![[logo] US EPA](https://cybercemetery.unt.edu/archive/oilspill/20120924214851im_/http://www.epa.gov/epafiles/images/logo_epaseal.gif){kind=logo}
Radiation Protection Basics
Radiation Protection Basics
Three basic concepts apply to all types of ionizing radiation. When we develop regulations or standards that limit how much radiation a person can receive in a particular situation, we consider how these concepts might affect a person's exposure.
Time 
The amount of radiation exposure increases and decreases with the time people spend near the source of radiation.
In general, we think of the exposure time as how long a person is near radioactive material. It's easy to understand how to minimize the time for external (direct) exposure. Gamma and x-rays are the primary concern for external exposure.
However, if radioactive material gets inside your body, you can't move away from it. You have to wait until it decays or until your body can eliminate it. When this happens, the biological half-life of the radionuclide controls the time of exposure. Biological half-life is the amount of time it takes the body to eliminate one half of the radionuclide initially present. Alpha and beta particles are the main concern for internal exposure.
How does EPA use the concept of time in radiation protection?
When we set a radiation standard that assumes an exposure over a certain period, we are applying the concept of time. For example, we often express exposures in terms of a committed dose. A committed dose is one that accounts for continuing exposures over long periods of time (such as 30, 50, or 70 years). It refers to the exposure received from radioactive material that enters and remains in the body for many years.
When we assess the potential for exposure in a situation, we consider the amount of time a person is likely to spend in the area of contamination. For example, in assessing the potential exposure from radon in a home, we estimate how much time people are likely to spend in the basement.
Distance 
The farther away people are from a radiation source, the less their exposure.
How close to a source of radiation can you be without getting a high exposure? It depends on the energy of the radiation and the size (or activity) of the source. Distance is a prime concern when dealing with gamma rays, because they can travel long distances. Alpha and beta particles don't have enough energy to travel very far.
As a rule, if you double the distance, you reduce the exposure by a factor of four. Halving the distance, increases the exposure by a factor of four.
Why does exposure change more rapidly than the distance?
The area of the circle depends on the distance from the center to the edge of the circle (radius). It is proportional to the square of the radius. As a result, if the radius doubles, the area increases four times.
Think of the radiation source as a bare light bulb. The bulb gives off light equally in every direction, in a circle. The energy from the light is distributed evenly over the whole area of the circle. When the radius doubles, the radiation is spread out over four times as much area, so the dose is only one fourth as much. (In addition, as the distance from the source increases so does the likelihood that some gamma rays will lose their energy.
Exposure at 2 feet
Exposure at 4 feet
The exposure of an individual sitting 4 feet from a radiation source will be 1/4 the exposure of an individual sitting 2 feet from the same source
How does EPA use the concept of distance in radiation protection?
We also consider distance in analyzing potential exposures from a source. If a person is at a contaminated site, or working around radioactive material, we assess how the exposures vary if the person is closer to, or farther away from, the source of radiation.
Shielding
The greater the shielding around a radiation source, the smaller the exposure.
Shielding simply means having something that will absorb radiation between you and the source of the radiation (but using another person to absorb the radiation doesn't count as shielding). The amount of shielding required to protect against different kinds of radiation depends on how much energy they have.
|
A thin piece of light material, such as paper, or even the dead cells in the outer layer of human skin provides adequate shielding because alpha particles can't penetrate it. However, living tissue inside body, offers no protection against inhaled or ingested alpha emitters. |
|
Additional covering, for example heavy clothing, is necessary to protect against beta-emitters. Some beta particles can penetrate and burn the skin. |
|
Thick, dense shielding, such as lead, is necessary to protect against gamma rays. The higher the energy of the gamma ray, the thicker the lead must be. X-rays pose a similar challenge, so x-ray technicians often give patients receiving medical or dental X-rays a lead apron to cover other parts of their body. |
How does EPA use the concept of shielding in radiation protection?
We take into account the type of shielding that might be provided by soil when we assess sites that have been contaminated or used for disposal of radioactive material. We also account for the shielding provided by buildings for a person working or living at a site that has been cleaned up.
