Lecture
In everyday life, the human body is constantly exposed to ionizing radiation from various sources. Ionizing radiation consists of streams of particles and
quanta of electromagnetic radiation, whose passage through matter leads to the ionization and excitation of its atoms and molecules.
The nuclei of certain chemical elements and their isotopes possess the property of radioactivity – the ability to decay spontaneously, without external influence (undergoing radioactive decay), with the emission of ionizing radiation. Such nuclei are called radionuclides.
Sources of ionizing radiation are divided into natural and artificial. Natural sources include cosmic rays and radionuclides contained in rocks, soil, water and air (primarily radon - 222Ra and its decay products). Artificial sources include the production, transport and reprocessing of nuclear fuel, monitoring and medical instruments in which radionuclides are used and ionizing radiation is applied, radioactive fallout after atomic weapons testing, radionuclides released into the atmosphere by nuclear power plants and other nuclear facilities, as well as radiation exposure in accidents at nuclear facilities. In rail transport, the source of ionizing radiation is the transport of radioactive cargo and construction materials with an elevated level of radiation, for example, granite.
Alpha, beta and gamma radiation are distinguished. Alpha radiation is a stream of helium nuclei. Its penetrating ability in air is 7 – 10 cm, in water – up to 0,1 cm, and in biological
tissue, 0,02 – 0,04 mm. Beta radiation is a stream of electrons, whose penetrating ability in air can reach 14 m, in aluminum and plastic – 7 mm, and in biological tissue –
2,5 cm. Gamma radiation is a stream of electromagnetic radiation with a very short wavelength. Gamma rays penetrate deeply into the human body and represent a great radiation hazard.
Radioactive decay occurs at a strictly defined rate, characteristic of each given radionuclide. The time during which the initial number of radioactive nuclei decreases by half is called the half-life (T½). It can vary within wide limits. Thus, for example, the half-life of uranium 238U is 4,5 billion years, of radium 236Ra – 1620 years, of radon 222Rn – 3,8 days.
The rate of decay of a radionuclide is called activity. The becquerel (Bq) is adopted as the unit of activity in the International System of Units (SI). One becquerel corresponds to one
decay per second for any radionuclide. Activity is also expressed in a non-system unit – the curie (Ci). 1 Ci = 3,7 · 1010Bq.
The amount of ionizing radiation energy absorbed per unit mass of a substance is called dose. This quantity is used to assess the effect of ionizing radiation on any substances and living organisms. Exposure, absorbed, equivalent, effective equivalent, expected and collective doses are distinguished.
Exposure dose – a measure of the ionization of air resulting from the action of photons on it, equal to the ratio of the total electric charge dQ of ions of one sign, formed by ionizing radiation absorbed in a certain mass of air, to the mass dM. DEXP=dQ/dM. In SI, the unit of exposure dose is the coulomb per kilogram (C/kg); the non-system unit of exposure dose is the roentgen. Exposure dose characterizes the radiation situation independent of the properties of the irradiated object.
Absorbed dose – the ratio of the total energy of ionizing radiation dE absorbed by a substance to the mass of that substance dM.
DABS = dE/dM. The unit of absorbed radiation dose in the SI system is the gray (Gy). The non-system unit is the rad. There is a relationship between the units: 1 Gy = J/kg = 100 rad.
The unit of equivalent dose in the SI system is the sievert (Sv). The non-system unit of equivalent radiation dose is the rem. 1 Sv = 100 rem. In addition, multiple units are used – microsieverts, microrems, etc.
Various additional coefficients are introduced, taking into account the differing sensitivity of the tissues and organs of a living being to the action of radiation. For example, at the same equivalent dose of exposure, the occurrence of a cancerous tumor in the lungs is more likely than in the thyroid gland. By multiplying the equivalent doses by the corresponding coefficients and summing the resulting values over all organs and tissues, the effective equivalent dose is obtained. It assesses the total effect of exposure in sieverts.
The sum of the individual equivalent doses received by a group of people is called the effective collective dose, which is measured in man-sieverts (man-Sv). The collective effective equivalent dose received by many generations of people from any radioactive source is called the expected total collective effective equivalent dose.
Operational monitoring of the radiation situation and assessment of its severity are carried out based on the value of the equivalent dose rate. The dose rate is determined by the value of the equivalent dose relative to the time of its accumulation. The unit of measurement is Sv/s or rem/s.
The most common methods for detecting radioactive radiation and particles are based on their ionizing, thermal, and chemical action. First and foremost, these are chemical, colorimetric, and photographic methods. Ionization chambers, Geiger-Muller counters, and scintillation counters are used to detect ionizing radiation.
The dose rate of external gamma radiation is measured with dosimeters of the DRG-01T type. For an approximate assessment of dose rate, indicator instruments of the SRP-68 type are used. Measurement of dose rate indoors is carried out at a height of 1 m in the center of the room, and in open areas – at least 30 m from the nearest building at a height of 1 m.
If the equivalent dose rate of external radiation inside residential buildings does not exceed the dose rate in open areas by more than 0,3 µSv/h (approximately 33 µR/h), then no intervention is required.
If the exposure dose rate of external radiation in residential buildings exceeds the dose rate in open areas by more than 0,3 µSv/h, then measures to reduce it are recommended. If it is not possible to reduce the external radiation dose rate to a level below 0,6 µSv/h (or approximately 65 µR/h) above the gamma background of open areas, the question of relocating residents (with their consent) and repurposing the buildings is decided.
Radioactive substances can enter the body through the respiratory organs, the digestive tract, and the skin. In emergency situations and under extraordinary conditions, radionuclides penetrate through scratches, wounds, and burned surfaces.
The most probable source of radioactive substances entering the human body is air contaminated with radioactive gases and aerosols, as well as food products. With inhalation intake, radioactive aerosols are absorbed in the lungs and in the gastrointestinal tract. The further fate of aerosols in the lungs depends on the physico-chemical properties of the substances: readily soluble compounds of radioactive substances are quickly absorbed into the bloodstream, insoluble particles are deposited in significant amounts on the walls of the respiratory organs and are then removed from the lungs by the cilia of the ciliated epithelium of the bronchi. The absorption of poorly soluble radionuclides in the lungs occurs to a significantly lesser degree than in the GI tract. A significant amount of aerosols passes from the nasopharynx and the tracheobronchial part of the lungs into the gastrointestinal tract.
The distribution of radioactive substances that have entered the body can vary. Some radionuclides are distributed evenly throughout the body across all organs, while others are deposited in specific organs and tissues. All radionuclides, according to the nature of their distribution, are divided into four groups:
1.osteotropic (32P, 45Ca, 90Sr, 95Zn, 140Ba, 226Ra, 238U);
2.radionuclides that accumulate in organs with reticuloendothelial tissue (140La, 144Ce,
227Ac);
3.radionuclides involved in specific metabolic exchange and accumulating in organs and tissues (131I – in the thyroid gland, 59Fe – in erythrocytes, 65Zn – in the pancreas, 99Mo – in the iris of the eye);
4.radionuclides uniformly distributed throughout all organs (3H, 40K, 86Rb, 95Nb, 106Ru,
137Cs).
Radionuclides possess different biological effectiveness. In terms of their biological action, radioactive substances differ from one another depending on the type and energy of radiation, the half-life, the extent of absorption and accumulation, and the rate of elimination from the body. The greatest biological effect upon entry into the body occurs with
exposure to alpha emitters. It turns out to be 10 times greater than the effectiveness of beta emitters. Gamma emitters pose a somewhat lesser danger to the body.
The main feature of the action of ionizing radiation is the ionization of the atoms and molecules of living matter. This process is the initial stage of the biological action of radiation and subsequently causes functional and organic damage to tissues, organs, and systems. Complex mechanisms of direct and indirect action of ionizing radiation on the body underlie the occurrence of radiation sickness.
The direct action of radiation (large doses) on protein molecules leads to their denaturation. As a result, the protein molecule coagulates and precipitates out of the colloidal solution, subsequently undergoing breakdown under the action of enzymes. In this process, a disruption of physicochemical processes occurs in the cell, accompanied by a change in the structure of its surface and the permeability of the membranes. Each cell has a sensitive region (a target) that perceives the action of ionizing radiation. It has been established that the chromosomes of the nuclei and the cytoplasm are especially sensitive to the action of radiation.
The indirect action of ionizing radiation occurs due to the radiolysis of water. As is known, water makes up about 80% of the mass of all organs and tissues of the human body. Upon ionization of water, radicals are formed that possess both oxidizing and reducing properties. The most important of these are atomic hydrogen (H), hydroxide (HO2), and hydrogen peroxide (H2O2). Free oxidizing radicals react with enzymes containing sulfhydryl groups (SH), which are converted into inactive disulfide compounds (S=S). As a result of these reactions and transformations, the catalytic activity of thiol enzyme systems, which take an active part in the synthesis of nucleoproteins and nucleic acids of enormous importance for the vital activity of the organism, is disrupted. The content of DNA and RNA in cell nuclei decreases sharply, and the process of their renewal is disrupted. This leads to various chromosomal abnormalities and, consequently, to disruption of the entire genetic system.
The course of biochemical processes in the nuclei of tissues affected by radioactive radiation is also influenced by the radiotoxins formed and by changes in the hormonal regulation of tissues and cells. Metabolic processes are disrupted, leading to the accumulation of substances foreign to the body, for example toxic amino acids. All this enhances the biological action of ionizing radiation and contributes to intoxication of the body.
Radioactive substances entering the body can cause acute, subacute, and chronic radiation injury. In acute injury, changes in the blood are noted (leukopenia, reticulopenia), hemorrhages into various organs, suppression of immunological reactivity, and loss of body weight. Death of animals occurs within the first two weeks. Subacute injury is characterized by changes in the lymph; the number of erythrocytes, hemoglobin, and reticulocytes decreases. Animals die 1 – 3 months after administration of the radionuclides. The chronic course of the process is associated with exposure to small doses of radionuclides. Qualitative changes occur in the blood, immunological reactivity decreases, vascular disorders occur, sexual function is suppressed, and premature aging occurs.
The processes of eliminating radionuclides from the body depend on the functional state of the excretory systems and proceed at different rates. The largest amount of radioactive substances is excreted through the gastrointestinal tract. These substances include transuranium elements and lanthanides. Soluble radionuclide compounds are readily excreted through the kidneys. Gaseous radioactive substances, such as 3H, 222Rn, 133Xe, 85Kr, are excreted mainly through the lungs and skin. A number of radionuclides (131I, 137Cs) are excreted through the sweat and salivary glands and with milk.
To remove radionuclides, the diet should include the following foods. First of all, it is necessary to consume as many vegetables, fruits, and berries as possible. The most beneficial are carrots, radish, pomegranates, raisins, chokeberry, cranberries, nuts, horseradish, garlic, onions, beets, potatoes, uryuk, and kuraga (dried apricots). Garlic, onions, and cottage cheese have the greatest ability to absorb radioactive elements. Vegetables should be peeled before consumption, and the outer leaves should be removed from cabbage, since most of the harmful substances accumulate in the outer layer of the produce. In vegetables, these substances also accumulate in the core.
The diet must include dairy products, cottage cheese, cream, and sour cream. The calcium they contain reduces the accumulation of radioactive strontium. Of meat products, poultry is preferable. Fish and seafood (kelp, squid, black caviar) are useful for eliminating radionuclides from the body.
Vegetable oil has an anti-radiation effect, as does a very small amount of alcohol (or vodka) and calcium tablets. Among cereals, preference should be given to oatmeal and buckwheat porridge. It is recommended to drink decoctions of prunes, nettle, and laxative herbs, as well as juices (grape, pomegranate, beet, tomato) and bread kvass. Preference should be given to juices with pulp. Such beverages sorb harmful substances well and promote accelerated elimination of radionuclides. Before meals, it is advisable to take one or two tablets of activated charcoal.
Foods hazardous to human health under conditions of increased radiation include coffee, aspic, bone fat, beef, and hard-boiled eggs (the strontium contained in the shell passes into the protein during boiling).
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