Gamma rays are attenuated by processes which are functions of atomic number and mass (that
is they all involve interactions near the nucleus or interactions with the electrons around the
nucleus). Gamma shielding is therefore more effectively performed by materials with high
atomic mass number and high density. One such material is lead. Lead is dense and has about
82 electrons for each nucleus. Thus, a gamma would interact more times in passing through
eight inches of lead then passing through the same thickness of a lighter material, such as water.
As the gamma interacts with the shielding material, it loses energy and eventually disappears.
Lead and lead alloys have been used to some extent in nuclear reactor shields and have an added
advantage of ease of fabrication. Because of its low melting point, lead can be used only where
the temperatures do not exceed its melting point.
Iron, although a medium weight element, also functions well as a gamma attenuator. For gamma
rays with energies of 2 MeV, roughly the same mass of iron as of lead is required to remove a
specific fraction of the radiation. At higher and lower energies, however, the mass-attenuation
efficiency of lead is appreciably greater than that of iron. In many cases, the selection of iron
is based on structural, temperature, and economic considerations.
Water is a poor material for shielding gamma rays; however, large amounts will serve to
attenuate gamma radiation.
Concrete, as discussed previously, is also a good attenuator of gamma rays and is superior to
water. This is mainly a result of the presence of moderately high mass number elements, such
as calcium and silicon. As a general shield material, there is much to recommend about
concrete; it is strong, inexpensive, and adaptable to both block and monolithic types of
Alpha and Beta Radiation
Alpha particles, being the largest particles of radiation and having a +2 charge, interact with
matter more readily than other types of radiation. Each interaction results in a loss of energy.
This is why the alpha has the shortest range of all the types of radiation. Alpha particles
generally are stopped by a thin sheet of paper. As a comparison, a 4 MeV alpha particle will
travel about 1 inch in air, whereas a 4 MeV beta particle will travel about 630 inches in air.
Because it deposits all of its energy in a very small area, the alpha particle travels only a short
The beta particle is more penetrating than the alpha. However, because of the -1 charge, the beta
particle interacts more readily than a non-charged particle. For this reason, it is less penetrating
than uncharged types of radiation such as the gamma or neutron. The beta particle can generally
be stopped by a sheet of aluminum. Because the beta travels farther than the alpha, it deposits
its energy over a greater area and is, therefore, less harmful than the alpha if taken internally.
All materials described under neutron and gamma radiation are also effective at attenuating beta