Ionising radiation at workplaces

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Dr. Gregor Omahen, ZVD, Slovenia

Introduction

Ionising radiation can be described as the transfer of energy in the form of particles (such as alpha and beta particles) or electromagnetic waves (such as X-rays and gamma rays) of a wavelength of 100 nanometres or less or a frequency of 3 x 1015 hertz or more capable of producing ions directly or indirectly. Ionising radiation can occur naturally (e.g. from the radioactive decay of natural radioactive substances such as radon gas and its decay products) or it can be generated artificially (e.g. man-made radioactive substances or the operation of certain electrical equipment, such as X-ray sets, which emit ionising radiations). Ionising radiation has many uses in industry, such as energy production, manufacturing, medicine and research and produces many benefits to society. However, ionising radiation attacks the cells of the body by producing chemical changes in the cell DNA, leading to abnormal cell growth, therefore it is important that exposures are managed sensibly to protect workers. The effect on body tissues will depend on: the type of radiation, the dose and duration of exposure and whether the source is internal or external to the body. A criterion and three principles for control (time, distance and shielding) are used to ensure that ionising radiation exposure is kept to a minimum.

Properties of ionising radiation

We are familiar with many types of radiations in our everyday lives. Even visible light is a form of radiation. The main difference between ionising and non-ionising radiation is in the amount of energy the radiation carries. Ionising radiation carries more energy than non-ionising radiation. It has such high energy that when travelling through matter, it knocks electrons from the atoms. This process is called ionisation. Every atom can be split into a negative charged particle called an electron and a positive charged particle called an ion.

Types of ionising radiation

Radioactive substances give out radiation all of the time. There are three types of nuclear radiation: alpha, beta and gamma. Alpha is the least penetrating, while gamma is the most penetrating. Nonetheless, all three are ionising radiation: they can knock electrons out of atoms and form charged particles.

Alpha radiation represents the nucleus of helium atoms that are often called alpha particles (α). They are positively charged particles with two protons and two neutrons. On an atomic scale, these particles are big. When such particles enter into matter, they see other atoms as small particles that are tightly bonded. Alpha particles cannot penetrate into these tightly bond structures. Since these particles are slow and heavy their range in solid matter is only a few µm and in the air a few mm. If an alpha source is outside the body, the alpha radiation will not cause any harm to the human body since radiation will not reach the skin. Alpha radiation cannot penetrate through a sheet of paper.

Beta radiation consists of small, high-energy, and high-speed electrons or positrons emitted by certain types of radioactive nuclei. These light and short range particles are also known as beta (β) particles and they are actually an ejected electron. Electrons are negatively charged particles and are usually bonded in the atom. If they are not bonded and travel through matter, this is beta radiation. Beta particles are a few thousand times smaller than alpha particles. From the perspective of an electron the human body appears as a net of huge balls that are very weakly bonded together at large distances providing large areas of empty space. They can travel in this space until they hit one of the huge balls (atoms). Their speed lowers and direction changes after the collision but the beta particle can continue travelling. After a few collisions, beta particles lose all their energy and stop. Their range in matter is much longer than those of alpha particles and is dependent on their energy. In air, their range can be a few meters. If beta radiation reaches the human body, it can cause skin burn or blindness if the eyes are exposed. Internal organs will not be damaged (unless ß-radiation emitters are deposited internally e.g. by ingestion) since beta radiation stops in 1 to 2 cm of tissue.

Gamma particles have no mass and no electric charge. We call them photons. They are the same particles as those that represent visible light but have much higher energy. Since they have no mass and no electric charge, it is difficult to stop them. In fact, we never can stop all of them. And we cannot define the range of gamma radiation as we were able for alpha or beta radiation. However, with distance and appropriate shielding, we can reduce gamma radiation. Since gamma radiation is very penetrating, we have to apply protection principles to minimise radiation damage to workers or to the general public.

X-rays also ionise atoms. They behave the same as gamma rays, except that their energy is lower. Neutrons are neutral particles with mass. Since they have no electrical charge, it is difficult to stop them. They can be stopped only by a collision with other particles in the matter.

The penetrating power of various ionising radiations is shown in Figure 1.

Figure 1: Penetrating power of ionising radiation
Figure11 GO.jpg
Source: Provided by the author

Use of ionising radiation sources

Ionising radiation sources are used in medicine (for diagnosis and treatment), industry (for measurement and other purposes as well as for producing energy) and in research and teaching. Maybe the most known use of ionising radiation is in diagnostic medicine. If someone has an internal injury, it is almost certain that he/she will be sent for an X-ray examination. X-rays are produced in X-ray tubes. They enter into the patient’s body on one side and leave on the opposite side, where a detector is installed. Since X-rays are attenuated differently in different tissues, we get an X-ray image of the patient’s internal organs and bones. From the image, a doctor can see where the anomaly is in the patient’s body.

Ionising radiation is also used for treatment in oncology. Cancerous growths can be killed if large doses of ionising radiation are delivered to it. Of course, every treatment with ionising radiation sources must be well planned since radiation can also kill healthy cells.

The use of ionising sources ionizing source in industry is very heterogeneous. Ionising radiation is used in level meters for example. It can be found in brewery where on the assembly line production an ionising radiation source is installed on one side of the line and a detector on the other. Radiation travels from the source to the detector. If a bottle passing between is full, the radiation is absorbed into the beer and only a small fraction of emitted radiation reaches the detector. This produces a “pass” indication and the bottle can proceed to another phase of production. It the bottle is not full, more radiation reaches the detector, giving a “fail” indication and the bottle is ejected from the production line.

Ionising radiation is often used for non-destructive testing. The method is similar to diagnostic use in medicine. Ionising radiation penetrates pipes, tubes, casts or other products where on the other side is a detector, usually to ionising radiation sensitive film. The image on the film shows if there are any defects in the object such as cracks, homogeneities or foreign material.

A nuclear density gauge is a tool used in civil construction and the petroleum industry as well as for mining and archaeology purposes. It consists of a radiation source that emits a directed beam of particles and a sensor that counts the received particles that are either reflected by the test material or pass through it. By calculating the percentage of particles that return to the sensor, the gauge can be calibrated to measure the density and inner structure of the test material.

In some industries, accumulation of naturally occurring radioactivity can occur. Everything around us is radioactive. In some cases, this natural radioactivity can accumulate. It accumulates in the Oil and Gas industries. We drill into solids to extract oil. Solids may contain naturally occurring radioactive material that can accumulate in vessels or deposit on internal surfaces. Workers working in the vicinity of such places are exposed to elevated levels of ionising radiation. In the zircon sand industry, workers might be exposed to naturally occurring radiation since elevated levels of uranium and thorium can be found in zircon.

Probably everyone has experienced an x-ray check of baggage at the airport. Passenger baggage is put on a conveyer and sent through an x-ray device. The security officer can see on the monitor if some suspicious objects are in the baggage. The method is very similar to that used in medicine.

In the nuclear industry, ionising radiation is not used but is a product of the nuclear reaction. For electricity production, we use heat generated during the reaction. The radioactive atoms that also originate from a nuclear reaction emit ionising radiation. Since these radioactive atoms and ionising radiation is the by product and cannot be used in the process, it becomes radioactive waste.

Basic radiation protection principles prevention of doses)

Workplaces with ionising radiation are designated as controlled and supervised areas. Designated Areas are a legal requirement and the responsibility to designate those areas is that of the employer. Their purpose is to help manage the radiation risk by identifying and segregating higher risk activities from lower and thus control the extent of radiation exposure. The higher category is Controlled Area, and thus tighter controls are required in Controlled Areas than in Supervised Areas.

A controlled area is one that has been designated by an employer to assist in controlling and restricting radiation exposures. Controlled areas will be designated because the employer has recognised the need for people entering an area to follow special procedures. In controlled areas, persons may receive an effective dose of more than 6 millisievert or organ doses higher than 45 millisievert for the eye lens or 150 millisievert for the skin, hands, forearms, feet and ankles in a calendar year. Entrance into controlled areas is strictly controlled by the employer. Employees designated as ‘classified persons’ and outside workers can freely enter the area provided they have been authorised to do so by the employer and have received appropriate training. Other employees should only be allowed conditional access and only in accordance with prior written arrangements.

Supervised areas are areas that do not belong to the controlled areas and in which persons may receive an effective dose of more than 1 millisievert or organ doses higher than 15 millisievert for the eye lens or 50 millisievert for the skin, hands, forearms, feet and ankles in a calendar year.

Radiation damage

Ionising radiation ionises atoms in all matter including the human body. Ionisation can cause cell damage. In the worst case, the radiation damages cell DNA, which contains genetic information. If the damaged cell survives, it can mutate and reproduce and cancer can occur. Such a harmful effect and its probability increases with the dose of ionising radiation absorbed in the tissue.

Deterministic effects only occur once a threshold of exposure has been exceeded. The severity of deterministic effects increases as the dose of exposure increases. Because of an identifiable threshold level, appropriate radiation protection mechanisms and occupational exposure dose limits can be put in place to reduce the likelihood of these effects occurring. Deterministic effects are caused by significant cell damage or death. The physical effects will occur when the cell death burden is large enough to cause obvious functional impairment of a tissue or organ. Such effects are skin erythema, necrosis or epilation. The deterministic effect includes eye cataracts or sterility. At doses above 1000 mSv, radiation sickness can occur. Signs of radiation sickness are nausea and vomiting, headaches, fatigue, fever and short periods of skin reddening. These symptoms are common to many illnesses and are often not recognised as the consequence of high radiation exposure. At doses above 6000 mSv, death occurs. Teratogenic mutations result from the exposure of foetuses (unborn children) to radiation. They can include smaller head or brain size, poorly formed eyes, abnormally slow growth and mental retardation. Studies indicate that foetuses are most sensitive between about eight to fifteen weeks after conception. They remain somewhat less sensitive between six and twenty-five weeks old.


Radiation effects can be categorised by when they appear as either prompt effects or delayed effects. Prompt effects are those seen immediately after large doses of radiation delivered over a short period of time. Such effects are radiation sickness or skin erythema. Delayed effects appear months or years after the exposure. Such effects are eye cataracts or cancer.

Justification

Every use of ionising radiation can cause harmful effects. Of course, we are using ionising radiation because it is useful, for example in medicine for diagnostics or treatment. Every use of ionising radiation must be justified [1][2]. The principle of justification is that no practice involving exposure to radiation should be adopted unless it produces sufficient benefit to the exposed individuals or to society to offset the radiation detriment it causes. It means that the benefits of the use of ionising radiation must be greater than the harm caused by it. In order to prevent unjustified uses of radiation, the radiation practice must by authorised by a competent authority. In the process of authorisation, the licensee should prove that the use of ionising radiation has benefits that outcome the risk due to exposure. In that process, all the aspects should be taken into account: doses to the workers, patients, impact to the environment, benefits and any other social or economic factors. The responsible party must reassess the justification for practices that have already begun whenever new information becomes available that could affect the justification. The justification must also be reassessed whenever suitable new alternative methods for achieving the same objective become available that does not involve exposure to ionising radiation. If a practice ceases to yield adequate benefits in relation to its drawbacks, its continuation is no longer justified.

Optimisation

In all exposure situations, radiation protection shall be optimised with the aim of keeping the magnitude and likelihood of exposure and the number of individuals exposed as low as reasonably achievable, taking into account economic and societal factors.

Whenever ionising radiation is used, there are workers that work with the ionising radiation source. They are exposed to ionising radiation, and since it is dangerous, the doses to the workers must be kept low. The question is how low the doses must be. No dose to workers at all? Every unnecessary dose must be prevented. In the majority of cases, gamma and X-ray sources are used. The nature of gamma and X-ray radiation means that we can never totally stop all the gamma or X-rays. Whatever we do to absorb them, some of them will penetrate and cause doses to workers or to the public. So, it is impossible to have radiation workers that will receive no dose. This does not mean that no effort is necessary to minimise doses. Optimisation means that doses of exposed workers must be kept as low as reasonably achievable using all the measures to control exposures, shielding, etc.

Dose limits

If the use of ionising radiation sources is justified and optimised, the final step to protect workers is the limitation of the doses. Every country should have legislation where these limits are defined. The limits are established in numerous epidemiological studies of survivors of Hiroshima and Nagasaki atomic bomb explosions, from many accidents with ionising radiation sources and from studies of the large cohorts of workers in the nuclear industry. The dose limits are defined in publications of the International Commission on Radiological Protection (ICRP) where the most eminent scientists from the radiation protection field are members. The last recommendations on dose limits were published in 2007 in ICRP publication 103: The 2007 Recommendations of the International Commission on Radiological Protection [3][4]. The most important dose limit is the annual dose limit of 20 mSv. It means that a worker can receive a dose of 20 mSv per year from ionising sources they are working with. To have a picture of what that dose is, we will use a comparison with natural background radiation. All over the world, there is natural background radiation due to radioactivity in soil, water, air, food, etc. Since we live in such a radioactive environment, every person on our planet receives annual doses of natural background. Of course, the natural background is not the same all over the world. There are some places where there is a really high natural background but on average the annual dose is around 2 mSv. So a worker using ionising radiation sources can receive ten times the dose of the natural background at the workplace.

For women, there are special limitations during pregnancy or breast feeding. Pregnant woman can work in a radiation area but the dose to the foetus must be below 1 mSv during pregnancy. Breast feeding woman can work in a radiation area when only exposure to external radiation is possible (X-ray devices or encapsulated radioactivity sources). In that case, the limit of 20 mSv per year applies. A breastfeeding mother is not allowed to work in an area where contamination and intake of radioactivity is possible. As soon as a breastfeeding woman informs the undertaking of her condition, she shall not be employed in work involving a significant risk of bodily radioactive contamination.

The dose limits for apprentices aged 18 years or over and students aged 18 years or over who, in the course of their studies, are obliged to use sources shall be the same as the dose limits for exposed workers. The limit for the effective dose for apprentices aged between 16 and 18 years and for students aged between 16 and 18 years who, in the course of their studies, are obliged to use sources shall be 6 mSv per year.

All organs and tissues are not equally sensitive to ionising radiation. Some tissues are more sensitive than others. Also, during the working process, only specific organs or tissues can be exposed to radiation and not the whole body. Due to these facts, the doses to the skin and eye lens are different. The annual skin dose is limited to 500 mSv and to the eye lens to 20 mSv.

Besides dose limits, in order to optimise radiation protection, dose constraints are also used. Dose constraints are doses that shall not be exceeded during the particular practice with the source. But dose constraints are not the dose limits. They are selected at some fraction of the dose limit and are based on good practice and on what can reasonably be achieved.

In order to establish what was the dose was that the worker received, the dose must be measured. Unfortunately, humans do not feel the radiation, i.e. there is no feeling of warmth, cold, pain or something similar when we are exposed to radiation. Therefore, we need instruments to measure the dose. Personal dose is measured using by the personal dosimeters (Figure 2). The worker wears it somewhere between their waist and neck during the whole working time. After a defined period of time, usually one month, the personal dosimeter is sent for reading to an authorised service. The report on received doses is send to the employee and to the competent regulatory authority. The most frequent dosimetry systems use thermoluminescent dosimeters. If these dosimeters are exposed to radiation, the material comes into a higher energy state. When such dosimeter is later heated, it emits light and returns to its previous state. The quantity of emitted light is connected with the dose the dosimeter (worker) received. Older dosimetry systems used films where the blackness is connected with the radiation dose. Some new systems use optically stimulated luminescence, where light is used instead of heat to return to the original state.

The mentioned dosimetry systems are passive. It means that a worker does not know what their dose is until the report comes. In some situations where workers work in high radiation areas, it is of utmost importance that workers knows their dose at every moment and can leave the area in case the dose approaches the predefined limit. Such workplaces can be found in the nuclear industry and in some therapy procedures in medicine. In that case, the worker, besides a passive dosimeter, wears an active dosimeter. Active dosimeters are so called electronic dosimeters that use semiconductors as detector material.

Figure 2: Panasonic personal dosimetry system
Figure2 GO.jpg
Source: Provided by the author

Shielding and protection

To keep radiation doses low, three methods are used: time, distance and shielding. The dose is proportional to the time of exposure. This means that if someone is exposed for two hours, the dose would be two times the dose compared to if the exposure was one hour. The radiation reduces with the distance from the source. If the distance is increased from 1 m to 2 m, the dose will be reduced by a factor of 4. If the distance is increased from 1 m to 3 m, the dose will be reduced by a factor of 9. We say that radiation is reduced by the square law by distance. Whenever necessary, we can reduce doses through the use of shields. Different shielding material is used depending on the nature of the ionising radiation. The most common material is lead due to its high density and convenient price.

Only in some very rare cases, we can achieve that workers are not exposed. Basic radiation protection principles are justification, optimisation and dose limitation. The principle of dose limits is not applied in medical exposures. When we use radiation in medicine, we primarily search for a disease or treat diseases that can be in some cases be fatal. Still, we are not allowed to expose patients to high doses, but they must be kept below so called reference levels. Radiation protection has improved over the last 20 years, and today doses to workers are normally low.

Time: The more time one is exposed to ionising radiation, the larger the dose that will be received and the more harmful the radiation will be. The relationship is linear: doubling the exposure time doubles the dose that is received.. It is very important that we minimise the exposure time in order to minimise the dose.

Distance: The second very efficient way of minimising the doses is increasing distance. The nature of ionising radiation is such that there is an inverse square law relationship between dose and distance. If we increase the distance from the source from one metre to two metres, the dose will decrease by a factor of four. If the distance is increased from one metre to three metres, the dose will decrease by a factor of nine. So whenever possible, we must be as far as possible from the source. Unfortunately, this is not always possible.

Shielding: There are activities that require workers to be close to the source and in a high radiation field. In that case, we minimise the doses by using shielding and protective clothing. When working with X-ray devices in medicine, the most common personal protective clothing is lead aprons. Led aprons made of 0.25 mm thick lead attenuate X-rays more than 100 times. In some cases when eyes are exposed, spectacles made of lead glass are used as protection. Also, lead gloves can be used, however such gloves are quite thick and not appropriate for detailed work.

Personal protection such as gloves is intended for protection against external radiation. That means that the ionising radiation source is outside the human body and radiation is coming from that source to the body. In case of internal irradiation, the person has an ionising radiation source in the body. When such a source is in the body, no protective clothing will help. So it is very important that we prevent ingestion or inhalation of radioactive material. This is especially important at workplaces were workers work with radioactive material in liquid, powder or gaseous form. In such workplaces, there is a possibility of skin contamination of workers or even of inhalation or ingestion of radioactive material. Suitable Personal protective clothing in these cases includes latex gloves, coveralls, gas masks, shoes covers, etc. When working in very contaminated areas, workers must wear even special overpressure suits.

Besides radiation protection of workers, it is important to protect the general public also. This is why the rooms where ionising radiation is used are usually shielded by thick concrete walls and doors with lead foil inside, have no windows and in some cases are arranged as a labyrinth. Such shielding of rooms is called structural shielding.

Doses to workers

The applications of ionising radiation in industry are varied. It may be that workers use very dangerous sources but receive low doses. On the other hand, there are practices where sources are not so dangerous but their use is simple and exposures are high. The practices where workers receive the highest doses are industrial radiography, where annual doses between 5 and 10 mSv are not uncommon. In medicine, the practice that requires special care is interventional cardiology. During an interventional cardiology procedure, the doctor inserts a catheter into a vein. The procedure has a long duration and the doctor has to use X-ray many times to see how it proceeds in the patient body. The whole medical team can be exposed to high doses. The annual doses to the team members are a few mSv. But of even greater concern are doses to the eye lens. Among cardiologists performing interventional cardiology procedures, a big increase in the incidence of cataracts has been observed [5]. In the nuclear industry, workers receive doses mainly during maintenance works during outages. A dose of a few mSv is not unusual and in some cases workers receive doses close to the regulatory limit.

The vast majority of workers will receive low doses. More than 80% of all doses are well below 0.5 mSv. The average annual worker dose is between 0.1 and 0.2 mSv if the nuclear industry is not taken into account and is approximately ten times lower than is the annual dose of the natural background.

For the purposes of monitoring and surveillance, a distinction shall be made between two categories of exposed workers:

  • category A: those exposed workers who are liable to receive an effective dose greater than 6 mSv per year or an equivalent dose greater than 3/10

of the dose limits for the lens of the eye, skin and extremities

  • category B: those exposed workers who are not classified as exposed category A workers.

Risk assessment

Prior to any practice with ionising radiation sources, a risk assessment should be done. In this document, the practice with ionising radiation sources must be described in detail. Sources and the radiation they emit must be described. The dose rates in the room with the source must be estimated as well as in adjacent rooms or outside the building if this is applicable. The working areas must be designated as controlled or supervised.

The time workers work in the radiation area, their location and distance from the source and protective measures should be explained. From that data, the workers’ doses should be estimated. From the dose they receive, the workers should be categorised as A or B category workers. The requirements for personal dosimetry or area monitoring must be described. In the case of work with radioactive liquids, powder or gas, the likelihood of contamination must be assessed as well as the possibility of spread of contamination. If there are previous results of personal dosimetry or area monitoring data, this must be included in the risk assessment. The doses to the public due to the practice must also be estimated.

Protective clothing needed must be prescribed as well as structural shielding (wall thickness, lead in doors, windows etc.) calculated and described.

Potential emergencies should be explained and procedures must be written in case such incidents happen. It must be known what actions should be taken to minimise doses to workers, the public or the impact on the environment. It must be known who to call to remove potential contamination as well as what to do in case of fire or in case the source is lost. For every specific practice, all potential incidents must be carefully examined.

The principle of optimisation must be included in the document. It must be clear how the dose records will be maintained as well as the results of area monitoring. The dose constraints must be stated in a clear and understandable manner, so worker doses and the dose constraints can easily be compared. In case the dose exceeds the dose constraint, it must be known what the further steps are such as temporarily stopping the practice and investigation of why it happened.

In the risk assessment, special requirements for the workers must be stated such as adequate training in radiation protection and health examination.

Conclusions

Ionising radiation is radiation that can ionise atoms into positively charged ions and negatively charged electrons. We use ionising radiation in medicine for diagnostics and treatment, in industry for level or density measurements or non-destructive testing, in research for labelling, microscopy, etc. and in many other applications. Ionising radiation can also ionise atoms in human cells. These cause deterministic and stochastic damage to human bodies. Deterministic effects appear only if the dose is above the threshold, usually above 1000 mSv. Stochastic effects may occur and the probability increases with higher doses. But we must be aware that they can occur even at small doses. A typical stochastic effect is cancer.

The basic principles are justification, optimisation and dose limits. Justification means that ionising radiation sources can only be used if the benefit from the use is larger than the harm caused by the use. Workers working with ionising radiation sources will receive a radiation dose. We have to keep the worker’s dose as low as reasonably achievable or optimised. This means that we will use all available measures in a reasonable way to keep the doses low. Dose limits are the third step in radiation protection. When the use of ionising sources is justified and optimised, we still have to take care that workers’ doses do not exceed the regulatory limit. In most countries, the dose limit is the annual dose of 20 mSv, which is approximately ten times the average natural worldwide background.

To minimise their doses, workers must wear protective clothing. In diagnostic medicine, the most common types of protective clothing are lead aprons and spectacles made of lead glass. Workers working with radioactive liquids, powders and gases must prevent contamination of their skin or inhalation and ingestion of radioactive material. The protective clothing for these workers includes cotton or plastic overalls, latex gloves, shoe covers and gas masks.

In recent times, radiation protection has been improved. The basis for our understanding of radiation risks has been based on information from population studies following nuclear accidents. We can say that today workers with ionising radiation receive small doses that are on average about 10% of the dose of the natural background.

References

  1. The 2007 Recommendations of the International Commission on Radiological Protection, ICRP Publication 103. Ann. ICRP 37 (2-4), Elsevier Ltd, 2007
  2. IAEA - International Atomic Energy Agency, Radiation Protection and Safety of Radiation Sources: International Basic Safety Standards, General Safety Requirements Part 3, Vienna, 2011.
  3. The 2007 Recommendations of the International Commission on Radiological Protection, ICRP Publication 103. Ann. ICRP 37 (2-4), Elsevier Ltd, 2007
  4. IAEA - International Atomic Energy Agency, Radiation Protection and Safety of Radiation Sources: International Basic Safety Standards, General Safety Requirements Part 3, Vienna, 2011.
  5. IAEA - International Atomic Energy Agency, IAEA Cataract Study, 2012. Available from: [1]

Links for further reading

European Commission, Directorate General for Energy, Nuclear Safety and Fuel Cycle, Radiation Protection (no publishing date), Nuclear energy – Radiation protection. Retrieved on 25 February 2013 from: [2]

Radiation Protection and Safety of Radiation Sources: International Basic Safety Standard, Interim edition, General Safety Requirements Part 3, No. GSR Part 3 (interim), International Atomic Energy Agency, Vienna, 2011

UNSCEAR - United Nations Scientific Committee on the Effects of Atomic Radiation (2013), 50+ years of service. Retrieved on 25 February 2013 from: [3]

Council Directive 96/29/EURATOM of 13 May 1996 laying down basic safety standards for the protection of the health of workers and the general public against dangers arising from ionising radiation. Available at: [4]

IRPA - International Radiation Protection Association (no publishing date), home page. Retrieved on 25 February 2013 from: [5]