Radiation in the environment : Types of Ionizing radiation

Types of Ionizing radiation
Introduction: Ionizing radiation enters our lives in a variety of ways. It arises from natural processes, such as the decay of uranium in the Earth, and from artificial procedures like the use of X rays in medicine. So we can classify radiation as natural or artificial according to its origin. Natural sources include cosmic rays, gamma rays from the Earth, radon decay products in the air, and various radionuclides found naturally in food and drink. Artificial sources include medical X rays, fallout from the testing of nuclear weapons in the atmosphere, discharges of radioactive waste from the nuclear industry, industrial gamma rays, and miscellaneous items such as consumer products.
Characteristics: Each source of radiation has two important characteristics, the dose that it delivers to human beings and the ease with which we can do something to affect such doses. Until recently, radiation from natural sources seemed both unremarkable and unalter­able - a background phenomenon. We now know, however, that doses from the decay products of radon gas (itself a product of uranium decay) in the home can be remarkably high in some areas, although it is fairly easy to reduce them in existing homes and to avoid high concentrations of the gas when building new homes. In contrast, we cannot do much to change our exposure to the other natural sources of radiation. This basic background of cosmic rays, gamma rays, and natural radioactivity within the body gives rise to an annual dose of about 1 mSv or more to an average citizen of the world. A comparable dose (at least) from radon decay products is also unavoidable in practice for most people.
It is easier, in most cases, to control artificial sources of radiation because we can alter or terminate the procedure producing the radiation, but there is always a balance to be made. It is important, for instance, to pay attention to the doses from medical X ray examinations, but it would be unwise to reduce them where this would lead to a loss of essential diagnostic information.
Dose Assessment: The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) was established In 1955 to estimate the potential health risks from radioactive fallout from atmospheric nuclear weapons tests. Today, UNSCEAR regularly publishes data on doses from all sources. The results of the latest review, published in 2000, are reflected in the pie chart . The annual dose, averaged over the population of the world, is about 2.8 mSv in total. Over 85 % of this total is from natural sources with about half coming from radon decay products in the home.
	Medical exposure of patients accounts for 14 % of the total, whereas all other artificial sources - fallout, consumer products, occupational exposure, and discharges from the nuclear industry - account for less than 1 % of the total value.
The greatest variations in dose arise from radon decay products in the home, which can give annual doses of 10 mSv or more. Annual doses for those exposed to radiation at work are, at present, limited by law in most countries to 50 mSv or less, but only a small fraction of the workforce exceeds 20 mSv. It is unlikely that many members of the public receive more than a fraction of 1 mSv in a year from incidental exposure to artificial sources. Doses to patients in some diagnostic procedures may be around 10 mSv. For consumer products that contain radioactive material, such as smoke alarms and luminous watches, annual doses are at most 1 µSv (1 millionth of a sievert), although less common items, such as gas mantles containing thorium, may cause as much as 0.1 mSv in a year in certain circumstances.
Source Dose (mSv) Natural Cosmic 0.4 Gamma rays 0.5 Internal 0.3 Radon 1.2 Artificial Medical 0.4 Atmospheric nuclear testing 0.005 Chernobyl 0.002 Nuclear Power 0.0002 Total (rounded) mSv 2.8 Table (8): Annual effective doses from natural radiation ( UNSCEAR report 2000 )
1- Natural radiation
Natural ionizing radiation pervades the whole environment. Cosmic rays reach the Earth from outer space. The Earth itself is radioactive. Natural activity is present in food and drink and in the air. We are all exposed to natural radiation to a greater or lesser extent, and for most people it is the major source of radiation exposure. Nevertheless, humans, animals and plants have evolved in this background of natural radiation, and the general view is that it is not a significant risk to health - but there are exceptions.
a) Cosmic radiation Cosmic rays are mainly protons of uncertain origin in space and very high energies that reach our atmosphere in fairly constant numbers. It is known, however, that some protons with lower energies come from the sun and are given off in bursts during solar flares. Protons are charged particles, so the number entering the atmosphere is affected by the Earth's magnetic field - more come in near the poles than the equator- so the dose rate increases with latitude. As they penetrate the atmosphere, the cosmic rays initiate complex reactions and are gradually absorbed so that the dose rate decreases as altitude decreases. Cosmic radiation is a mixture of many different types of radiation, including protons, alpha particles. electrons and other various exotic (high energy) particles. At ground level, cosmic radiation is primarily muons, neutrons, electrons, positrons and photons, and most of the dose comes from muons and electrons. UNSCEAR has calculated that the annual effective dose from cosmic rays at ground level is about 0.4 mSv, on average, allowing for variations in altitude and latitude.
The type of building in which a person lives may also affect the dose from cosmic rays to a slight degree. The intensity of cosmic rays at altitudes where aircraft fly is much greater than on the ground. At cruising altitude on an intercontinental flight, the dose rate can reach 100 times that on the ground. General air travel gives rise to a further annual dose of 0.01 mSv on average to some populations (the doses to some. individual 'frequent fliers' will be much higher than this aver­age), but this does not affect the world average of 0.4 mSv.
b) Gamma radiation All materials in the Earth's crust contain radionuclides. Indeed, energy from natural activity deep in the Earth contributes to the shaping of the crust and the mainte­nance of internal temperatures. This energy comes mainly from the decay of the radio­active isotopes of uranium, thorium and potassium.
Uranium is dispersed throughout rocks and soils in low concentrations of a few parts per million (ppm). Where it exceeds 1000 ppm or so in an ore, it may be economical to mine it for use in nuclear reactors. Uranium-238 is the parent of a long series of radionuclides of several elements, which decay in suc­cession until the stable nuclide lead-206 is reached.
	Among the decay products in the series is an isotope of the radioactive gas radon, namely radon-222, which can reach the atmosphere, where it continues to decay. Thorium is similarly dispersed in the ground. Thorium-232 is the parent of another radioactive series, which gives rise to radon-220, another isotope of radon, sometimes called thoron. Potassium is far more common than either uranium or thorium and makes up 2.4 per cent by weight of the Earth's crust. The radionuclide potassium-40, however, constitutes only 120 ppm of stable potassium.
The radionuclides in the ground emit penetrating gamma rays that irradiate us more or less uniformly. Since most building materials are extracted from the Earth, they too are mildly radioactive, and people are irradiated indoors as well as out of doors. The doses they receive are affected both by the geology of the area where they live and the structure of the buildings in which they live, but the average effective dose from natural gamma rays is about 0.5 mSv in a year. Actual values vary appreciably. Some people may receive doses a few times higher or lower than the average. In a few places where the ground naturally contains relatively high concentrations of radionuclides, such as Kerala in India and parts of France and Brazil, the dose- can be up to 20 times the global average. Although in general there is little that can be done to affect this dose, it would be sensible where possible to avoid building in locations or with materials with unusually high activity.
c) Radon inhalation Radon gas is a particularly significant source of exposure to natural radiation. This is because the immediate decay products of radon-222 are radionuclides with short half-lives, which attach themselves to fine particles in the air, are inhaled, irradiate the tissues of the lung with alpha particles, and increase the risk of lung cancer. The same is true of radon-220 (thoron), but the degree of exposure of the lung is much less. When radon gas enters the atmosphere from the ground, it disperses in the air, so concentrations out of doors are Iow. When the gas enters a building, predominantly through the floor from the ground, the concentration of activity builds up within the enclosed space.
If buildings are well ventilated this accumulation of radon will not be marked. How­ever, in many - generally colder - countries, buildings are constructed with more emphasis on retaining heat and preventing draughts. They are, therefore, often poorly ventilated, and radon concentrations indoors can be many times higher than those outdoors. Radon con­centrations in buildings are also very dependent on the local geology and can vary a great deal between different parts of a country and even from building to building in the same area.
The worldwide average annual effective dose from the decay products of radon is esti­mated to be about 1.2 mSv. There are, however, pronounced variations about this value. In' some countries (e.g. Finland) the national average is several times higher, and in particular homes in many countries occupants have received effective doses of the order of hundreds of mSv in a year. Given this, ICRP and IAEA have recommended the use of Action Levels (expressed in Bq m-3 ) above which householders are advised to reduce radon levels in their homes. Typically these Action Levels should be in the range 200-600 Bq m-3, which is about ten times the average value for the radon concentration in homes.
	Anyone finding high radon levels in their homes can reduce it by preventing air from the ground entering the building. The most effective way to do so is to reduce the air pressure under the house with a small fan. This circumstance is an example of intervention, in the ICRP sense, to reduce human exposure to ionizing radiation.
d) Internal irradiation Other radionuclides from the uranium and thorium series, in particular lead-210 and polonium-210, are present in air, food, and water, and so irradiate the body internally. Potassium-40 also comes into the body with the normal diet. It is the main source of internal irradiation apart from the radon decay products. In addition, the interactions of cosmic rays with the atmosphere create a number of radionuclides, such as carbon-14, which also contribute to internal irradiation.
	The average effective dose from these sources of internal irradiation is estimated to be 0.3 mSv in a year, with potassium-40 contributing about half. Information on how the total varies from one person to another is limited, although it is known that the potas­sium content of the human body is controlled by biological processes. The amount of potassium, and hence potassium-40, varies with the amount of muscle in the body, and is about twice as high in young men as in older women. There is little anyone could do to affect internal irradiation from the other radionuclides except by avoiding any food and water with a high radioactive content.
The total average effective dose from natural radiation is about 2.4 mSv in a year, but doses can vary a great deal. Some national averages exceed 10 mSv in a year, and in some regions individual doses may exceed 100 mSv in a year, usually because of homes with particularly high levels of radon and its decay products. Occupational exposure to enhanced natural sources of radiation occurs mainly in mines, buildings and aircraft. Almost 4 million coal miners are monitored for radiation exposure. Fewer people (about a million worldwide) work in mines other than coal mines and in the processing of ores with levels of natural activity appreciably above average. The doses incurred are, nevertheless, monitored routinely. Radon levels - and doses - are low in coal mines because the ventilation is usually good. Few if any miners exceed 15 mSv in a year. The state of ventilation in metal and other mines is not always as satisfactory, so the average dose is much higher and a fraction of the workforce does exceed this dose. About one-fifth of the people considered to be occupationally exposed to enhanced natural radiation work in shops, offices, schools, and other premises in radon-prone areas. Within these areas, the average dose is appreciable. The average dose for such workers is almost 5 mSv per Year - higher than for the other groups of occupation­ally exposed workers. However, it should be remembered that this group is unusual in that its members are identified, precisely because they receive high doses, rather than because they have the same occupation. Radon levels vary markedly from day to day because of the way buildings are heated and ventilated, so short measurements of radon in air may be misleading. The best remedy for high radon levels is the same as in houses - reduced air pressure under the floor. Doses to aircrew from cosmic rays depend on the routes flown and the amount of flying time. On average, the annual dose is around 3 mSv, but it could be twice as much for long flights continually at high altitudes. By the nature of the radiation and the operations, such doses are unavoidable.
2- Artificial source of radiation
Artificial sources of radiation are commonly used in the manufacturing and service industries ( Radiography of welds and joints , Level gauging of container contents , Static elimination in paper production , Analysis of specimens for quality control ) , in areas of defense ( Security inspection of bags and parcels ) in research institutions, and in universities, as well as in the nuclear power industry. Moreover, they are extensively used by physi­cians and health professionals (Sterilization of some medical supplies , nuclear medicine ) . Exposure to ionizing radiation occurs in many occupations from the different sources .
	There are about 800 000 work­ers in the nuclear industry worldwide, and over 2 million workers exposed in medical facilities. UNSCEAR has com­piled data on doses received by these workers and others such as industrial radiogra­phers. The collective dose to nuclear industry workers is about 1400 man Sv, while that for medical radiation workers is about 800 man Sv.
There are fewer workers in industrial uses of radiation, therefore the collective dose is lower at about 400 man Sv. However, these workers get the high­est individual doses in some countries. The average dose overall to occupationally exposed work­ers from artificial sources is less than 1 mSv in a year. The average in the nuclear industry tends to be a little higher than this, while the average for med­ical staff is slightly less. Doses have declined steeply in the last decade primarily because of the widespread introduction of ICRP recommendations and the BSS. With the exception of mining, average doses from most types of occupational exposure from artificial sources, including the nuclear industry, are now below about 2 mSv in a year. Doses in the health professions - medical, dental and veterinary - are generally very low, but there are still matters of concern. Some clinical procedures with diagnostic radio­logy require the physician to be close to the patient and at risk of appreciable exposure. X ray equipment and procedures in veterinary practices are frequently inadequate.
Source Dose (mSv) Artificial sources Nuclear industry Uranium mining 4.5 Uranium milling 3.3 Enrichment 0.1 Fuel fabrication 1.0 Nuclear reactors 1.4 Reprocessing 1.5 Medical uses Radiology 0.5 Dentistry 0.06 Nuclear medicine 0.8 Radiotherapy 0.6 Industrial sources Irradiation 0.1 Radiography 1.6 Isotope production 1.9 Well-logging 0.4 Accelerators 0.8 Natural sources Radon sources Coal mines 0.7 Metal mines 2.7 Premises above ground (radon) 4.8 Cosmic sources Civil aircrew 3.0 Table (9) : Average annual effective doses in different occupations (UNSCEAR Data for 1990-1994)