kinetic energy, and are called elastic.