Radiation protection
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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.
Ionizing radiation is widely used in industry and medicine, but presents a significant health hazard. It causes microscopic damage to living tissue, resulting in skin burns and radiation sickness at high exposures and cancer, tumors and genetic damage at low exposures.
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[edit] Principles of radiation protection
Radiation protection can be divided into occupational radiation protection, which is the protection of workers; medical radiation protection, which is the protection of patients; and public radiation protection, which is protection of individual members of the public, and of the population as a whole. The types of exposure, as well as government regulations and legal exposure limits are different for each of these groups, so they must be considered separately.
There are three factors that control the amount, or dose, of radiation received from a source. Radiation exposure can be managed by a combination of these factors:
- Time: 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.
- Distance: Increasing distance reduces dose due to the inverse square law. Distance can be as simple as handling a source with forceps rather than fingers.
- Shielding: Adding shielding can also reduce radiation doses. The effectiveness of a material as a radiation shield is related to its cross-section for scattering and absorption, and to a first approximation is proportional to the total mass of material per unit area interposed along the line of sight between the radiation source and the region to be protected. Hence, shielding strength or "thickness" is conventionally measured in units of gm/cm2. The radiation that manages to get through falls exponentially with the thickness of the shield. 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.
Practical radiation protection tends to be a job of juggling the three factors to identify the most cost effective solution.
[edit] Types of radiation
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 particles (helium nuclei) are the least penetrating. Even very energetic alpha particles can be stopped by a single sheet of paper.
- Beta particles (electrons) are more penetrating, but still can be absorbed by a few millimeters of aluminum. However, in cases where high energy beta particles are emitted shielding must be accomplished with low density materials, e.g. plastic, wood, water or acrylic glass (Plexiglas, Lucite) [1]. In the 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.
- 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 and frequent fliers are also at a slight risk. Cosmic radiation is extremely high energy, and is very penetrating.
- 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; several centimeters are often required. 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.
- Ultraviolet (UV) radiation is ionizing but it is not penetrating, so it can be shielded by thin opaque layers such as sunscreen, clothing, and protective eyewear. Protection from UV is simpler than for the other forms of radiation above, so it is often considered separately.
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.
[edit] Shielding design
Shielding reduces the intensity of radiation exponentially depending on the thickness.
This means when added thicknesses are used, the shielding multiplies. For example, a practical shield in a fallout shelter is ten halving-thicknesses of packed dirt, which is 90 cm (3 ft) of dirt. This reduces gamma rays by a factor of 1/1,024, which is 1/2 multiplied by itself ten times. Halving thicknesses of some materials, that reduce gamma ray intensity by 50% (1/2) include[1] (see also Kearney, ref):
| Material | Halving Thickness, inches | Halving Thickness, cm | Density, g/cm³ | Halving Mass, g/cm² |
|---|---|---|---|---|
| lead | 0.4 | 1.0 | 11.3 | 12 |
| concrete | 2.4 | 6.1 | 3.33 | 20 |
| steel | 0.99 | 2.5 | 7.86 | 20 |
| packed soil | 3.6 | 9.1 | 1.99 | 18 |
| water | 7.2 | 18 | 1.00 | 18 |
| lumber or other wood | 11 | 29 | 0.56 | 16 |
| depleted uranium | 0.08 | 0.2 | 19.1 | 3.9 |
| air | 6000 | 15000 | 0.0012 | 18 |
Column Halving Mass in the chart above indicates mass of material, required to cut radiation by 50%, in grams per square centimetre of protected area.
The effectiveness of a shielding material in general increases with its density.
[edit] ALARP
ALARP, is an acronym for an important principle in exposure to radiation and other occupational health risks and stands for "As Low As Reasonably Practicable". The aim is to minimize the risk of radioactive exposure or other hazard while keeping in mind that some exposure may be acceptable in order to further the task at hand. The equivalent term ALARA, "As Low As Reasonably Achievable", is also commonly used.
This compromise is well illustrated in radiology. The application of radiation can aid the patient by providing doctors and other health care professionals 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 ALARP 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.
[edit] References
- Harvard University Radiation Protection Office Providing radiation guidance to Harvard University and affiliated institutions.
- ^ ""Halving-thickness for various materials"". "The Compass DeRose Guide to Emergency Preparedness - Hardened Shelters". http://www.derose.net/steve/guides/emergency/hardened.html.
[edit] External links
- IRPA, International Radiation Protection Association A world-wide association of individuals engaged in radiation protection.
- Radiation protection of patients International Atomic Energy Agency information on the safe use of radiation in medicine.
- ICRP, International Commission on Radiation Protection
- ICRU, International Commission on Radiation Units
- IAEA, International Atomic Energy Agency
- UNSCEAR, United Nations Scientific Committee on the effects of Ionizing Radiations
- HPA (ex NRPB), Health Protection Agency, UK
- NCRP, National Council on Radiation Protection and Measurements, USA
- IRSN, Institute for Radioprotection and Nuclear Safety, France
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