N A T U R A L R A D I A T I O N

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N A T U R A L R A D I A T I O N

  1. 1. NATURAL RADIATION Introduction In the previous section, we discussed the deleterious effects that radiation can have on human beings (and indeed animals). This rather gives the impression that one should strive at all costs to avoid any sort of exposure to radiation! However, this is quite impossible, as radiation is all around us, and we are continually bombarded with radioactive particles. The level of radiation exposure from natural sources occurs at the level of stochastic effects, discussed in the previous section, and the principal sources are as follows:1. Cosmic rays;2. Naturally occurring radionuclides in soils and rock;3. Naturally occurring radionuclides in our bodies;4. Radon gas in our homes We will discuss each of these in turn. Cosmic Rays Cosmic rays are a source of radiation that originates from outer space. In fact there are two sources; the first is the sun, with the remainder originating from outside the solar system. Primary cosmic rays (those that are incident on the atmosphere of the earth) consist mainly of protons (hydrogen nuclei), and alpha particles (helium nuclei). Also present are electrons and a small number of heavier nuclei. The energy of the primary cosmic rays can be very high, and when these cosmic rays interact with the atoms and molecules in the atmosphere, a number of additional secondary particles are produced, including protons, electrons, neutrons, positrons and a variety of more ‘exotic’ particles that will not be discussed here. Many of these secondary cosmic rays are sufficiently energetic to reach the surface of the earth. At sea level, it has been found that the number of ions produced by cosmic rays is about 2 ions per cubic centimeter per second. Using this information, it can be calculated that the absorbed dose rate due to these cosmic rays is about 32 nGy/hr (0.000000032 Gy/hr). The world average annual dose rate due to cosmic rays is about 0.4 mSv/yr (0.0004 Sv/yr). It is of interest to note that the dose rate due to cosmic rays increases with altitude, because the flux of cosmic ray particles increases (they have less distance to travel through the earth’s atmosphere). Thus, regular travellers
  2. 2. by airplane, and indeed the pilots of those planes, will be subject toconsiderably higher radiation doses. The dose rate doubles for every 1500m increase in altitude, and so at a cruising height of 10000 m, the doserate could exceed 32 nGy/hr × 2(10/1.5) ~ 3.2 µGy/hr. This is 100 timesgreater than the dose rate at sea level.Supersonic aircraft often fly at much higher altitudes, perhaps as high as15 km. The dose rate at this altitude could be as high as 210 × 32 nGy/hr =33 µGy/hr. It is reassuring to note that supersonic aircraft carry monitoringequipment to measure the cosmic ray dose rate, so that during periods ofintense cosmic radiation (for example during solar flares – the primarysource of solar cosmic radiation), the pilot can adopt a lower flying height,resulting in a reduction of the radiation dose to the passengers and crew.Radionuclides in Soils and RockA number of radioactive materials occur naturally in the earth itself. Theseradioactive materials all have very long half lives, and have been present inthe earth since its creation. At the creation of the earth during the BigBang, perhaps about 15 billion years ago, virtually all possible atoms werecreated. Most of the radioactive atoms will have decayed away, but thereare three important radionuclides that have such long half lives that evenafter 15 billion years, they are still present in the earth. Theseradionuclides are: Uranium-238 half life = 4.46 billion years; Thorium-232 half life = 14 billion years Potassium-40 half life = 1.3 billion yearsOther primordial radionuclides include rubidium-87 and uranium-235, butthese are not so important in considering radiation doses to man.Uranium-235 is an important source of fuel for many types of nuclearreactor, and the extraction of concentrated amounts of uranium-235 fromnatural uranium in the ground is a major technological challenge.Uranium-238 and thorium-232 have the important property that theirradioactive decay products are also radioactive. Both of these nuclides areat the beginning of long sequences of radionuclides (containing about 10 or15 members) in which each member is radioactive, and decays to the nextmember in the series. Only the final member of the series consists ofstable atoms. Uranium-235 is also at the head of a long sequence ofradionuclides.The following diagram illustrates these two radionuclides and theradioactive progeny that arise from their decay.
  3. 3. It follows that any of the members of the decay series mentioned abovecan give rise to a radiation dose to man. However, the nature of theradiation dose is usually considered in two categories, namely externaldoses, and doses that arise internally, for example because the nuclideshave been inhaled or ingested. It turns out that the external doses fromthese natural nuclides are dominated by bismuth-214, which is a stronggamma emitter and occurs in the uranium-238 series, and thallium-208,which is also a gamma emitter and occurs in the thorium-232 series. Theexternal dose, averaged over the world population, is about 0.5 mSv/yr.The internal doses arise when radioactive materials are ingested orinhaled. In this case, the nuclides that emit alpha particles assume agreater importance (for example radium-226 in the uranium-238 series).The dose arising from internal contamination (averaged over the worldpopulation) is about 0.3 mSv/yr.Radionuclides in the BodyAn important radionuclide that can be found in the human body ispotassium-40 (it is also found in soils and rocks, and contributes to thedoses considered in the previous section). The human body of a 70 kgman contains about 140 g of potassium, most of which is located inmuscle. Of this potassium, about 0.018% is radioactive potassium-40.This corresponds to a mass of potassium-40 of about 0.03 g. It can beshown by calculation that the decay of this potassium-40 produces about6000 radioactive decays every second, and therefore body tissues willacquire a radiation dose. The dose arising from potassium-40 in the body(and a contribution due to naturally occurring potassium-40 in rocks andsoils) is about 0.1 mSv/yr.Radon in HomesOne of the most important sources of natural radiation arises from anelement called radon. The two most important isotopes of radon areradon-220 and radon-222. The former is often called thoron and the latteris usually given the name of the element, i.e. radon. To avoid confusion,we will reserve the term radon for the name of the element, and we willrefer to each isotope explicitly.
  4. 4. Radon has two important qualities from the perspective of radiationprotection:1. Radon is a gas;2. Both isotopes of radon are radioactive and are derived either fromthe decay of uranium-238 (radon-222) or thorium-232 (radon-220).Thus, because uranium-238 and thorium-232 occur to some extent in allsoils and rocks, it follows that radon gas is constantly emanating from theground. In areas that are particularly rich in uranium and/or thorium ores,the rate of emanation can be quite high. However, the radiological hazardfrom radon-220 (thoron) is usually smaller than that from radon-222,because radon-220 has a half-life of only 55 seconds, and so decays awaybefore radiological problems can arise. Radon is colourless and odourless,so we cannot perceive the presence of radon in the atmosphere. However,in most circumstances, only radon-222 is problematic.Of particular importance is the fact radon can accumulate in enclosedstructures such as homes and offices. The radon either seeps intobuildings through the foundations, or in parts of the world where buildingmaterials are derived from local sources, the radon may enter the buildingas a result of emanation from the foundation and building materialsthemselves. If the degree of ventilation of the building is negligible, radoncan accumulate to substantial levels. The radiological hazard isexacerbated by the fact that buildings such as homes tend to be occupiedfor substantial periods of time.Strictly speaking, radon-222 itself is not particularly dangerous, as radon isan inert gas, and so radon breathed in tends to be breathed straight backout again. However, the decay products of radon-222 are isotopes ofbismuth and polonium, which are ‘heavy’ metals that attach themselves todust particles, and tend to accumulate in the lung. These isotopes thenundergo further radioactive decay, and bombard the lungs with alphaparticles, resulting in radiological damage to the cells of the lung. Thus,lung cancer is a possible outcome from exposure to radon. It should benoted that cigarette smoking tends to increase the likelihood of lung cancerin regions where radon levels are high.The hazard of radon is taken so seriously that action levels are defined inmost countries. These action levels specify the maximum concentration ofradon and its decay products that is allowed before intervention is requiredto lower concentrations. The most common type of intervention is the useof fans under the floor of an affected building to ‘suck’ radon out of theunderfloor region, before it can enter the main volume of the building.
  5. 5. Ionising Radiation - The EndAt this point, it was my intention to describe some of the more practicaluses of radiation. However, it became clear to me that (1) there are somany uses that I could not do justice to them all; (2) I may never actuallyfinish writing this article if I started out to do that, and; (3) people’s viewsdiffer on what are acceptable, unacceptable or controversial uses.There are a multitude of stories (both happy and sad) about how people’slives have been changed as a result of radiation. But that’s for anothertime!

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