Answer to Question #1094 Submitted to "Ask the Experts"

Q

If a product (like lead) protects against x-ray radiation, does it also protect against comparable radiation from other sources, or do certain substances only attenuate certain types of radiation?

A

In general, different materials exhibit different abilities to shield against different radiations. It is true, however, that radiations of a given type are well shielded by the same material, regardless of the source of the radiation. Thus, x rays, whether they arise from operation of an x-ray machine or come from a radioactive material that emits so-called characteristic x rays during radioactive decay, are well-shielded by lead. Lead and other high-mass density materials are also good shielding materials for gamma radiation. Gamma radiation is the same type of radiation as x rays (called electromagnetic radiation); it may differ significantly in energy from some x rays, but the same kinds of materials effective against x rays are also good for gamma rays.

It turns out that one of the most important properties of a material for shielding against x rays and gamma rays is its electron density, and high-mass density materials like lead have high electron densities. It turns out that such materials are also effective against some common particulate radiations, such as alpha particles and beta particles, although these particles are much more easily stopped by materials than are x rays or gamma rays, and much smaller thicknesses of the materials are generally required. In fact, partly because these particulate radiations are so easily stopped, the type of material is often not very critical. For example, a sheet of ordinary paper will stop most all alpha radiation coming from the radioactive decay of radionuclides.

Most beta radiation from radioactive materials will require less than one-half inch of paper or plastic to stop it. Less than one-tenth of an inch of lead would be required to stop the same beta particles, although we frequently do not use lead or other materials that have high atomic numbers for routine beta shielding because of a secondary process that occurs when beta radiation interacts in the vicinity of the nuclei of atoms. This process is called bremsstrahlung radiation production and results in bremsstrahlung x rays being produced (the process is actually similar to what occurs in the production of x rays in an x-ray tube when electrons are made to bombard a high-atomic-number target material). The bremsstrahlung production process increases with increasing atomic number of the material. The x rays are more difficult to shield against than the beta particles and, as a consequence, we often choose to use lower-atomic-number materials, such as plastic, to shield beta radiation.

One type of radiation that is not sufficiently shielded by typical high-mass density materials is neutron radiation. Neutrons are electrically neutral particles that arise from the nucleus of atoms, often as a result of nuclear reactions that have taken place (for example, the fission process in a nuclear reactor). Neutrons from most common sources are born with significant energy and can pass fairly easily through many materials. They have no charge and do not interact with the electrons of materials through which they pass (as do x rays, gamma rays, alpha particles, and beta radiation) but, rather, interact with the nuclei of the atoms. For this reason it happens that materials that have low-mass nuclei are good for shielding the neutrons because the neutrons can transfer large amounts of their energy to the light nuclei through collisions (referred to as elastic collisions). The closer in mass the nucleus is to that of the neutron, the more efficient is the energy transfer. Hydrogen is the atom whose nucleus (a single proton) is closest in mass to the neutron. Thus, material with high hydrogen content is desirable for shielding energetic neutrons.

Examples of good neutron shields are water and many plastics. Concrete is frequently used for neutron shielding because it does have a reasonable amount of hydrogen incorporated into the concrete as water of hydration, and the concrete can be poured to form large structurally sound shields for large sources. After neutrons have been slowed down by collision processes they finally disappear by being absorbed into the nuclei of materials. Hydrogen has a moderate propensity for absorbing slow neutrons. When it does, the capture process leads to excitation of the nucleus and emission of the excitation energy as a rather high-energy gamma ray which may also require shielding. Other materials are sometimes used that have much greater affinities for capturing the slow neutrons than does hydrogen and do not lead to significant secondary gamma ray production. One example of such a material is boron, particularly the isotope ¹⁰B, which undergoes a nuclear reaction when it captures a neutron and produces an alpha particle and the residual nucleus (which is ⁷Li) which is easily stopped in very small amounts of material. This avoids the production of penetrating gamma radiation.

While there is much more that could be said about the appropriateness and desirability of various materials for shielding against radiations of different types, I hope the above is sufficient to provide you with an appreciation of the similarities and differences that exist among requirements for radiation protection using selected shielding materials.

George Chabot, CHP, PhD