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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.
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Types
Occupational
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 act as a shield from gamma or x-rays if used in sufficient amounts (see below).
Practical
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 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. Even very energetic 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) [1].
- 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.[2][3] 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.
Design
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):
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- 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.
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ALARA
ALARA is an acronym for an important principle in radiation protection and stands for "As Low As Reasonably Achievable". The aim is to minimize the risk of radioactive exposure or amount of dose while keeping in mind that some exposure may be acceptable in order to further the task at hand.
This compromise is well illustrated in radiology. The application of radiation can aid the patient by providing doctors with a medical diagnosis, but the exposure should be reasonably low enough to keep the statistical probability of cancers or sarcomas (stochastic effects) below an acceptable level, and to eliminate deterministic effects (eg. skin reddening or cataracts). An acceptable level of incidence of stochastic effects is considered to be equal for a worker to the risk in another work generally considered to be safe.
This policy is based on the principle that any amount of radiation exposure, no matter how small, can increase the chance of negative biological effects such as cancer, though perhaps by a negligible amount. It is also based on the principle that the probability of the occurrence of negative effects of radiation exposure increases with cumulative lifetime dose. These ideas are combined to form the linear no-threshold model. At the same time, radiology and other practices that involve use of radiations bring benefits to population, so reducing radiation exposure can reduce the efficacy of a medical practice. The economic cost, for example of adding a barrier against radiation, must also be considered when applying the ALARA principle.
There are four major ways to reduce radiation exposure to workers or to population:
- Shielding. Use proper barriers to block or reduce ionizing radiation.
- Time. Spend less time in radiation fields.
- Distance. Increase distance between radioactive sources and workers or population.
- Amount. Reduce the quantity of radioactive material for a practice.
See also
- Demron, a radiation shielding polymer
- Ducrete
- Fallout shelter
- Lead shielding
- List of nuclear accidents
- Nuclear safety
- Stopping power (particle radiation)
- Radiological protection of patients
References
- Oregon Institute of Science and Medicine This website offers the entire online version of Nuclear War Survival Skills with full graphics and web navigation, created with the permission of the author Cresson Kearny. This manual has proven technical info on expedient fallout shelters, radiation shielding for it, the nature of radiation, shelter habitation, and assorted shelter system needs that can be created from common household items. OISM also offers free downloads of other civil defense and shelter information as well.
- Harvard University Radiation Protection Office Providing radiation guidance to Harvard University and affiliated institutions.
External links
- International Radiation Protection Association A world-wide association of individuals engaged in radiation protection.
- Radiological protection of patients International Atomic Energy Agency information on the safe use of radiation in medicine.
Wikipedia content modification information:
- This page was last modified on 20 June 2008, at 10:57.
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