Radiation protection

Radiation protection, sometimes known as radiological protection, is the science of protecting people and the environment from the harmful effects of ionizing radiation, which includes both particle radiation and high energy electromagnetic radiation.

It includes occupational radiation protection, which is the protection of workers; medical radiation protection, which is the protection of patients; and public radiation protection, which is about protection of individual members of the public, and of the population as a whole.

There are mainly three principles to radiation protection: those of time, distance and shielding. Radiation exposure can be managed by one or more of these:
 * Reducing the time of an exposure reduces the effective dose proportionally.
 * An example of reducing radiation doses by reducing the time of exposures might be improving operator training to reduce the time they take to handle a source.
 * Increasing distance reduces dose due to the inverse square law.
 * Distance can be as simple as handling a source with forceps rather than fingers.
 * Adding shielding can also reduce radiation doses.
 * In x-ray facilities, the plaster on the rooms with the x-ray generator contains barium sulfate and the operators stay behind a leaded glass screen and wear lead aprons.
 * Almost any material can shield from gamma or x-rays in sufficient amounts (see below).

Practical radiation protection tends to be a job of juggling the three factors to identify the most cost effective solution.

In some cases, improper shielding can actually make the situation worse, when the radiation interacts with the shielding material and creates secondary radiation that absorbs in the organisms more readily.

Different types of ionizing radiation behave in a different ways, so different shielding techniques are used.
 * Particle radiation consists of a stream of charged or neutral particles, both charged ions and subatomic elementary particles. This includes solar wind, cosmic radiation, and neutron flux in nuclear reactors.
 * Alpha radiation (helium nuclei) is the easiest to shield, because the very massive alpha particles can be stopped even with a leaf of paper.
 * Beta radiation (electrons) is more difficult, but still a relatively thin layer of aluminum can usually do the job. However, in cases where high energy beta particles are emitted (e.g. 32P), the Bremsstrahlung produced by shielding this radiation with the normally used materials is itself dangerous; in such cases, shielding must be accomplished with low density materials, e.g. plastic, wood, water or acrylic glass (Plexiglas, Lucite).
 * In case of beta+ radiation (positrons) the gamma radiation from the electron-positron annihilation reaction poses additional concern.
 * Neutron radiation is not as readily absorbed as charged particle radiation. Neutrons are absorbed by nuclei of atoms in a nuclear reaction (which often leads to emission of gamma photons, causing additional shielding concerns), but fast neutrons have first to be slowed down (moderated) to slower speeds, by inelastic collisions with heavy nuclei or by elastic collisions with light ones. A large mass of hydrogen-rich material, eg. water (or concrete, which contains a lot of chemically-bound water), polyethylene, or paraffin wax is commonly used. It can be further combined with boron for more efficient absorption of the thermal neutrons.
 * Cosmic radiation is not a common concern, as the Earth's atmosphere absorbs it and the magnetosphere acts as a shield, but it poses a problem for satellites and astronauts. While satellite electronics can be radiation hardened, astronauts can't, so they have to be shielded. Because weight is a premium on space technology, methods alternative to absorption are being proposed, such as magnetic shielding using superconductors. Aircrews and frequent flyers are also at a slight risk.
 * Electromagnetic radiation consists of emissions of electromagnetic waves, the properties of which depend on the wavelength.
 * X-ray and gamma radiation are best absorbed by atoms with heavy nuclei; the heavier the nucleus, the better the absorption. In some special applications, depleted uranium is used, but lead is much more common. Barium sulfate is used in some applications too. However, when cost is important, almost any material can be used, but it must be far thicker. Most nuclear reactors use thick concrete shields to create a bioshield with a thin water cooled layer of lead on the inside to protect the porous concrete from the coolant inside.

One standard design practice is to measure the halving thickness of a material, the thickness that reduces gamma or x-ray radiation by half. When multiple thicknesses are built, the shielding multiplies. For example, a practical shield in a fallout shelter is ten halving-thicknesses of packed dirt. This reduces gamma rays by a factor of 1/1,024, which is 1/2 multiplied by itself ten times. This multiplies out to 90 cm (3 ft) of dirt. Shields that reduce gamma ray intensity by 50% (1/2) include (see Kearney, ref):
 * 9 cm (3.6 inches) of packed soil or
 * 6 cm (2.4 inches) of concrete,
 * 1 cm (0.4 inches) of lead,
 * 0.2 cm (0.08 inches) of depleted uranium,
 * 150 m (500 ft) of air.
 * Ultraviolet radiation may or may not be ionizing, depending on the wavelength. It is not penetrating, so it can be shielded by any material which is opaque to it such as sunscreen. Anything that stops X-ray radiation will do the job as well. The ozone layer absorbs UV radiation, but its depletion considerably lowers its effectiveness, especially in extreme northern and southern areas of the globe.