This article describes aspects of radiation safety programmes. The objective of radiation safety is to eliminate or minimize harmful effects of ionizing radiation and radioactive material on workers, the public and the environment while allowing their beneficial uses.

Most radiation safety programmes will not have to implement every one of the elements described below. The design of a radiation safety programme depends on the types of ionizing radiation sources involved and how they are used.

Radiation Safety Principles

The International Commission on Radiological Protection (ICRP) has proposed that the following principles should guide the use of ionizing radiation and the application of radiation safety standards:

1. No practice involving exposures to radiation should be adopted unless it produces sufficient benefit to the exposed individuals or to society to offset the radiation detriment it causes (the justification of a practice).
2. In relation to any particular source within a practice, the magnitude of individual doses, the number of people exposed, and the likelihood of incurring exposures where these are not certain to be received should all be kept as low as reasonably achievable (ALARA), economic and social factors being taken into account. This procedure should be constrained by restrictions on the doses to individuals (dose constraints), so as to limit the inequity likely to result from the inherent economic and social judgements (the optimization of protection).
3. The exposure of individuals resulting from the combination of all the relevant practices should be subject to dose limits, or to some control of risk in the case of potential exposures. These are aimed at ensuring that no individual is exposed to radiation risks that are judged to be unacceptable from these practices in any normal circumstances. Not all sources are susceptible of control by action at the source and it is necessary to specify the sources to be included as relevant before selecting a dose limit (individual dose and risk limits).

Radiation Safety Standards

Standards exist for radiation exposure of workers and the general public and for annual limits on intake (ALI) of radionuclides. Standards for concentrations of radionuclides in air and in water can be derived from the ALIs.

The ICRP has published extensive tabulations of ALIs and derived air and water concentrations. A summary of its recommended dose limits is in table 1.

Table 1. Recommended dose limits of the International Commission on Radiological Protection¹

¹ The limits apply to the sum of the relevant doses from external exposure in the specified period and the 50-year committed dose (to age 70 years for children) from intakes in the same period.

² With the further provision that the effective dose should not exceed 50 mSv in any single year. Additional restrictions apply to the occupational exposure of pregnant women.

³ In special circumstances, a higher value of effective dose could be allowed in a single year, provided that the average over 5 years does not exceed 1 mSv per year.

⁴ The limitation on the effective dose provides sufficient protection for the skin against stochastic effects. An additional limit is needed for localized exposures in order to prevent deterministic effects.

Dosimetry

Dosimetry is used to indicate dose equivalents that workers receive from external radiation fields to which they may be exposed. Dosimeters are characterized by the type of device, the type of radiation they measure and the portion of the body for which the absorbed dose is to be indicated.

Three main types of dosimeters are most commonly employed. They are thermoluminescent dosimeters, film dosimeters and ionization chambers. Other types of dosimeters (not discussed here) include fission foils, track-etch devices and plastic “bubble” dosimeters.

Thermoluminescent dosimeters are the most commonly used type of personnel dosimeter. They take advantage of the principle that when some materials absorb energy from ionizing radiation, they store it such that later it can be recovered in the form of light when the materials are heated. To a high degree, the amount of light released is directly proportional to the energy absorbed from the ionizing radiation and hence to the absorbed dose the material received. This proportionality is valid over a very wide range of ionizing radiation energy and absorbed dose rates.

Special equipment is necessary to process thermoluminescent dosimeters accurately. Reading the thermoluminescent dosimeter destroys the dose information contained in it. However, after appropriate processing, thermoluminescent dosimeters are reusable.

The material used for thermoluminescent dosimeters must be transparent to the light it emits. The most common materials used for thermoluminescent dosimeters are lithium fluoride (LiF) and calcium fluoride (CaF₂). The materials may be doped with other materials or made with a specific isotopic composition for specialized purposes such as neutron dosimetry.

Many dosimeters contain several thermoluminescent chips with different filters in front of them to allow discrimination between energies and types of radiation.

Film was the most popular material for personnel dosimetry before thermoluminescent dosimetry became common. The degree of film darkening depends on the energy absorbed from the ionizing radiation, but the relationship is not linear. Dependence of film response on total absorbed dose, absorbed dose rate and radiation energy is greater than that for thermoluminescent dosimeters and can limit film’s range of applicability. However, film has the advantage of providing a permanent record of the absorbed dose to which it was exposed.

Various film formulations and filter arrangements may be used for special purposes, such as neutron dosimetry. As with thermoluminescent dosimeters, special equipment is needed for proper analysis.

Film is generally much more sensitive to ambient humidity and temperature than thermoluminescent materials, and can give falsely high readings under adverse conditions. On the other hand, dose equivalents indicated by thermoluminescent dosimeters may be affected by the shock of dropping them on a hard surface.

Only the largest of organizations operate their own dosimetry services. Most obtain such services from companies specializing in providing them. It is important that such companies be licensed or accredited by appropriate independent authorities so that accurate dosimetry results are assured.

Self-reading, small ionization chambers, also called pocket chambers, are used to obtain immediate dosimetry information. Their use is often required when personnel must enter high or very high radiation areas, where personnel could receive a large absorbed dose in a short period of time. Pocket chambers often are calibrated locally, and they are very sensitive to shock. Consequently, they should always be supplemented by thermoluminescent or film dosimeters, which are more accurate and dependable but do not provide immediate results.

Dosimetry is required for a worker when he or she has a reasonable probability of accumulating a certain percentage, usually 5 or 10%, of the maximum permissible dose equivalent for the whole-body or certain parts of the body.

A whole-body dosimeter should be worn somewhere between the shoulders and the waist, at a point where the highest exposure is anticipated. When conditions of exposure warrant, other dosimeters may be worn on fingers or wrists, at the abdomen, on a band or hat at the forehead, or on a collar, to assess localized exposure to extremities, a foetus or embryo, the thyroid or the lenses of the eyes. Refer to appropriate regulatory guidelines about whether dosimeters should be worn inside or outside protective garments such as lead aprons, gloves and collars.

Personnel dosimeters indicate only the radiation to which the dosimeter was exposed. Assigning the dosimeter dose equivalent to the person or organs of the person is acceptable for small, trivial doses, but large dosimeter doses, especially those greatly exceeding regulatory standards, should be analysed carefully with respect to dosimeter placement and the actual radiation fields to which the worker was exposed when estimating the dose that the worker actually received. A statement should be obtained from the worker as part of the investigation and included in the record. However, much more often than not, very large dosimeter doses are the result of deliberate radiation exposure of the dosimeter while it was not being worn.

Bioassay

Bioassay (also called radiobioassay) means the determination of kinds, quantities or concentrations, and, in some cases, the locations of radioactive material in the human body, whether by direct measurement (in vivo counting) or by analysis and evaluation of materials excreted or removed from the human body.

Bioassay is usually used to assess worker dose equivalent due to radioactive material taken into the body. It also can provide an indication of the effectiveness of active measures taken to prevent such intake. More rarely it may be used to estimate the dose a worker received from a massive external radiation exposure (for example, by counting white blood cells or chromosomal defects).

Bioassay must be performed when a reasonable possibility exists that a worker may take or has taken into his or her body more than a certain percentage (usually 5 or 10%) of the ALI for a radionuclide. The chemical and physical form of the radionuclide sought in the body determines the type of bioassay necessary to detect it.

Bioassay can consist of analysing samples taken from the body (for example, urine, faeces, blood or hair) for radioactive isotopes. In this case, the amount of radioactivity in the sample can be related to the radioactivity in the person’s body and subsequently to the radiation dose that the person’s body or certain organs have received or are committed to receive. Urine bioassay for tritium is an example of this type of bioassay.

Whole or partial body scanning can be used to detect radionuclides that emit x or gamma rays of energy reasonably detectable outside the body. Thyroid bioassay for iodine-131 (¹³¹I) is an example of this type of bioassay.

Bioassay can be performed in-house or samples or personnel can be sent to a facility or organization that specializes in the bioassay to be performed. In either case, proper calibration of equipment and accreditation of laboratory procedures is essential to ensure accurate, precise, and defensible bioassay results.

Protective Clothing

Protective clothing is supplied by the employer to the worker to reduce the possibility of radioactive contamination of the worker or his or her clothing or to partially shield the worker from beta, x, or gamma radiation. Examples of the former are anti-contamination clothing, gloves, hoods and boots. Examples of the latter are leaded aprons, gloves and eyeglasses.

Respiratory Protection

A respiratory protection device is an apparatus, such as a respirator, used to reduce a worker’s intake of airborne radioactive materials.

Employers must use, to the extent practical, process or other engineering controls (for example, containment or ventilation) to limit the concentrations of the radioactive materials in air. When this is not possible for controlling the concentrations of radioactive material in air to values below those that define an airborne radioactivity area, the employer, consistent with maintaining the total effective dose equivalent ALARA, must increase monitoring and limit intakes by one or more of the following means:

• control of access
• limitation of exposure times
• use of respiratory protection equipment
• other controls.

Respiratory protection equipment issued to workers must comply with applicable national standards for such equipment.

The employer must implement and maintain a respiratory protection programme that includes:

• air sampling sufficient to identify the potential hazard, permit proper equipment selection and estimate exposures
• surveys and bioassays, as appropriate, to evaluate actual intakes
• testing of respirators for operability immediately prior to each use
• written procedures regarding selection, fitting, issuance, maintenance and testing of respirators, including testing for operability immediately prior to each use; supervision and training of personnel; monitoring, including air sampling and bioassays; and record-keeping
• determination by a physician prior to the initial fitting of respirators, and periodically at a frequency determined by a physician, that the individual user is medically fit to use the respiratory protection equipment.

The employer must advise each respirator user that the user may leave the work area at any time for relief from respirator use in the event of equipment malfunction, physical or psychological distress, procedural or communication failure, significant deterioration of operating conditions, or any other conditions that might require such relief.

Even though circumstances may not require routine use of respirators, credible emergency conditions may mandate their availability. In such cases, the respirators also must be certified for such use by an appropriate accrediting organization and maintained in a condition ready for use.

Occupational Health Surveillance

Workers exposed to ionizing radiation should receive occupational health services to the same extent as workers exposed to other occupational hazards.

General preplacement examinations assess the overall health of the prospective employee and establish baseline data. Previous medical and exposure history should always be obtained. Specialized examinations, such as of lens of the eye and blood cell counts, may be necessary depending on the nature of the expected radiation exposure. This should be left to the discretion of the attending physician.

Contamination Surveys

A contamination survey is an evaluation of the radiological conditions incident to the production, use, release, disposal or presence of radioactive materials or other sources of radiation. When appropriate, such an evaluation includes a physical survey of the location of radioactive material and measurements or calculations of levels of radiation, or concentrations or quantities of radioactive material present.

Contamination surveys are performed to demonstrate compliance with national regulations and to evaluate the extent of radiation levels, concentrations or quantities of radioactive material, and the potential radiological hazards that could be present.

The frequency of contamination surveys is determined by the degree of potential hazard present. Weekly surveys should be performed in radioactive waste storage areas and in laboratories and clinics where relatively large amounts of unsealed radioactive sources are used. Monthly surveys suffice for laboratories that work with small amounts of radioactive sources, such as laboratories that perform in vitro testing using isotopes such as tritium, carbon-14 (¹⁴C), and iodine-125 (¹²⁵I) with activities less than a few kBq.

Radiation safety equipment and survey meters must be appropriate for the types of radioactive material and radiations involved, and must be properly calibrated.

Contamination surveys consist of measurements of ambient radiation levels with a Geiger-Mueller (G-M) counter, ionization chamber or scintillation counter; measurements of possible α or βγ surface contamination with appropriate thin-window G-M or zinc sulphide (ZnS) scintillation counters; and wipe tests of surfaces to be later counted in a scintillation (sodium iodide (NaI)) well counter, a germanium (Ge) counter or a liquid scintillation counter, as appropriate.

Appropriate action levels must be established for ambient radiation and contamination measurement results. When an action level is exceeded, steps must be taken immediately to mitigate the detected levels, restore them to acceptable conditions and prevent unnecessary personnel exposure to radiation and the uptake and spread of radioactive material.

Environmental Monitoring

Environmental monitoring refers to collecting and measuring environmental samples for radioactive materials and monitoring areas outside the environs of the workplace for radiation levels. Purposes of environmental monitoring include estimating consequences to humans resulting from the release of radionuclides to the biosphere, detecting releases of radioactive material to the environment before they become serious and demonstrating compliance with regulations.

A complete description of environmental monitoring techniques is beyond the scope of this article. However, general principles will be discussed.

Environmental samples must be taken that monitor the most likely pathway for radionuclides from the environment to man. For example, soil, water, grass and milk samples in agricultural regions around a nuclear power plant should be taken routinely and analysed for iodine-131 (¹³¹I) and strontium-90 (⁹⁰Sr) content.

Environmental monitoring can include taking samples of air, ground water, surface water, soil, foliage, fish, milk, game animals and so on. The choices of which samples to take and how often to take them should be based on the purposes of the monitoring, although a small number of random samples may sometimes identify a previously unknown problem.

The first step in designing an environmental monitoring programme is to characterize the radionuclides being released or having the potential for being accidentally released, with respect to type and quantity and physical and chemical form.

The possibility of transport of these radionuclides through the air, ground water and surface water is the next consideration. The objective is to predict the concentrations of radionuclides reaching humans directly through air and water or indirectly through food.

The bioaccumulation of radionuclides resulting from deposition in aquatic and terrestrial environments is the next item of concern. The goal is to predict the concentration of radionuclides once they enter the food chain.

Finally, the rate of human consumption of these potentially contaminated foodstuffs and how this consumption contributes to human radiation dose and resultant health risk are examined. The results of this analysis are used to determine the best approach to environmental sampling and to ensure that the goals of the environmental monitoring programme are met.

Leak Tests of Sealed Sources

A sealed source means radioactive material that is encased in a capsule designed to prevent leakage or escape of the material. Such sources must be tested periodically to verify that the source is not leaking radioactive material.

Each sealed source must be tested for leakage before its first use unless the supplier has provided a certificate indicating that the source was tested within six months (three months for α emitters) before transfer to the present owner. Each sealed source must be tested for leakage at least once every six months (three months for α emitters) or at an interval specified by the regulatory authority.

Generally, leak tests on the following sources are not required:

• sources containing only radioactive material with a half-life of less than 30 days
• sources containing only radioactive material as a gas
• sources containing 4 MBq or less of βγ-emitting material or 0.4 MBq or less of α-emitting material
• sources stored and not being used; however, each such source must be tested for leakage before any use or transfer unless it has been leakage-tested within six months before the date of use or transfer
• seeds of iridium-192 (¹⁹²Ir) encased in nylon ribbon.

A leak test is performed by taking a wipe sample from the sealed source or from the surfaces of the device in which the sealed source is mounted or stored on which radioactive contamination might be expected to accumulate or by washing the source in a small volume of detergent solution and treating the entire volume as the sample.

The sample should be measured so that the leakage test can detect the presence of at least 200 Bq of radioactive material on the sample.

Sealed radium sources require special leak test procedures to detect leaking radon (Rn) gas. For example, one procedure involves keeping the sealed source in a jar with cotton fibres for at least 24 hours. At the end of the period, the cotton fibres are analysed for the presence of Rn progeny.

A sealed source found to be leaking in excess of allowable limits must be removed from service. If the source is not repairable, it should be handled as radioactive waste. The regulatory authority may require that leaking sources be reported in case the leakage is a result of a manufacturing defect worthy of further investigation.

Inventory

Radiation safety personnel must maintain an up-to-date inventory of all radioactive material and other sources of ionizing radiation for which the employer is responsible. The organization’s procedures must ensure that radiation safety personnel are aware of the receipt, use, transfer and disposal of all such material and sources so that the inventory can be kept current. A physical inventory of all sealed sources should be done at least once every three months. The complete inventory of ionizing radiation sources should be verified during the annual audit of the radiation safety programme.

Posting of Areas

Figure 1 shows the international standard radiation symbol. This must appear prominently on all signs denoting areas controlled for the purposes of radiation safety and on container labels indicating the presence of radioactive materials.

Figure 1. Radiation symbol

Areas controlled for the purposes of radiation safety are often designated in terms of increasing dose rate levels. Such areas must be conspicuously posted with a sign or signs bearing the radiation symbol and the words “CAUTION, RADIATION AREA,” “CAUTION (or DANGER), HIGH RADIATION AREA,” or “GRAVE DANGER, VERY HIGH RADIATION AREA,” as appropriate.

1. A radiation area is an area, accessible to personnel, in which radiation levels could result in an individual receiving a dose equivalent in excess of 0.05 mSv in 1 h at 30 cm from the radiation source or from any surface that the radiation penetrates.
2. A high radiation area is an area, accessible to personnel, in which radiation levels could result in an individual receiving a dose equivalent in excess of 1 mSv in 1 h at 30 cm from the radiation source or from any surface that the radiation penetrates.
3. A very high radiation area is an area, accessible to personnel, in which radiation levels could result in an individual receiving an absorbed dose in excess of 5 Gy in 1 h at 1 m from a radiation source or from any surface that the radiation penetrates.

If an area or room contains a significant amount of radioactive material (as defined by the regulatory authority), the entrance to such area or room must be conspicuously posted with a sign bearing the radiation symbol and the words “CAUTION (or DANGER), RADIOACTIVE MATERIALS”.

An airborne radioactivity area is a room or area in which airborne radioactivity exceeds certain levels defined by the regulatory authority. Each airborne radioactivity area must be posted with a conspicuous sign or signs bearing the radiation symbol and the words “CAUTION, AIRBORNE RADIOACTIVITY AREA” or “DANGER, AIRBORNE RADIOACTIVITY AREA”.

Exceptions for these posting requirements may be granted for patients’ rooms in hospitals where such rooms are otherwise under adequate control. Areas or rooms in which the sources of radiation are to be located for periods of eight hours or less and are otherwise constantly attended under adequate control by qualified personnel need not be posted.

Access Control

The degree to which access to an area must be controlled is determined by the degree of the potential radiation hazard in the area.

Control of access to high radiation areas

Each entrance or access point to a high radiation area must have one or more of the following features:

• a control device that, upon entry into the area, causes the level of radiation to be reduced below that level at which an individual might receive a dose of 1 mSv in 1 h at 30 cm from the radiation source or from any surface that the radiation penetrates
• a control device that energizes a conspicuous visible or audible alarm signal so that the individual entering the high radiation area and the supervisor of the activity are made aware of the entry
• entryways that are locked, except during periods when access to the area is required, with positive control over each individual entry.

In place of the controls required for a high radiation area, continuous direct or electronic surveillance that is capable of preventing unauthorized entry may be substituted.

The controls must be established in a way that does not prevent individuals from leaving the high radiation area.

Control of access to very high radiation areas

In addition to the requirements for a high radiation area, additional measures must be instituted to ensure that an individual is not able to gain unauthorized or inadvertent access to areas in which radiation levels could be encountered at 5 Gy or more in 1 h at 1 m from a radiation source or any surface through which the radiation penetrates.

Markings on Containers and Equipment

Each container of radioactive material above an amount determined by the regulatory authority must bear a durable, clearly visible label bearing the radiation symbol and the words “CAUTION, RADIOACTIVE MATERIAL” or “DANGER, RADIOACTIVE MATERIAL”. The label must also provide sufficient information - such as the radionuclide(s) present, an estimate of the quantity of radioactivity, the date for which the activity is estimated, radiation levels, kinds of materials and mass enrichment - to permit individuals handling or using the containers, or working in the vicinity of the containers, to take precautions to avoid or minimize exposures.

Prior to removal or disposal of empty uncontaminated containers to unrestricted areas, the radioactive material label must be removed or defaced, or it must be clearly indicated that the container no longer contains radioactive materials.

Containers need not be labelled if:

1. the containers are attended by an individual who takes the precautions necessary to prevent the exposure of individuals in excess of the regulatory limits
2. containers, when they are in transport, are packaged and labelled in accordance with appropriate transportation regulations
3. containers are accessible only to individuals authorized to handle or use them, or to work in the vicinity of the containers, if the contents are identified to these individuals by a readily available written record (examples of containers of this type are containers in locations such as water-filled canals, storage vaults or hot cells); the record must be retained as long as the containers are in use for the purpose indicated on the record; or
4. the containers are installed in manufacturing or process equipment, such as reactor components, piping and tanks.

Warning Devices and Alarms

High radiation areas and very high radiation areas must be equipped with warning devices and alarms as discussed above. These devices and alarms can be visible or audible or both. Devices and alarms for systems such as particle accelerators should be automatically energized as part of the start-up procedure so that personnel will have time to vacate the area or turn off the system with a “scram” button before radiation is produced. “Scram” buttons (buttons in the controlled area that, when pressed, cause radiation levels to drop immediately to safe levels) must be easily accessible and prominently marked and displayed.

Monitor devices, such as continuous air monitors (CAMs), can be preset to emit audible and visible alarms or to turn off a system when certain action levels are exceeded.

Instrumentation

The employer must make available instrumentation appropriate for the degree and kinds of radiation and radioactive material present in the workplace. This instrumentation may be used to detect, monitor or measure the levels of radiation or radioactivity.

The instrumentation must be calibrated at appropriate intervals using accredited methods and calibration sources. The calibration sources should be as much as possible like the sources to be detected or measured.

Types of instrumentation include hand-held survey instruments, continuous air monitors, hand-and-feet portal monitors, liquid scintillation counters, detectors containing Ge or NaI crystals and so on.

Radioactive Material Transportation

The International Atomic Energy Agency (IAEA) has established regulations for the transportation of radioactive material. Most countries have adopted regulations compatible with IAEA radioactive shipment regulations.

Figure 2. Category I - WHITE label

Figure 2, figure 3 and figure 4 are examples of shipping labels that IAEA regulations require on the exterior of packages presented for shipment that contain radioactive materials. The transport index on the labels shown in figure 3 and figure 4 refer to the highest effective dose rate at 1 m from any surface of the package in mSv/h multiplied by 100, then rounded up to the nearest tenth. (For example, if the highest effective dose rate at 1 m from any surface of a package is 0.0233 mSv/h, then the transport index is 2.4.)

Figure 3. Category II - YELLOW label

Figure 5 shows an example of a placard that ground vehicles must prominently display when carrying packages containing radioactive materials above certain amounts.

Figure 5. Vehicle placard

Packaging intended for use in shipping radioactive materials must comply with stringent testing and documentation requirements. The type and quantity of radioactive material being shipped determines what specifications the packaging must meet.

Radioactive material transportation regulations are complicated. Persons who do not routinely ship radioactive materials should always consult experts experienced with such shipments.

Radioactive Waste

Various radioactive waste disposal methods are available, but all are controlled by regulatory authorities. Therefore, an organization must always confer with its regulatory authority to ensure that a disposal method is permissible. Radioactive waste disposal methods include holding the material for radioactive decay and subsequent disposal without regard to radioactivity, incineration, disposal in the sanitary sewerage system, land burial and burial at sea. Burial at sea is often not permitted by national policy or international treaty and will not be discussed further.

Radioactive waste from reactor cores (high-level radioactive waste) presents special problems with regard to disposal. Handling and disposal of such wastes is controlled by national and international regulatory authorities.

Often radioactive waste may have a property other than radioactivity that by itself would make the waste hazardous. Such wastes are called mixed wastes. Examples include radioactive waste that is also a biohazard or is toxic. Mixed wastes require special handling. Refer to regulatory authorities for proper disposition of such wastes.

Holding for radioactive decay

If the half-life of the radioactive material is short (generally less than 65 days) and if the organization has enough storage space, the radioactive waste can be held for decay with subsequent disposal without regard to its radioactivity. A holding period of at least ten half-lives usually is sufficient to make radiation levels indistinguishable from background.

The waste must be surveyed before it may be disposed of. The survey should employ instrumentation appropriate for the radiation to be detected and demonstrate that radiation levels are indistinguishable from background.

Incineration

If the regulatory authority allows incineration, then usually it must be demonstrated that such incineration does not cause the concentration of radionuclides in air to exceed permissible levels. The ash must be surveyed periodically to verify that it is not radioactive. In some circumstances it may be necessary to monitor the stack to ensure that permissible air concentrations are not being exceeded.

Disposal in the sanitary sewerage system

If the regulatory authority allows such disposal, then usually it must be demonstrated that such disposal does not cause the concentration of radionuclides in water to exceed permissible levels. Material to be disposed of must be soluble or otherwise readily dispersible in water. The regulatory authority often sets specific annual limits to such disposal by radionuclide.

Land burial

Radioactive waste not disposable by any other means will be disposed of by land burial at sites licensed by national or local regulatory authorities. Regulatory authorities control such disposal tightly. Waste generators usually are not allowed to dispose of radioactive waste on their own land. Costs associated with land burial include packaging, shipping and storage expenses. These costs are in addition to the cost of the burial space itself and can often be reduced by compacting the waste. Land burial costs for radioactive waste disposal are rapidly escalating.

Programme Audits

Radiation safety programmes should be audited periodically for effectiveness, completeness and compliance with regulatory authority. The audit should be done at least once a year and be comprehensive. Self-audits are usually permissible but audits by independent outside agencies are desirable. Outside agency audits tend to be more objective and have a more global point of view than local audits. An auditing agency not associated with day-to-day operations of a radiation safety programme often can identify problems not seen by the local operators, who may have become accustomed to overlooking them.

Training

Employers must provide radiation safety training to all workers exposed or potentially exposed to ionizing radiation or radioactive materials. They must provide initial training before a worker begins work and annual refresher training. In addition, each female worker of child-bearing age must be provided special training and information about the effects of ionizing radiation on the unborn child and about appropriate precautions she should take. This special training must be given when she is first employed, at annual refresher training, and if she notifies her employer that she is pregnant.

All individuals working in or frequenting any portion of an area access to which is restricted for the purposes of radiation safety:

• must be kept informed of the storage, transfer or use of radioactive materials or of radiation in such portions of the restricted area
• must be instructed in the health protection problems associated with exposure to such radioactive materials or radiation, in precautions or procedures to minimize exposure, and in the purposes and functions of protective devices employed
• must be instructed in, and instructed to observe, to the extent within the worker’s control, the applicable provisions of national and employer regulations for the protection of personnel from exposures to radiation or radioactive materials occurring in such areas
• must be instructed of their responsibility to report promptly to the employer any condition which may lead to or cause a violation of national or employer regulations or unnecessary exposure to radiation or to radioactive material
• must be instructed in the appropriate response to warnings made in the event of any unusual occurrence or malfunction that may involve exposure to radiation or radioactive material
• must be advised as to the radiation exposure reports that workers may request.

The extent of radiation safety instructions must be commensurate with potential radiological health protection problems in the controlled area. Instructions must be extended as appropriate to ancillary personnel, such as nurses who attend radioactive patients in hospitals and fire-fighters and police officers who might respond to emergencies.

Worker Qualifications

Employers must ensure that workers using ionizing radiation are qualified to perform the work for which they are employed. The workers must have the background and experience to perform their jobs safely, particularly with reference to exposure to and use of ionizing radiation and radioactive materials.

Radiation safety personnel must have the appropriate knowledge and qualifications to implement and operate a good radiation safety programme. Their knowledge and qualifications must be at least commensurate with the potential radiological health protection problems that they and the workers are reasonably likely to encounter.

Emergency Planning

All but the smallest operations that use ionizing radiation or radioactive materials must have emergency plans in place. These plans must be kept current and exercised on a periodic basis.

Emergency plans should address all credible emergency situations. The plans for a large nuclear power plant will be much more extensive and involve a much larger area and number of people than the plans for a small radioisotope laboratory.

All hospitals, especially in large metropolitan areas, should have plans for receiving and caring for radioactively contaminated patients. Police and fire-fighting organizations should have plans for dealing with transportation accidents involving radioactive material.

Record Keeping

The radiation safety activities of an organization must be fully documented and appropriately retained. Such records are essential if the need arises for past radiation exposures or radioactivity releases and for demonstrating compliance with regulatory authority requirements. Consistent, accurate and comprehensive record keeping must receive high priority.

Organizational Considerations

The position of the person primarily responsible for radiation safety must be placed in the organization so that he or she has immediate access to all echelons of workers and management. He or she must have free access to areas to which access is restricted for purposes of radiation safety and the authority to halt unsafe or illegal practices immediately.

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Ionizing Radiation Types

Alpha particles

An alpha particle is a tightly bound collection of two protons and two neutrons. It is identical to a helium-4 (⁴He) nucleus. Indeed, its ultimate fate after it loses most of its kinetic energy is to capture two electrons and become a helium atom.

Alpha-emitting radionuclides are generally relatively massive nuclei. Almost all alpha emitters have atomic numbers greater than or equal to that of lead (⁸²Pb). When a nucleus decays by emitting an alpha particle, both its atomic number (number of protons) and its number of neutrons are reduced by two and its atomic mass number is reduced by four. For example, the alpha decay of uranium-238 (²³⁸U) to thorium-234 (²³⁴Th) is represented by:

The left superscript is the atomic mass number (number of protons plus neutrons), the left subscript is the atomic number (number of protons), and the right subscript is the number of neutrons.

Common alpha emitters emit alpha particles with kinetic energies between about 4 and 5.5 MeV. Such alpha particles have a range in air of no more than about 5 cm (see figure 1). Alpha particles with an energy of at least 7.5 MeV are required to penetrate the epidermis (the protective layer of skin, 0.07 mm thick). Alpha emitters generally do not pose an external radiation hazard. They are hazardous only if taken within the body. Because they deposit their energy in a short distance, alpha particles are high linear energy transfer (LET) radiation and have a large radiation weighting factor; typically, w _R=20.

Figure 1. Range-energy radiation of slow alpha particles in air at 15 and 760 m

Beta particles

A beta particle is a highly energetic electron or positron. (A positron is the anti-particle of the electron. It has the same mass and most other properties of an electron except for its charge, which is exactly the same magnitude as that of an electron but is positive.) Beta-emitting radionuclides can be of high or low atomic weight.

Radionuclides that have an excess of protons in comparison with stable nuclides of about the same atomic mass number can decay when a proton in the nucleus converts to a neutron. When this occurs, the nucleus emits a positron and an extremely light, very non-interacting particle called a neutrino. (The neutrino and its anti-particle are of no interest in radiation protection.) When it has given up most of its kinetic energy, the positron ultimately collides with an electron and both are annihilated. The annihilation radiation produced is almost always two 0.511 keV (kiloelectron volt) photons travelling in directions 180 degrees apart. A typical positron decay is represented by:

where the positron is represented by β⁺ and the neutrino by n. Note that the resulting nuclide has the same atomic mass number as the parent nuclide and an atomic (proton) number larger by one and a neutron number lesser by one than those of the original nuclide.

Electron capture competes with positron decay. In electron capture decay, the nucleus absorbs an orbital electron and emits a neutrino. A typical electron capture decay is given by:

Electron capture is always possible when the resulting nucleus has a lower total energy than the initial nucleus. However, positron decay requires that the total energy of the initial atom is greater than that of the resulting atom by more than 1.02 MeV (twice the rest mass energy of the positron).

Similar to positron and electron capture decay, negatron (β^–) decay occurs for nuclei that have an excess of neutrons compared to stable nuclei of about the same atomic mass number. In this case, the nucleus emits a negatron (energetic electron) and an anti-neutrino. A typical negatron decay is represented by:

where the negatron is represented by β^– and the anti-neutrino by`n Here the resulting nucleus gains one neutron at the expense of one proton but again does not change its atomic mass number.

Alpha decay is a two-body reaction, so alpha particles are emitted with discrete kinetic energies. However, beta decay is a three-body reaction, so beta particles are emitted over a spectrum of energies. The maximum energy in the spectrum depends on the decaying radionuclide. The average beta energy in the spectrum is approximately one-third of the maximum energy (see figure 2).

Figure 2. Energy spectrum of negatrons emitted from ³²P

Typical maximum beta energies range from 18.6 keV for tritium (³H) to 1.71 MeV for phosphorus-32 (³²P).

The range of beta particles in air is approximately 3.65 m per MeV of kinetic energy. Beta particles of at least 70 keV energy are required to penetrate the epidermis. Beta particles are low-LET radiation.

Gamma radiation

Gamma radiation is electromagnetic radiation emitted by a nucleus when it undergoes a transition from a higher to a lower energy state. The number of protons and neutrons in the nucleus does not change in such a transition. The nucleus may have been left in the higher energy state following an earlier alpha or beta decay. That is, gamma rays are often emitted immediately following alpha or beta decays. Gamma rays can also result from neutron capture and inelastic scattering of subatomic particles by nuclei. The most energetic gamma rays have been observed in cosmic rays.

Figure 3 is a picture of the decay scheme for cobalt-60 (⁶⁰Co). It shows a cascade of two gamma rays emitted in nickel-60 (⁶⁰Ni) with energies of 1.17 MeV and 1.33 MeV following the beta decay of ⁶⁰Co.

Figure 3. Radioactive decay scheme for ⁶⁰Co

Figure 4 is a picture of the decay scheme for molybdenum-99 (⁹⁹Mo). Note that the resulting technetium-99 (⁹⁹Tc) nucleus has an excited state that lasts for an exceptionally long time (t_½ = 6 h). Such an excited nucleus is called an isomer. Most excited nuclear states have half-lives between a few picoseconds (ps) and 1 microsecond (μs).

Figure 4. Radioactive decay scheme for ⁹⁹Mo

Figure 5 is a picture of the decay scheme for arsenic-74 (⁷⁴As). It illustrates that some radionuclides decay in more than one way.

Figure 5. Radioactive decay scheme for ⁷⁴As, illustrating competing processes of negatron emission, positron emission and electron capture (m₀ is the rest mass of the electron)

While alpha and beta particles have definite ranges in matter, gamma rays are attenuated exponentially (ignoring build-up that results from scattering within a material) as they pass through matter. When build-up can be ignored the attenuation of gamma rays is given by:

where I(x) is the gamma ray intensity as a function of distance x into the material and μ is the mass attenuation coefficient. The mass attenuation coefficient depends on gamma-ray energy and on the material with which the gamma rays are interacting. Mass attenuation coefficient values are tabulated in many references. Figure 6 shows the absorption of gamma rays in matter in conditions of good geometry (build-up can be ignored).

Figure 6. Atenuation of 667 keV gamma rays in Al and Pb under conditions of good geometry (dashed line represents attenuation of a poly-energetic photon beam)

Build-up occurs when a broad gamma-ray beam interacts with matter. The measured intensity at points within the material is increased relative to the expected “good geometry” (narrow beam) value due to gamma rays scattered from the sides of the direct beam into the measuring device. The degree of build-up depends on the geometry of the beam, on the material and on the energy of the gamma rays.

Internal conversion competes with gamma emission when a nucleus transforms from a higher energy state to a lower one. In internal conversion, an inner orbital electron is ejected from the atom instead of the nucleus emitting a gamma ray. The ejected electron is directly ionizing. As outer orbital electrons drop to lower electronic energy levels to fill the vacancy left by the ejected electron, the atom emits x rays. Internal conversion probability relative to gamma emission probability increases with increasing atomic number.

X rays

X rays are electromagnetic radiation and, as such, are identical to gamma rays. The distinction between x rays and gamma rays is their origin. Whereas gamma rays originate in the atomic nucleus, x rays result from electron interactions. Although x rays often have lower energies than gamma rays, this is not a criterion for differentiating them. It is possible to produce x rays with energies much higher than gamma rays resulting from radioactive decay.

Internal conversion, discussed above, is one method of x ray production. In this case, the resulting x rays have discrete energies equal to the difference in the energy levels between which the orbital electrons transit.

Charged particles emit electromagnetic radiation whenever they are accelerated or decelerated. The amount of radiation emitted is inversely proportional to the fourth power of the particle’s mass. As a result, electrons emit much more x radiation than heavier particles such as protons, all other conditions being equal. X-ray systems produce x rays by accelerating electrons across a large electric potential difference of many kV or MV. The electrons are then quickly decelerated in a dense, heat-resistant material, such as tungsten (W).

The x rays emitted from such systems have energies spread over a spectrum ranging from about zero up to the maximum kinetic energy possessed by the electrons before deceleration. Often superimposed on this continuous spectrum are x rays of discrete energy. They are produced when the decelerating electrons ionize the target material. As other orbital electrons move to fill vacancies left after ionization, they emit x rays of discrete energies similar to the way x rays are emitted following internal conversion. They are called characteristic x rays because they are characteristic of the target (anode) material. See figure 7 for a typical x ray spectrum. Figure 8 depicts a typical x ray tube.

Figure 7. X-ray spectrum illustrating the contribution of characteristic x rays produced as electrons fill holes in the K shell of W (the wavelength of x rays is inversely proportional to their energy)

X rays interact with matter the same way gamma rays do, but a simple exponential attenuation equation does not adequately describe the attenuation of x rays with a continuous range of energies (see figure 6). However, as lower energy x rays are removed more rapidly from the beam than higher energy x rays as they pass through material, the description of attenuation approaches an exponential function.

Figure 8. A simplified x-ray tube with a stationary anode and a heated filament

Neutrons

Generally, neutrons are not emitted as a direct result of natural radioactive decay. They are produced during nuclear reactions. Nuclear reactors produce neutrons in the greatest abundance but particle accelerators and special neutron sources, called (α, n) sources, also can yield neutrons.

Nuclear reactors produce neutrons when uranium (U) nuclei in nuclear fuel split, or fission. Indeed, the production of neutrons is essential in maintaining nuclear fission in a reactor.

Particle accelerators produce neutrons by accelerating charged particles, such as protons or electrons, to high energies to bombard stable nuclei in a target. Neutrons are only one of the particles that can result from such nuclear reactions. For example, the following reaction produces neutrons in a cyclotron that is accelerating deuterium ions to bombard a beryllium target:

Alpha emitters mixed with beryllium are portable sources of neutrons. These (α, n) sources produce neutrons via the reaction:

The source of the alpha particles can be such isotopes as polonium-210 (²¹⁰Po),
plutonium-239 (²³⁹Pu) and americium-241 (²⁴¹Am).

Neutrons are generally classified according to their energy as illustrated in table 1. This classification is somewhat arbitrary and may vary in different contexts.

Table 1. Classification of neutrons according to kinetic energy

A number of possible modes of neutron interaction with matter exist, but the two main modes for the purposes of radiation safety are elastic scattering and neutron capture.

Elastic scattering is the means by which higher-energy neutrons are reduced to thermal energies. Higher-energy neutrons interact primarily by elastic scattering and generally do not cause fission or produce radioactive material by neutron capture. It is thermal neutrons that are primarily responsible for the latter types of interaction.

Elastic scattering occurs when a neutron interacts with a nucleus and bounces off with reduced energy. The interacting nucleus takes up the kinetic energy the neutron loses. After being excited in this manner, the nucleus soon gives up this energy as gamma radiation.

When the neutron eventually reaches thermal energies (so-called because the neutron is in thermal equilibrium with its environment), it is easily captured by most nuclei. Neutrons, having no charge, are not repelled by the positively charged nucleus as are protons. When a thermal neutron approaches a nucleus and comes within the range of the strong nuclear force, on the order of a few fm (fm = 10^–15 metres), the nucleus captures the neutron. The result can then be a radioactive nucleus that emits a photon or other particle or, in the case of fissionable nuclei such as ²³⁵U and ²³⁹Pu, the capturing nucleus can fission into two smaller nuclei and more neutrons.

The laws of kinematics indicate that neutrons will reach thermal energies more rapidly if the elastic scattering medium includes a large number of light nuclei. A neutron rebounding off a light nucleus loses a much larger percentage of its kinetic energy than when it bounces off of a heavy nucleus. For this reason, water and hydrogenous materials are the best shielding material to slow down neutrons.

A monoenergetic beam of neutrons will attenuate exponentially in material, obeying an equation similar to that given above for photons. The probability of a neutron interacting with a given nucleus is described in terms of the quantity cross section. Cross section has units of area. The special unit for cross section is the barn (b), defined by:

It is extremely difficult to produce neutrons without accompanying gamma and x rays. It may be generally assumed that if neutrons are present, so are high energy photons.

Ionizing Radiation Sources

Primordial radionuclides

Primordial radionuclides occur in nature because their half-lives are comparable with the age of the earth. Table 2 lists the most important primordial radionuclides.

Table 2. Primordial radionuclides

Uranium and thorium isotopes head a long chain of progeny radioisotopes that, as a result, are also naturally occurring. Figure 9, A-C, illustrates the decay chains for ²³²Th, ²³⁸U and ²³⁵U, respectively. Because alpha decay is common above atomic mass number 205 and an alpha particle’s atomic mass number is 4, there are four distinct decay chains for heavy nuclei. One of these chains (see figure 9, D), that for ²³⁷Np, does not occur in nature. This is because it does not contain a primordial radionuclide (that is, no radionuclide in this chain has a half-life comparable with the age of the earth).

Figure 9. Decay series (Z = atomic number; N = atomic mass number)    

 Note that radon (Rn) isotopes occur in each chain (²¹⁹Rn, ²²⁰Rn and ²²²Rn). Since Rn is a gas, once Rn is produced, it has a chance of escape to the atmosphere from the matrix in which it was formed. However, the half-life of ²¹⁹Rn is much too short to allow significant amounts of it to reach a breathing zone. The relatively short half-life of ²²⁰Rn usually makes it a lesser health hazard concern than ²²²Rn.

Not including Rn, primordial radionuclides external to the body deliver on the average about 0.3 mSv annual effective dose to the human population. The actual annual effective dose varies widely and is determined primarily by the concentration of uranium and thorium in the local soil. In some parts of the world where monazite sands are common, the annual effective dose to a member of the population is as high as about 20 mSv. In                                                                                                                           other places such as on coral atolls and near seashores, the                                                                                                                           value may be as low as 0.03 mSv (see figure 9).

Radon is usually considered separately from other naturally occurring terrestrial radionuclides. It seeps into the air from the soil. Once in the air, Rn further decays to radioactive isotopes of Po, bismuth (Bi) and Pb. These progeny radionuclides attach themselves to dust particles that may be breathed in and trapped in the lungs. Being alpha emitters, they deliver almost all of their radiation energy to the lungs. It is estimated that the average annual lung equivalent dose from such exposure is about 20 mSv. This lung equivalent dose is comparable to a whole body effective dose of about 2 mSv. Clearly, Rn and its progeny radionuclides are the most significant contributors to background radiation effective dose (see figure 9).

Cosmic rays

Cosmic radiation includes energetic particles of extraterrestrial origin that strike the atmosphere of the earth (primarily particles and mostly protons). It also includes secondary particles; mostly photons, neutrons and muons, generated by interactions of primary particles with gases in the atmosphere.

By virtue of these interactions, the atmosphere serves as a shield against cosmic radiation, and the thinner this shield, the greater the effective dose rate. Thus, the cosmic-ray effective dose rate increases with altitude. For example, the dose rate at an altitude of 1,800 metres is about double that at sea level.

Because primary cosmic radiation consists mostly of charged particles, it is influenced by the earth’s magnetic field. People living in higher latitudes receive greater effective doses of cosmic radiation than those closer to the earth’s equator. Variation due to this effect is of the order
of 10%.

Finally, the cosmic-ray effective dose rate varies according to modulation of the sun’s cosmic-ray output. On the average, cosmic rays contribute about 0.3 mSv to background radiation whole-body effective dose.

Cosmogenic radionuclides

Cosmic rays produce cosmogenic radionuclides in the atmosphere. The most prominent of these are tritium (³H), beryllium-7 (⁷Be), carbon-14 (¹⁴C) and sodium-22 (²²Na). They are produced by cosmic rays interacting with atmospheric gases. Cosmogenic radionuclides deliver about 0.01 mSv annual effective dose. Most of this comes from ¹⁴C.

Nuclear fallout

From the 1940s through the 1960s, extensive testing of nuclear weapons above ground occurred. This testing produced large quantities of radioactive materials and distributed them to the environment throughout the world as fallout. Although much of this debris has since decayed to stable isotopes, small amounts that remain will be a source of exposure for many years to come. In addition, nations that continue to occasionally test nuclear weapons in the atmosphere add to the worldwide inventory.

The primary fallout contributors to effective dose currently are strontium-90 (⁹⁰Sr) and caesium-137 (¹³⁷Cs), both of which have half-lives around 30 years. The average annual effective dose from fallout is about 0.05 mSv.

Radioactive material in the body

The deposition of naturally occurring radionuclides in the human body results primarily from the inhalation and ingestion of these materials in air, food and water. Such nuclides include radioisotopes of Pb, Po, Bi, Ra, K (potassium), C, H, U and Th. Of these, ⁴⁰K is the largest contributor. Naturally occurring radionuclides deposited in the body contribute about 0.3 mSv to the annual effective dose.

Machine-produced radiation

The use of x rays in the healing arts is the largest source of exposure to machine-produced radiation. Millions of medical x ray systems are in use around the world. The average exposure to these medical x ray systems is greatly dependent on a population’s access to care. In developed countries, the average annual effective dose from medically prescribed radiation from x rays and radioactive material for diagnosis and therapy is on the order of 1 mSv.

X rays are a by-product of most high-energy physics particle accelerators, especially those that accelerate electrons and positrons. However, appropriate shielding and safety precautions plus the limited population at risk make this source of radiation exposure less significant than the above sources.

Machine-produced radionuclides

Particle accelerators can produce a large variety of radionuclides in varying quantities by way of nuclear reactions. Accelerated particles include protons, deuterons (²H nuclei), alpha particles, charged mesons, heavy ions and so on. Target materials can be made of almost any isotope.

Particle accelerators are virtually the only source for positron-emitting radioisotopes. (Nuclear reactors tend to produce neutron-rich radioisotopes that decay by negatron emission.) They are also being increasingly used to produce short-lived isotopes for medical use, especially for positron-emission tomography (PET).

Technologically enhanced material and consumer products

X rays and radioactive materials appear, wanted and unwanted, in a great number of modern-day operations. Table 3 lists these radiation sources.

Table 3. Sources and estimates of associated population effective doses from technologically enhanced material and consumer products

Source: NCRP 1987.

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Ionizing radiation is everywhere. It arrives from outer space as cosmic rays. It is in the air as emissions from radioactive radon and its progeny. Naturally occurring radioactive isotopes enter and remain in all living things. It is inescapable. Indeed, all species on this planet evolved in the presence of ionizing radiation. While humans exposed to small doses of radiation may not immediately show any apparent biological effects, there is no doubt that ionizing radiation, when given in sufficient amounts, can cause harm. These effects are well known both in kind and in degree.

While ionizing radiation can cause harm, it also has many beneficial uses. Radioactive uranium generates electricity in nuclear power plants in many countries. In medicine, x rays produce radiographs for diagnosis of internal injuries and diseases. Nuclear medicine physicians use radioactive material as tracers to form detailed images of internal structures and to study metabolism. Therapeutic radiopharmaceuticals are available to treat disorders such as hyperthyroidism and cancer. Radiotherapy physicians use gamma rays, pion beams, electron beams, neutrons and other types of radiation to treat cancer. Engineers use radioactive material in oil well logging operations and in soil moisture density gauges. Industrial radiographers use x rays in quality control to look at internal structures of manufactured devices. Exit signs in buildings and aircraft contain radioactive tritium to make them glow in the dark in the event of a power failure. Many smoke detectors in homes and commercial buildings contain radioactive americium.

These many uses of ionizing radiation and radioactive materials enhance the quality of life and help society in many ways. The benefits of each use must always be compared with the risks. The risks may be to workers directly involved in applying the radiation or radioactive material, to the public, to future generations and to the environment or to any combination of these. Beyond political and economic considerations, benefits must always outweigh risks when ionizing radiation is involved.

Ionizing Radiation

Ionizing radiation consists of particles, including photons, which cause the separation of electrons from atoms and molecules. However, some types of radiation of relatively low energy, such as ultraviolet light, can also cause ionization under certain circumstances. To distinguish these types of radiation from radiation that always causes ionization, an arbitrary lower energy limit for ionizing radiation usually is set around 10 kiloelectron volts (keV).

Directly ionizing radiation consists of charged particles. Such particles include energetic electrons (sometimes called negatrons), positrons, protons, alpha particles, charged mesons, muons and heavy ions (ionized atoms). This type of ionizing radiation interacts with matter primarily through the Coulomb force, repelling or attracting electrons from atoms and molecules by virtue of their charges.

Indirectly ionizing radiation consists of uncharged particles. The most common kinds of indirectly ionizing radiation are photons above 10 keV (x rays and gamma rays) and all neutrons.

X-ray and gamma-ray photons interact with matter and cause ionization in at least three different ways:

1. Lower-energy photons interact mostly via the photoelectric effect, in which the photon gives all of its energy to an electron, which then leaves the atom or molecule. The photon disappears.
2. Intermediate-energy photons mostly interact through the Compton effect, in which the photon and an electron essentially collide as particles. The photon continues in a new direction with reduced energy while the released electron goes off with the remainder of the incoming energy (less the electron’s binding energy to the atom or molecule).
3. Pair production is possible only for photons with energy in excess of 1.02 MeV. (However, near 1.02 MeV, the Compton effect still dominates. Pair production dominates at higher energies.) The photon disappears and an electron-positron pair appears in its place (this occurs only in the vicinity of a nucleus because of conservation of momentum and energy considerations). The total kinetic energy of the electron-positron pair is equal to the energy of the photon less the sum of the rest-mass energies of the electron and positron (1.02 MeV). These energetic electrons and positrons then proceed as directly ionizing radiation. As it loses kinetic energy, a positron will eventually encounter an electron, and the particles will annihilate each other. Two (usually) 0.511 MeV photons are then emitted from the annihilation site at 180 degrees from each other.

a given photon any of these can occur, except that pair production is possible only for photons with energy greater than 1.022 MeV. The energy of the photon and the material with which it interacts determine which interaction is the most likely to occur.

Figure 1 shows the regions in which each type of photon interaction dominates as a function of photon energy and atomic number of absorber.

Figure 1. Relative importance of the three principal interactions of photons in matter

The most common neutron interactions with matter are inelastic collisions, neutron capture (or activation) and fission. All of these are interactions with nuclei. A nucleus colliding inelastically with a neutron is left at a higher energy level. It can release this energy in the form of a gamma ray or by emitting a beta particle, or both. In neutron capture, an affected nucleus may absorb the neutron and eject energy as gamma or x rays or beta particles, or both. The secondary particles then cause ionization as discussed above. In fission, a heavy nucleus absorbs the neutron and splits into two lighter nuclei that are almost always radioactive.

Quantities, Units and Related Definitions

The International Commission on Radiation Units and Measurements (ICRU) develops internationally accepted formal definitions of quantities and units of radiation and radioactivity. The International Commission on Radiological Protection (ICRP) also sets standards for definition and use of various quantities and units used in radiation safety. A description of some quantities, units and definitions commonly used in radiation safety follows.

Absorbed dose. This is the fundamental dosimetric quantity for ionizing radiation. Basically, it is the energy ionizing radiation imparts to matter per unit mass. Formally,

where D is the absorbed dose, de is the mean energy imparted to matter of mass dm. Absorbed dose has units of joules per kilogram (J kg^–1). The special name for the unit of absorbed dose is the gray (Gy).

Activity. This quantity represents the number of nuclear transformations from a given nuclear energy state per unit time. Formally,

where A is the activity, dN is the expectation value of the number of spontaneous nuclear transitions from the given energy state in the time interval dt. It is related to the number of radioactive nuclei N by:

where l is the decay constant. Activity has units of inverse seconds (s^–1). The special name for the unit of activity is the becquerel (Bq).

Decay constant (l). This quantity represents the probability per unit time that a nuclear transformation will occur for a given radionuclide. The decay constant has units of inverse seconds (s^–1). It is related to the half-life t_½ of a radionuclide by:

The decay constant l is related to the mean lifetime, t, of a radionuclide by:

The time dependence of activity A(t) and of the number of radioactive nuclei N(t) can be expressed by and  respectively.

Deterministic biological effect. This is a biological effect caused by ionizing radiation and whose probability of occurrence is zero at small absorbed doses but will increase steeply to unity (100%) above some level of absorbed dose (the threshold). Cataract induction is an example of a stochastic biological effect.

Effective dose. The effective dose E is the sum of the weighted equivalent doses in all the tissues and organs of the body. It is a radiation safety quantity, so its use is not appropriate for large absorbed doses delivered in a relatively short period of time. It is given by:

where w _T is the tissue weighting factor and H_T is the equivalent dose for tissue T. Effective dose has units of J kg^–1. The special name for the unit of effective dose is the sievert (Sv).

Equivalent dose. The equivalent dose H_T is the absorbed dose averaged over a tissue or organ (rather than at a point) and weighted for the radiation quality that is of interest. It is a radiation safety quantity, so its use is not appropriate for large absorbed doses delivered in a relatively short period of time. The equivalent dose is given by:

where D_T,R is the absorbed dose averaged over the tissue or organ T due to radiation R and w _R
is the radiation weighting factor. Equivalent dose has units of J kg^–1. The special name for the unit of equivalent dose is the sievert (Sv).

Half-life. This quantity is the amount of time required for the activity of a radionuclide sample to reduce by a factor of one-half. Equivalently, it is the amount of time required for a given number of nuclei in a given radioactive state to reduce by a factor of one-half. It has fundamental units of seconds (s), but is also commonly expressed in hours, days and years. For a given radionuclide, half-life t_½ is related to the decay constant l by:

Linear energy transfer. This quantity is the energy a charged particle imparts to matter per unit length as it traverses the matter. Formally,

where L is the linear energy transfer (also called linear collision stopping power) and de is the mean energy lost by the particle in traversing a distance dl. Linear energy transfer (LET) has units of J m^–1.

Mean lifetime. This quantity is the average time a nuclear state will survive before it undergoes a transformation to a lower energy state by emitting ionizing radiation. It has fundamental units of seconds (s), but may also be expressed in hours, days or years. It is related to the decay constant by:

where t is the mean lifetime and l is the decay constant for a given nuclide in a given energy state.

Radiation weighting factor. This is a number w _R that, for a given type and energy of radiation R, is representative of values of the relative biological effectiveness of that radiation in inducing stochastic effects at low doses. The values of w _R are related to linear energy transfer (LET) and are given in table 1. Figure 2 (overleaf) shows the relationship between w _R and LET for neutrons.

Table 1. Radiation weighting factors w_R

¹ All values relate to the radiation incident on the body or, for internal sources, emitted from the source.

² Excluding Auger electrons emitted from nuclei bound to DNA.

Relative biological effectiveness (RBE). The RBE of one type of radiation compared with another is the inverse ratio of the absorbed doses producing the same degree of a defined biological end point.

Figure 2. Radiation weighting factors for neutrons (the smooth curve is to be treated  as an approximation)

Stochastic biological effect. This is a biological effect caused by ionizing radiation whose probability of occurrence increases with increasing absorbed dose, probably with no threshold, but whose severity is independent of absorbed dose. Cancer is an example of a stochastic biological effect.

Tissue weighting factor w _T. This represents the contribution of tissue or organ T to the total detriment due to all of the stochastic effects resulting from uniform irradiation of the whole body. It is used because the probability of stochastic effects due to an equivalent dose depends on the tissue or organ irradiated. A uniform equivalent dose over the whole body should give an effective dose numerically equal to the sum of effective doses for all tissues and organs of the body. Therefore, the sum of all tissue weighting factors is normalized to unity. Table 2 gives values for tissue weighting factors.

Table 2. Tissue weighting factors w_T

¹ The values have been developed from a reference population of equal numbers of both sexes and a wide range of ages. In the definition of effective dose they apply to workers, to the whole population, and to either sex.

² For purposes of calculation, the remainder is composed of the following additional tissues and organs: adrenals, brain, upper large intestine, small intestine, kidneys, muscle, pancreas, spleen, thymus and uterus. The list includes organs that are likely to be selectively irradiated. Some organs in the list are known to be susceptible to cancer induction.

³ In those exceptional cases in which a single one of the remainder tissues or organs receives an equivalent dose in excess of the highest dose in any of the twelve organs for which a weighting factor is specified, a weighting factor of 0.025 should be applied to that tissue or organ and a weighting factor of 0.025 to the average dose in the rest of the remainder as defined above.

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