Ionizing radiation is emitted by radioactive substances, and it can be produced electronically by X-ray machines, for example. The properties and effects of ionizing radiation are difficult to perceive on the basis of everyday experiences, because human senses cannot detect radiation.
The basic component of any matter is the atom. The nucleus of the atom consists of protons and neutrons. The number of protons in a given element is always the same. The number of neutrons may vary, meaning that the element has different isotopes.
The nucleus of an atom may be in an excited state. Such a nucleus often has too many or too few neutrons. Substances containing excited nuclei are radioactive. Almost every element has both stable and radioactive isotopes. The isotope is indicated by the mass number following the abbreviation of the substance, as in Sr-90. The mass number is the sum of the protons and neutrons in the nucleus. Sooner or later, the nucleus de-excites, releasing a particle and energy from the nucleus. This is when the substance radiates. The original atom, the nuclide, is called the parent nuclide, and the resulting new nuclide is called the daughter nuclide.
Radioactive isotopes behave in nature in the same way as the stable isotopes of the same substance. For example, both travel through natural food chains and the body in the same way.
Ionizing radiation has enough energy to remove electrons from a substance exposed to radiation or to break molecules. Radioactive substances emit ionizing radiation. In addition, ionizing radiation is produced by X-ray equipment, for example.
Alpha, beta and gamma radiation
Alpha and beta radiation are particle radiation. An alpha or beta particle leaves the nucleus at high speed. An alpha particle consists of two protons and two neutrons. Alpha decay is common in heavy nuclides. Natural uranium and thorium are alpha emitters. Beta particles can be electrons or positrons. Electrons are negatively charged and positrons are positively charged. Examples of beta emitters include cesium-137, iodine-131 and strontium-90.
Alpha particles are heavier than beta particles. An alpha particle cannot penetrate human skin or a sheet of paper. Alpha radiation can only be dangerous if the radioactive substances emitting alpha radiation enter the body in, for example, respiratory air. Beta particles have a higher penetration ability, and they are able to penetrate the skin, for example. Beta emitters are dangerous on the skin or when they enter the body. The daughter nuclide resulting from alpha or beta decay is often excited, and gamma rays are generated when it de-excites.
Gamma radiation is not particle radiation. It can be described as energy packets sent by an excited nucleus. Gamma radiation is electromagnetic wave motion.
Gamma radiation is usually very penetrating. Protecting oneself against external gamma radiation is more difficult than against other forms of radiation. Depending on the energy of gamma radiation, a thick layer of concrete, steel or lead may be needed to absorb it. If the energy of gamma radiation is low, a layer of lead about one millimetre thick is enough to absorb it.
Neutrons are released, for example, as a result of spontaneous splitting (fission) of the uranium nucleus or a reaction in a neutron source. Cosmic radiation from space is also rich in neutrons, which cause the majority of the radiation dose to high-flying aircrew and passengers.
The nuclei of uranium-235, the nuclear fuel in the reactor of a nuclear power plant, split both in spontaneous fission reactions and in new fissions caused by the slow neutrons released in the fission reaction. Since several neutrons are released in each fission, this eventually leads to a chain reaction in the nuclear fuel. The fission reactions also release a lot of energy. Lots of fission products of uranium are generated in the nuclear fuel, many of which are radioactive.
Because fast-moving neutrons are dangerous to living tissue, neutron sources must be shielded properly.
X-rays are electromagnetic radiation produced by an X-ray tube. The X-ray tube is a vacuum tube with a heated cathode and an anode made of a highly heat-resistant material. A voltage of 5 to 400 kV is applied between the cathode and anode. Under the influence of the voltage, electrons released from the heated cathode move towards the anode at high speed and eventually collide with it. As the speed of the electrons decreases, some of the kinetic energy of the electrons is converted into electromagnetic radiation, which is called X-rays.
The activity of a radioactive substance indicates how many nuclear transformations occur in that amount of substance within one second.
The unit of radioactivity is the becquerel (Bq). One becquerel is defined as one nuclear transformation in the radioactive substance (de-excitation of the excited state of the nucleus). The more nuclear transformations occur, the more radiation is generated.
The becquerel is a very small unit of radiation. Because of this, the kilobecquerel (kBq), equal to 1,000 Bq, and the megabecquerel (MBq), equal to 1,000,000 Bq, are also used.
Activity is often expressed as activity per unit of weight or volume, representing activity concentration. The unit of measurement is the becquerel per litre (Bq/l), the becquerel per kilogram (Bq/kg) or the becquerel per cubic metre (Bq/m³). A radon concentration of 400 Bq/l in drinking water, for example, means that 400 radon atom decays per second take place in one litre of water.
Examples of activity
The activity of a predatory fish caught in a small lake is 1,000 becquerels. The fish weighs two kilos. Its activity concentration is 1,000 becquerels per two kilograms, or 500 becquerels per kilogram (Bq/kg). In this case, the radioactive substance concentration of the fish is 500 Bq/kg.
The radon concentration is usually significantly higher in bore well water than in ring well or water supply system water. In Finland, the average radon concentration in bore well water is approximately 500 becquerels per litre (Bq/l). When a person drinks two litres of such water per day, the activity in their body increases by 1,000 becquerels.
The body of an adult normally contains approximately 5,000 becquerels of the radioactive isotope of natural potassium, potassium-40.
The half-life of a radioactive substance refers to the time it takes for the activity of the substance to decrease by half. If the half-life of a substance is two years and the original activity is 1,000 becquerels, the activity will be 500 becquerels after two years. Another two years later, the activity will be 250 becquerels, and so on.
Half-lives of radioactive substances vary a lot. Half-lives of short-lived substances are seconds or fractions of seconds. The most long-lived substances decrease by half over millions of years. Krypton-94, for example, which is a gas, decreases by half in 1.4 seconds. Iodine-131 decreases by half in about eight days. Cesium-137 decreases by half in 30 years. The natural uranium-235 needed in the production of nuclear energy only decreases by half in 700 million years. The length of the half-life does not indicate how dangerous the substance is.
Radioactive substances usually leave the human body quicker than one might assume based on the half-life of the radionuclide in question. The amount of radioactive substance in the body decreases as the radioactive substance decays. The body’s biological functions also remove radioactive substances from the body. The physical half-life of cesium-137, for instance, is 30 years, but its biological half-life is only 3 months.
Radiation dose and dose rate
The radiation dose is a quantity describing the harmful impacts of radiation on a person. The unit of radiation dose is the sievert (Sv). Unlike the becquerel, the unit of activity, the sievert is a very large unit of measurement, and millisieverts (mSv) or microsieverts (µSv) are therefore usually used when talking about doses. One sievert equals 1,000 millisieverts, or 1,000,000 microsieverts. The radiation dose is often simply referred to as the dose.
The dose caused by an X-ray examination of the lungs, for example, is 0.07 mSv on average, and normal dental X-ray imaging generates a dose of approximately 0.01 mSv.
The external radiation dose refers to the radiation dose caused by a radiation source outside the body. The internal dose, on the other hand, refers to the dose caused by the radioactive substances found in the body. The amount of the internal radiation dose depends on the amount of the radioactive substance and its radiation characteristics. The organs or tissues where the radioactive substance travels to also have an impact on the dose.
The dose rate indicates the size of the radiation dose received by a person over a given period of time. The unit of dose rate is sieverts per hour (Sv/h). Using millisieverts or microsieverts usually makes sense, which means using the units of measurement millisievert per hour (mSv/h) or microsievert per hour (µSv/h). This means that one sievert per hour equals 1,000 millisieverts per hour, or 1,000,000 microsieverts per hour.
The average radiation dose received by a Finn from various radiation sources is approximately 5.9 mSv per year. About 4 mSv is caused by indoor air radon, and approximately 1.1 mSv by other types of natural background radiation than indoor air radon. On average, the medical use of radiation causes a Finnish person an annual effective dose of 0.76 mSv. It is estimated that the Chernobyl nuclear power plant accident and the fallout from nuclear weapon tests cause a radiation dose of approximately 0.01 mSv per year.
The dose rate is usually used to describe how dangerous it is to stay in a specific place exposed to a specific type of radiation. If the dose rate is high, even a short stay will cause a high radiation dose.
In Finland, the dose rate due to background radiation is 0.04–0.30 µSv/h.
From becquerel to sievert
If an adult digests 63,000 Bq of cesium-137 with food, this causes a radiation dose of 1 mSv for them. This relationship only applies to cesium-137, not to other radioactive substances. The average cesium-137 concentration in reindeer meat, for example, is 500 Bq/kg. A meal containing 500 grams of reindeer meat results in an internal radiation dose of approximately 0.004 mSv.
If the concentration of iodine-131 in the air is 10,000 Bq/m³, this causes a dose of 1 mSv when inhaled for approximately ten hours.
Natural radiation and artificial radiation
Radiation has always existed and will continue to exist in nature regardless of human activities. Finns receive the highest radiation dose from radon in indoor air. There is some radiation everywhere. The ground below our feet and the concrete and brick walls around us emit radiation. We are exposed to radiation from space everywhere; the exposure is higher aboard an airplane than on the ground. We also eat, drink and breathe radioactive substances.
Man-made (artificial) radioactive substances have been released into the living environment by, for example, nuclear tests carried out in the atmosphere and by the Chernobyl nuclear power plant accident.
In addition to the ionizing radiation found in nature, ionizing radiation can be generated by means of electronic devices, such as particle accelerators and X-ray machines. Particle accelerators and nuclear reactors can be used to produce radionuclides not found in nature. Such man-made radiation caused by radionuclides generated by machines is called artificial radiation.