Ionizing Radiation - Limiting Exposure

Limiting Exposure

There are three standard ways to limit exposure:

1. Time: For people who are exposed to radiation in addition to natural background radiation, limiting or minimizing the exposure time will reduce the dose from the radiation source.
2. Distance: Radiation intensity decreases sharply with distance, according to an inverse-square law (in an absolute vacuum).
3. Shielding: Air or skin can be sufficient to substantially attenuate low-energy alpha and beta radiation. Barriers of lead, concrete, or water give effective protection from more energetic particles such as gamma rays and neutrons. Some radioactive materials are stored or handled underwater or by remote control in rooms constructed of thick concrete or lined with lead. There are special plastic shields that stop beta particles, and air will stop most alpha particles. The effectiveness of a material in shielding radiation is determined by its half-value thicknesses, the thickness of material that reduces the radiation by half. This value is a function of the material itself and of the type and energy of ionizing radiation. Some generally accepted thicknesses of attenuating material are 5 mm of aluminum for most beta particles, and 3 inches of lead for gamma radiation.

Containment is a combination of shielding and distance: Radioactive materials are confined in the smallest possible space and kept out of the environment. Radioactive isotopes for medical use, for example, are dispensed in closed handling facilities, while nuclear reactors operate within closed systems with multiple barriers that keep the radioactive materials contained. Rooms have a reduced air pressure so that any leaks occur into the room and not out of it. An example of containment is an effective fallout shelter, which in a nuclear war, reduces human exposure at least 1,000 times.

Other civil defense measures can help reduce exposure of populations by reducing ingestion of isotopes and occupational exposure during war time and nuclear reactor accidents. One available measure is the use of potassium iodide (KI) tablets, which effectively blocks the uptake of radioactive iodine (one of the major radioisotope products of nuclear fission) into the human thyroid gland.