Radiation protection

From Wikipedia, the free encyclopedia

Radiation protection, sometimes known as radiological protection, is the science and practice of protecting people and the environment from the harmful effects of ionizing radiation.

Fundamental to radiation protection is the reduction of expected dose and the measurement of human dose uptake.

Ionizing radiation is widely used in industry and medicine and can present a significant health hazard. It causes microscopic damage to living tissue, which can result in skin burns and radiation sickness at high exposures (known as "tissue effects"), and statistically elevated risks of cancer at low exposures ("stochastic effects"). The ICRP produces international recommendations which are fundamental to the measurement of dose and its effects.

Principles of radiation protection

External dose quantities used in radiation protection and dosimetry

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:

  1. 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.
  2. 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.
  3. 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, walls surrounding the room with the x-ray generator may contain lead sheets, or the plaster may contain barium sulfate. Operators view the target through a leaded glass screen, or if they must remain in the same room as the target, 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.

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.

Types of radiation

Diagram showing various forms of ionizing radiation, and the sort of material that is used to stop or reduce that type.
The total absorption coefficient of lead (atomic number 82) for gamma rays, plotted versus gamma energy, and the contributions by the three effects. Here, the photoelectric effect dominates at low energy. Above 5 MeV, pair production starts to dominate.

Different types of ionizing radiation interact in different ways with shielding material. The effectiveness of shielding is dependent on the Stopping power of radiation particles, which varies with the type and energy of radiation and the shielding material used. Different shielding techniques are therefore used dependent on the application and the type and energy of the radiation.

  • 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).[2] 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. 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 or thorium[3] are 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. Gamma rays are better absorbed by materials with high atomic numbers and high density, although neither effect is important compared to the total mass per area in the path of the gamma ray.
    • 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. For example, although high atomic number materials are very effective in shielding photons, using them to shield beta particles may cause higher radiation exposure due to the production of bremsstrahlung x-rays, and hence low atomic number materials are recommended. Also, using material with a high neutron activation cross section to shield neutrons will result in the shielding material itself becoming radioactive and hence more dangerous than if it were not present.

Shielding design

A lead castle built to shield a radioactive sample in a lab

To first approximation 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 1/1024 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:[4]

Material Halving Thickness/the Half value layer,
[cm] 		Halving Thickness,
[inches] 		Density,
[g/cm³] 		Halving Mass,
[g/cm²]
Lead 1.0 0.4 11.3 12
Steel 2.5 0.99 7.86 20
Concrete 6.1 2.4 3.33 20
Packed soil 9.1 3.6 1.99 18
Water 18 7.2 1.00 18
Lumber or other wood 29 11 0.56 16
Air 15 000 6000 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 except for neutron shielding which is more readily shielded by the likes of Neutron absorbers and moderators such as compounds of Boron e.g boric acid, cadmium and Carbon & Hydrogen respectively.

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%.[5] It is commonly used 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 as polypropylene or boron carbide are used. [6][7]

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 and Auger 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.

Radiation protection instruments

Practical radiation measurement is essential in evaluating the effectiveness of protection measures, and in assessing the radiation dose likely to be received by individuals. The measuring instruments for radiation protection are both "installed" (in a fixed position) and portable (hand-held or transportable).

Installed instruments

Installed instruments are fixed in positions which are known to be important in assessing the general radiation hazard in an area. Examples are installed "area" radiation monitors, Gamma interlock monitors, personnel exit monitors, and airborne particulate monitors.

The area radiation monitor will measure the ambient radiation, usually X-Ray, Gamma or neutrons; these are radiations which can have significant radiation levels over a range in excess of tens of metres from their source, and thereby cover a wide area.

Gamma radiation "interlock monitors" are used in applications to prevent inadvertent exposure of workers to an excess dose by preventing personnel access to an area when a high radiation level is present. These interlock the process access directly.

Airborne contamination monitors measure the concentration of radioactive particles in the ambient air to guard against radioactive particles being ingested, or deposited in the lungs of personnel. These instruments will normally give a local alarm, but are often connected to an integrated safety system so that areas of plant can be evacuated and personnel are prevented from entering an air of high airborne contamination.

Personnel exit monitors (PEM) are used to monitor workers who are exiting a "contamination controlled" or potentially contaminated area. These can be in the form of hand monitors, clothing frisk probes, or whole body monitors. These monitor the surface of the workers body and clothing to check if any radioactive contamination has been deposited. These generally measure alpha or beta or gamma, or combinations of these.

The UK National Physical Laboratory publishes a good practice guide through its Ionising Radiation Metrology Forum concerning the provision of such equipment and the methodology of calculating the alarm levels to be used.[8]

Portable instruments

Hand-held ion chamber survey meter in use

Portable instruments are hand-held or transportable. The hand-held instrument is generally used as a survey meter to check an object or person in detail, or assess an area where no installed instrumentation exists. They can also be used for personnel exit monitoring or personnel contamination checks in the field. These generally measure alpha, beta or gamma, or combinations of these.

Transportable instruments are generally instruments that would have been permanently installed, but are temporarily placed in an area to provide continuous monitoring where it is likely there will be a hazard. Such instruments are often installed on trolleys to allow easy deployment, and are associated with temporary operational situations.

In the United Kingdom the HSE has issued a user guidance note on selecting the correct radiation measurement instrument for the application concerned.[9] This covers all radiation instrument technologies, and is a useful comparative guide.

Instrument types

A number of commonly used detection instruments are listed below.

The links should be followed for a fuller description of each.

Radiation dosimeters

Dosimeters are devices worn by the user which measure the absorbed dose the user receives. Common types of wearable dosimeters for ionizing radiation include:

ALARP

Main 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".[10] 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 outside the UK.

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.

Outer space

Space radiation produced by the Sun and other galactic sources, and trapped in radiation "belts" is more dangerous and hundreds of times more intense than radiation sources such as medical X-rays or normal cosmic radiation usually experienced on Earth.[11] When the intensely ionizing particles found in space strike human tissue, it can result in cell damage and may eventually lead to cancer.

The Space Radiation Laboratory makes use of a particle accelerator that produces beams of protons or heavy ions. These ions are typical of those accelerated in cosmic sources and by the Sun. The beams of ions move through a 100-meter (328-foot) transport tunnel to the 37-square-meter (400-square-foot) shielded target hall. There, they hit the target, which may be a biological sample or shielding material.[11] In a 2002 NASA study, it was determined that materials that have high hydrogen contents, such as polyethylene, can reduce primary and secondary radiation to a greater extent than metals, such as aluminum.[12] but there are other possible methods to protect from hazardous radiations in space, including magnetic field radiation protection, like mini-magnetosphere and semi-dipole magnetic field, although the technical and cost issues associated make them unrealistic for the foreseeable future.

Spacecraft radiation protection

The usual method for radiation protection is material shielding by spacecraft and equipment structures (usually aluminium), possibly augmented by polyethylene in human spaceflight where the main concern is high energy protons and cosmic ray ions. On unmanned spacecraft in high electron dose environments such as Jupiter missions, or medium Earth orbit (MEO), additional shielding with high atomic number can be effective. On long duration manned missions, advantage can be taken of the good shielding characteristics of liquid hydrogen fuel and water. The problem with this "passive shielding" method is that radiation interactions in the material generate secondary radiation. For many years it has been speculated whether "active shielding" with magnets, high voltages, or artificial magnetospheres might be feasible. All serious studies conclude that, even if effectiveness can be demonstrated, the size, power, safety, reliability and cost problems with them render them unusable in all realistic scenarios for the foreseeable future (~50 years). Dipolar magnetic fields are not homogenous and the intensity shows variations. High-energy particles penetrate to the magnetic field from low-intensity parts, like cusps in dipolar magnetic field of Earth. Also this method is acceptable only for habitable volumes of spacecraft. The torus-solenoid rings method is lighter than other methods and the effect of this quadrupole magnetic field on energetic particles is stable in order to shield spacecraft during Mars mission.

See also

Notes

  1. ↑ "Biological shield". United States Nuclear Regulatory Commission. Retrieved 13 August 2010. 
  2. ↑ http://www.oseh.umich.edu/TrainP32.pdf
  3. ↑ Historical Use of Thorium at Hanford
  4. ↑ "Halving-thickness for various materials". "The Compass DeRose Guide to Emergency Preparedness - Hardened Shelters". 
  5. ↑ 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. 
  6. ↑ Smith, D.M. et al. (2002). Solar Physics 210: 33–60. Bibcode:2002SoPh..210...33S. doi:10.1023/A:1022400716414. 
  7. ↑ 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. 
  8. ↑ Operational Monitoring Good Practice Guide "The Selection of Alarm Levels for Personnel Exit Monitors" Dec 2009 - National Physical Laboratory, Teddington UK
  9. ↑ http://www.hse.gov.uk/pubns/irp7.pdf
  10. ↑ 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'" 
  11. ↑ 11.0 11.1 "Behind the scenes - NASA's Space Radiation Laboratory". NASA. 2003. Retrieved 2012-07-25. 
  12. ↑ "Understanding Space Radiation" (PDF). Lyndon B. Johnson Space Center. NASA. October 2002. Retrieved 2012-07-25. "FS-2002-10-080-JSC" 

References

External links

Wikimedia Commons has media related to Radiation protection.
Radiation poisoning
General
Conditions
Treatments
This article is issued from Wikipedia. The text is available under the Creative Commons Attribution/Share Alike; additional terms may apply for the media files.