Answer to Question #12014 Submitted to "Ask the Experts"

Category: Radiation Basics — Radiation Effects

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Q

When radiation encounters shielding of any kind does it maintain its motion; for example, does it bounce off and keep going? Or does it get trapped in the shielding? Or does this depend on the shielding? When it encounters shielding, does it lose energy, and does this occur at different rates depending on the type of radiation and shielding? If radiation does bounce off a shielding material (maintaining its motion and energy), then can it be redirected? Or does it always move in a linear trajectory?

A

Some of your inferences regarding the fate of radiation as it enters shielding materials are true. I will attempt to elaborate on these and to point out pertinent and modifying factors.

You recognized as a possibility that particular interactions associated with energy loss by the radiation as it encounters shielding material may depend on the type of radiation involved. This is certainly true.

For example, alpha radiation consists of individual particles, each containing two neutrons and two protons and having an electrical charge of +2 (associated with the two protons). As ionizing radiation goes, these particles are quite massive and rather highly charged, and both properties contribute to very strong energy interactions with materials.

As alpha particles enter shielding material, the particles' energy degrades very quickly because of the strong electric field interactions with the electrons of the material through which they pass. Their large mass causes the alpha particles to have high inertia, which tends to keep the alpha particles moving in a straight line. Occasionally an alpha particle may penetrate the electron cloud of an atom of the shielding material and get close to the nucleus. This can result in a strong interaction between the electric field of the alpha particle and that of the nucleus, and because the nucleus itself may be rather massive, the alpha particle may be deflected from its original direction, scattering off in a new direction. Because the energy loss per unit path length traversed in shielding material is very high for alpha particles, very small thicknesses of material are able to easily stop them; for example, even a single sheet of paper will stop most alpha particles incident on it.

If one is dealing with beta radiation (which is just high-speed electrons), beta particles have masses much smaller than alpha particles (about 7,000 times less), and they carry an electrical charge of -1 (as a negatively charged electron). Because of their lower mass and lower charge, beta particles interact less strongly than do alpha particles.

Beta particles are much more subject to scattering off of electrons of the material through which they pass, so they may frequently undergo some changes in direction. Even so, they are fairly easy to stop; 1–2 centimeters (cm) of common plastic material, for instance, is sufficient to stop almost all beta radiation.

High-density materials such as lead will stop the beta radiation with considerably smaller thicknesses. However, because of a particular type of interaction, a high-speed electron may accelerate in the vicinity of a nucleus and result in the emission of what is called bremsstrahlung radiation (which is the same as x rays). The frequency of this type of interaction increases with increasing atomic number of the shielding material, so it is often preferable to use lower atomic number materials (such as plastic) to shield against beta radiation.

The last radiation I will mention here is gamma radiation, which is a form of electromagnetic radiation (like light, radio waves, microwaves, etc.), but which is higher in energy. Gamma radiation is much more penetrating than either alpha or beta radiation, but its interactions with shielding material are much weaker.

The weaker interactions of gamma radiation are attributable to the facts that the electromagnetic radiation has no mass and no electrical charge associated with it. Gamma radiation interacts with the electronic portions of atoms of shielding material; that is, the oscillating electric field of the electromagnetic radiation interacts with the electric fields of the electrons in the shielding material. If the energy of the gamma radiation is high enough (greater than 1.022 million electronvolts [MeV]), interactions may also occur in the electric field of the shielding material's nucleus.

In some interactions, the gamma radiation may give up all of its energy to the material through which it passes. Gamma rays, especially lower energy ones, may lose all their energy in single events. More commonly, however, they lose only part of their energy in scattering events—interactions in which gamma rays scatter off in different directions from their original paths. As a general rule, the higher the energy of the gamma ray, the more likely it is the gamma ray will scatter off the shielding material at small angles and the scattered gamma ray will travel in the overall forward direction (that is, in the same direction as the original gamma ray).

In general, the attenuation of gamma radiation through shielding material follows an exponential reduction law, with intensity decreasing markedly with increasing thickness of shielding material. In general, high atomic number and density materials are more effective in shielding gamma radiation than are lower atomic number and density materials.

There is more that could be said here, but I believe this is sufficient to address your questions.

George Chabot, PhD, CHP

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
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