辐射防护（来自Wikipedia）概念及相关内容

0052020105li · 发表于 2011-12-25 22:03

Radiation protection

Radiation protection, sometimes known as radiological protection, is the science[citation needed] 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 statistically elevated risks of 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 the radiographer, 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: The term 'biological shield' refers to a mass of absorbing material placed around a reactor, or other radioactive source, to reduce the radiation to a level safe for humans.[1] The effectiveness of a material as a biological 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 g/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 containsbarium 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 effectivesolution.

In most countries a national regulatory authority works towards ensuring a secure radiation environment in society by setting requirements that are also based on the international recommendations for ionizing radiation (ICRP - International Commission on Radiological Protection):

Justification: No unnecessary use of radiation is permitted, which means that the advantages must outweigh the disadvantages.
Limitation: Each individual must be protected against risks that are far too large through individual radiation dose limits.
Optimization: Radiation doses should all be kept as low as reasonably achievable. This means that it is not enough to remain under the radiation dose limits. As permit holder, you are responsible for ensuring that radiation doses are as low as reasonably achievable, which means that the actual radiation doses are often much lower than the permitted limit.

[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]. This is to reduce generation of Bremsstrahlung X-rays. 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, which makes this type highly penetrating. Neutrons are absorbed by nuclei of atoms in a nuclear reaction. This most-often creates a secondary radiation hazard, as the absorbing nuclei transmute to the next-heavier isotope, many of which are unstable.
- 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 thewavelength.
- 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. The concrete is also made with heavy aggregates, such as Baryte, to aid in the shielding properties of the concrete.
- 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 to 1/1,024 of their original intensity (1/2 multiplied by itself ten times). Halving thicknesses of some materials, that reduce gamma ray intensity by 50% (1/2) include[2]:

MaterialHalving Thickness, inchesHalving Thickness, cmDensity, g/cm3Halving Mass, g/cm2
lead0.41.011.312
concrete2.46.13.3320
steel0.992.57.8620
packed soil3.69.11.9918
water7.2181.0018
lumber or other wood11290.5616
air6000150000.001218

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]Graded-Z shielding

Graded-Z shielding is a laminate of several materials with different Z values (atomic numbers) designed to protect against ionizing radiation. Compared to single-material shielding, the same mass of graded-Z shielding has been shown to reduce electron penetration over 60%.[3] It is commonly used to in satellite-based particle detectors, offering several benefits:

protection from radiation damage
reduction of background noise for detectors
lower mass compared to single-material shielding

Designs vary, but typically involve a gradient from high-Z (usually tantalum) through successively lower-Z elements such as tin, steel, and copper, usually ending with aluminium. Sometimes even lighter materials such aspolypropylene or boron carbide are used. [4][5]

In a typical graded-Z shield, the high-Z layer effectively scatters protons and electrons. It also absorbs gamma rays, which produces X-ray fluorescence. Each subsequent layers absorbs the X-ray fluorescence of the previous material, eventually reducing the energy to a suitable level. Each decrease in energy produces bremsstrahlung andAuger electrons, which are below the detector's energy threshold. Some designs also include an outer layer of aluminium, which may simply be the skin of the satellite.

[edit]ALARPMain article: 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".[6] 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 more commonly used in the United States and Canada.

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 (e.g. 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. 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]Notes

^ "Biological shield". United States Nuclear Regulatory Commission. Retrieved 13 August 2010.
^ "Halving-thickness for various materials". "The Compass DeRose Guide to Emergency Preparedness - Hardened Shelters".
^ Fan, W.C. et al. (1996). "Shielding considerations for satellite microelectronics". IEEE Transactions on Nuclear Science 43(6): 2790–2796. Bibcode 1996ITNS...43.2790F. doi:10.1109/23.556868.
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^ Smith, D.M. et al. (2002). Solar Physics 210: 33–60. Bibcode 2002SoPh..210...33S. doi:10.1023/A:1022400716414.
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^ Pia, Maria Grazia et al. (2009). "PIXE Simulation with Geant4". IEEE Transactions on Nuclear Science 56 (6): 3614–3649.Bibcode 2009ITNS...56.3614P. doi:10.1109/TNS.2009.2033993.
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^ This is the wording used by the national regulatory authority that coined the term, in turn derived from its enabling legislation: Health and Safety at Work etc. Act 1974: "Risk management: ALARP at a glance". London: Health and Safety Executive. Retrieved 13 February 2011. "'ALARP' is short for 'as low as reasonably practicable'"