Time, Distance and Shielding
There are three general guidelines for controlling exposure to ionizing radiation:
- minimizing exposure time,
- maximizing distance from the radiation source,
- shielding yourself from the radiation source.
Time is an important factor in limiting exposure to the public and to radiological emergency responders. The amount of radiation exposure increases and decreases with the time people spend near the source of radiation. The maximum time to be spent in the radiation environment is defined as the “stay time.” The stay time can be calculated using the following equation:
Stay Time = Exposure Limit/Dose Rate
Distance can be used to reduce exposure. The farther away people are from a radiation source, the less their exposure. Doubling the distance from a point source of radiation decreases the exposure rate to 1/4 the original exposure rate. Halving the distance increases the exposure by a factor of four.
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 at the speed of light. Alpha particles can only travel a few inches and beta particles around 10 feet.
Shielding: As ionizing radiation passes through matter, the intensity of the radiation is diminished. Shielding is the placement of an “absorber” between you and the radiation source. An absorber is a material that reduces radiation from the radiation source to you. Alpha, beta, or gamma radiation can all be stopped by different thicknesses of absorbers.
Shielding material can include barrels, boards, vehicles, buildings, gravel, water, lead or whatever else is immediately available.
α ALPHA – can be stopped after traveling through about 1.2 inches of air, about 0.008 inches of water, or a piece of paper or skin. A thin piece of 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 the body offers no protection against inhaled or ingested alpha emitters.
β BETA – can only be stopped after traveling through about 10 feet of air, less than 2 inches of water, or a thin layer of glass or metal. Additional covering, for example heavy clothing, is necessary to protect against beta-emitters. Some beta particles can penetrate and burn the skin.
γ GAMMA: To reduce typical gamma rays by a factor of a billion, thicknesses of shield need to be about 13.8 feet of water, about 6.6 feet of concrete, or about 1.3 feet of lead. Thick, dense shielding is necessary to protect against gamma rays. The higher the energy of the gamma ray, the thicker the shield must be. X-rays pose a similar challenge. This is why x-ray technicians often give patients receiving medical or dental X-rays a lead apron to cover other parts of their body.