The biological effect of radiation is the law of radioactive decay. “Presentation in physics on the topic“ Biological effects of radiation. The law of radioactive decay "(grade 9). Sources of radiation are

Radiation has a detrimental effect on living things. Alpha, beta, gamma radiation, when passing through a substance, can ionize it, that is, knock electrons out of its atoms and molecules.

Ionization - the process of formation of ions from neutral atoms and molecules.

Ionization of living tissues disrupts their proper functioning, which leads to a destructive effect on living cells.

At any point in the world, a person is always under the influence of radiation, such an effect is called background radiation.

Radiation background - ionizing radiation of terrestrial and space origin. The degree of exposure to radiation on the body depends on several factors:

• absorbed radiation energy;
• the mass of a living organism and the amount of energy per kilogram of its weight.

Absorbed radiation dose (D ) - the energy of ionizing radiation absorbed by the irradiated substance and calculated per unit mass.

where E - energy of absorbed radiation, m - body mass.

is a unit of measurement named after the English physicist Lewis Gray.

To measure exposure to low radiation, use the off-system unit of measurement - X-ray. One hundred x-rays are equal to one gray:

With the same absorbed dose of radiation, its effect on living organisms depends on the type of radiation and on the organ that is exposed to this radiation.

It is customary to compare the effects of various radiation with X-ray radiation or with gamma radiation. For alpha radiation, the efficacy is 20 times that of gamma radiation. The efficiency of the action of fast neutrons is 10 times higher than that of gamma radiation. To describe the characteristics of the impact, a quantity was introduced, which is called the quality factor (for alpha radiation it is 20, for fast neutrons - 10).

Quality factor (K) shows how many times the radiation hazard from exposure to a living organism of this type of radiation is greater than from exposure to gamma radiation (γ-radiation) at the same absorbed doses.

In order to take into account the quality factor, the concept was introduced - equivalent dose of radiation (H ) , which is equal to the product of the absorbed dose and the quality factor.

is a unit of measurement named after the Swedish scientist Rolf Maximilian Sievert.

Different organs of living organisms have different sensitivity to ionizing radiation. To estimate this parameter, the value - radiation risk coefficient.

When assessing the impact of radiation on living organisms, it is important to take into account the time of its action. In the process of radioactive decay, the number of radioactive atoms in a substance decreases, therefore, the intensity of radiation decreases. To be able to estimate the amount of remaining radioactive atoms in a substance, a value is used, which is called the half-life.

Half life (T ) is the period of time during which the initial number of radioactive nuclei is halved on average. Using the half-life, is introduced radioactive decay law (half-life law), which shows how many atoms of a radioactive substance will remain after a certain decay time.

,

where is the number of non-decayed atoms;

Initial number of atoms;

t - past tense;

T - half life.

The half-lives for the various substances are already calculated and known tabular values.

Calculate the radiation dose absorbed by two liters of water, if due to the absorption of this dose the water is heated by.

Given:, - specific heat capacity of water (tabular value).

To find:D - radiation dose.

Decision:

The radiation heated the water, that is, its absorbed energy passed into the internal energy of the water. Let's write it down as the transfer of a certain amount of heat.

The formula for the amount of heat transferred to water when heated:

The radiation energy, which has been converted into a given amount of heat, can be expressed from the formula for the absorbed radiation dose:

Let's equate these two expressions (energy and amount of heat):

From here we obtain the required formula for calculating the radiation dose:

Answer:

The safe equivalent dose of ionizing radiation is 15 mSv / year. What absorbed dose rate for γ-radiation does this correspond to?

Given:; ;

The quality factor of γ-radiation.

To find: is the absorbed dose rate.

Decision:

We translate the data into SI:

Let us express the absorbed dose from the equivalent dose formula:

Let us substitute the resulting expression into the expression for the absorbed dose rate:

Answer:.

There was some radioactive isotope silver. The mass of radioactive silver has decreased 8 times in 810 days. Determine the half-life of radioactive silver.

Given: - the ratio of the initial mass to the remaining;

To find:T.

Decision: Let's write down the half-life law:

The ratio of the initial and final mass will be equal to the ratio of the initial and final number of silver atoms:

Let's solve the resulting equation:

Answer: days.

At a minimum, during the study, you should not take radiation samples in your hands; for this, special holders are used. If there is a danger of getting into the radiation zone, it is necessary to use respiratory protection: masks and gas masks, as well as special suits (see Fig. 2).

Figure: 2. Protective equipment The effect of alpha radiation, although dangerous, is delayed even by a sheet of paper (see Fig. 3). To protect against this radiation, clothing is sufficient, which covers all parts of the body, the main thing is to prevent alpha particles from entering the lungs with radioactive dust.

Figure: 3. Exposure to α-radiation Beta radiation has a much higher penetrating ability (it penetrates into the body tissues by 1-2 cm). Protection against this radiation is difficult. For isolation from β-radiation, you will need, for example, a plate of aluminum several millimeters thick or a plate of glass (Fig. 4).

Figure: 4. Exposure to β-radiation Gamma radiation has the highest penetrating power. It is retained by a thick layer of lead or concrete walls several meters thick, therefore, personal protective equipment for a person from such radiation is not provided (Fig. 5).

Figure: 5. Exposure to γ-radiation

Homework

1. Questions at the end of paragraph 78, p. 263 (Peryshkin A.V., Gutnik E.M. Physics 9th grade ().
2. The average absorbed dose of radiation by an employee working with an X-ray unit is 7 μGy per hour. Is it dangerous for an employee to work for 200 days a year for 6 hours a day if the maximum permissible radiation dose is 50 mGy per year?
3. What is the half-life of one of the isotopes of france, if in 6 s the number of nuclei of this isotope decreases to 8 times?

Radiation. Radioactivity is called the instability of the nuclei of some atoms, which manifests itself in their ability to spontaneous transformation (according to scientific - decay), which is accompanied by the release of ionizing radiation (radiation). The energy of such radiation is large enough, therefore, it is able to affect the substance, creating new ions of different signs. It is impossible to cause radiation by means of chemical reactions, it is a completely physical process.

There are several types of radiation: -Alpha particles are relatively heavy particles, positively charged, are helium nuclei. Beta particles are ordinary electrons. - Gamma radiation - has the same nature as visible light, but much more penetrating power. -Neutrons are electrically neutral particles that arise mainly near a working nuclear reactor, access there must be limited. -X-rays - Similar to gamma rays, but with lower energy. By the way, the Sun is one of the natural sources of such rays, but the Earth's atmosphere provides protection from solar radiation.

The most dangerous for humans is Alpha, Beta and Gamma radiation, which can lead to serious diseases, genetic disorders and even death. The fact is that A., B. and G. particles, passing through a substance, ionize it, knocking out electrons from molecules and atoms. The more energy a person receives from the flow of particles acting on him and the less the mass of a person, the more serious disturbances in his body this will lead.

The amount of ionizing radiation energy transferred to a substance is expressed as the ratio of the radiation energy absorbed in a given volume to the mass of the substance in this volume, called the absorbed dose. D \u003d E / m The unit of absorbed dose is Gray (Gy). The off-system unit Rad was defined as the absorbed dose of any ionizing radiation equal to 100 erg per 1 gram of the irradiated substance.

But for a more accurate assessment of the possible damage to human health under conditions of chronic exposure in the field of radiation safety, the concept of an equivalent dose is introduced, which is equal to the product of the absorbed dose created by irradiation and averaged over the analyzed organ or throughout the body by the quality factor. H \u003d DK The unit for dose equivalent is Joule per kilogram. It has a special name z. Ivert (Sv).

Energy, as we already know, is one of the factors that determine the degree of negative impact of radiation on a person. Therefore, it is important to find a quantitative dependence (formula) by which it would be possible to calculate how many radioactive atoms remain in a substance at any given time. To derive this dependence, it is necessary to know that the rate of decrease in the number of radioactive nuclei for different substances is different and depends on a physical quantity called the half-life.

The law of radioactive decay - a physical law describing the dependence of the intensity of radioactive decay on time and the number of radioactive atoms in the sample. Discovered by Frederick Soddy and Ernest Rutherford, each of whom was subsequently awarded the Nobel Prize. They discovered it experimentally and published it in 1903 in the works "Comparative study of the radioactivity of radium and thorium" and "Radioactive transformation", formulating as follows:

In all cases, when one of the radioactive products was separated and its activity was investigated regardless of the radioactivity of the substance from which it was formed, it was found that the activity in all studies decreases with time according to the law of a geometric progression, from which, using Bernoulli's theorem, scientists concluded:

The rate of transformation is always proportional to the number of systems that have not yet undergone transformation. There are several formulations of the law, for example, in the form of a differential equation:

which means the number of decays? dNthat happened in a short period of time dt, proportional to the number of atoms N in the sample.

In the above math expression - decay constant, which characterizes the probability of radioactive decay per unit of time and has the dimension c − 1. The minus sign indicates a decrease in the number of radioactive nuclei over time.

The solution to this differential equation is:

where is the initial number of atoms, that is, the number of atoms for

Thus, the number of radioactive atoms decreases exponentially with time. Decay rate, that is, the number of decays per unit time

also falls exponentially. Differentiating the expression for the dependence of the number of atoms on time, we get:

where is the decay rate at the initial moment of time

Thus, the time dependence of the number of non-decayed radioactive atoms and the decay rate is described by the same constant.

In addition to the decay constant, radioactive decay is characterized by two more constants derived from it, discussed below.

Average lifetime

From the law of radioactive decay, one can obtain an expression for the average lifetime of a radioactive atom. The number of atoms that underwent decay at the time instant within the interval is equal to their lifetime equals.The average lifetime is obtained by integrating over the entire decay period:

Substituting this value into the exponential time dependences for and, it is easy to see that over time the number of radioactive atoms and the activity of the sample (the number of decays per second) decrease by a factor of e.

Half life

In practice, another time characteristic has become more widespread - half life equal to the time during which the number of radioactive atoms or the activity of the sample is reduced by a factor of 2. The connection of this quantity with the decay constant can be derived from the relation

Research into the biological effects of radioactive radiation began immediately after the discovery of X-rays (1895) and radioactivity (1896). In 1896 the Russian physiologist I.R. Tarkhanov showed that X-ray radiation, passing through living organisms, disrupts their vital activity. Research into the biological effect of radioactive radiation began to develop especially intensively with the beginning of the use of atomic weapons (1945), and then the peaceful use of atomic energy. The biological action of radioactive radiation is characterized by a number of general laws:

• 1) Deep disturbances in life are caused by negligible amounts of absorbed energy. So, the energy absorbed by the body of a mammal, animal or human during irradiation with a lethal dose, when converted into heat, would lead to heating of the body by only 0.001 ° C. An attempt to explain the “inconsistency” of the amount of energy with the results of exposure led to the creation of a target theory, according to which radiation damage develops when energy enters a particularly radiosensitive part of the cell - the “target”.
• 2) The biological effect of radioactive radiation is not limited to the exposed organism, but can spread to subsequent generations, which is explained by the effect on the hereditary apparatus of the organism. It is this feature that poses a very acute problem for humanity to study the biological effects of radioactive radiation and to protect the body from radiation.
• 3) The biological action of radioactive radiation is characterized by a latent (latent) period, i.e., the development of radiation damage is not observed immediately. The duration of the latent period can vary from several minutes to tens of years, depending on the radiation dose, the body's radiosensitivity, and the observed function. So, when exposed to very high doses (tens of thousands of glad) it is possible to cause "death under the beam", while long-term irradiation in low doses leads to a change in the state of the nervous and other systems, to the appearance of tumors years after irradiation.

The radiosensitivity of different types of organisms is different. The death of half of the irradiated animals (with total irradiation) within 30 days after irradiation (lethal dose - LD 50/30) is caused by the following X-ray doses: guinea pigs 250 r, dogs 335 r, monkey 600 r,mice 550-650 r, crucian carp (at 18 ° С) 1800 r, snakes 8000-20000 r. Unicellular organisms are more resistant: yeast dies at a dose of 30,000 r, amoeba - 100,000 r, and ciliates can withstand radiation at a dose of 300,000 r. The radiosensitivity of higher plants is also different: lily seeds completely lose their germination at a dose of 2000 r, cabbage seeds are not affected by a dose of 64,000 r.

Also of great importance are age, physiological state, intensity of the body's metabolic processes, as well as the conditions of irradiation. At the same time, in addition to the radiation dose to the body, the following factors play a role: the power, rhythm and nature of radiation (single, multiple, intermittent, chronic, external, total or partial, internal), its physical characteristics that determine the depth of energy penetration into the body (X-ray and gamma radiation penetrates to great depths, alpha particles up to 40 μm, beta particles - a few mm), the density of ionization caused by radiation (under the influence of alpha particles it is higher than under the action of other types of radiation). All these features of the influencing radiation agent determine the relative biological efficiency of radiation. If the source of radiation is the radioactive isotopes that have entered the body, then their chemical characteristics, which determine the participation of the isotope in metabolism, the concentration in one or another organ, and, consequently, the nature of the body's irradiation, are of great importance for the biological effect of radioactive radiation emitted by these isotopes. The primary effect of radiation of any kind on any biological object begins with the absorption of radiation energy, which is accompanied by the excitation of molecules and their ionization. When water molecules are ionized (indirect effect of radiation) in the presence of oxygen, active radicals (OH- and others), hydrated electrons, and hydrogen peroxide molecules appear, which are then included in the chain of chemical reactions in the cell. When organic molecules are ionized (direct action of radiation), free radicals arise, which, being included in the chemical reactions occurring in the body, disrupt the course of metabolism and, causing the appearance of compounds unusual for the body, disrupt vital processes. At a dose of 1000 r in a cell of medium size (10-9 r) there are about 1 million such radicals, each of which? in the presence of atmospheric oxygen, it can give rise to chain oxidation reactions, which many times increase the number of altered molecules in the cell and cause further changes in supramolecular (submicroscopic) structures. Elucidation of the large role of free oxygen in chain reactions leading to radiation injury, the so-called. oxygen effect, contributed to the development of a number of effective radioprotective substances causing artificial hypoxia in body tissues. The migration of energy through the molecules of biopolymers is also of great importance, as a result of which the absorption of energy that occurs anywhere in the macromolecule leads to the defeat of its active center (for example, to inactivation of the protein-enzyme). The physical and physicochemical processes underlying the biological action of radioactive radiation, i.e., the absorption of energy and the ionization of molecules, take a fraction of a second. Subsequent biochemical processes of radiation damage develop more slowly. The resulting active radicals disrupt normal enzymatic processes in the cell, which leads to a decrease in the amount of energy-rich (high-energy) compounds. The synthesis of deoxyribonucleic acids (DNA) in rapidly dividing cells is especially sensitive to radiation. Thus, as a result of chain reactions arising from the absorption of radiation energy, many components of the cell change, including macromolecules (DNA, enzymes, etc.) and relatively small molecules (adenosine triphosphoric acid, coenzymes, etc.). This leads to disruption of enzymatic reactions, physiological processes and cellular structures. Exposure to ionizing radiation causes cell damage. The most important violation of cell division is mitosis. Under irradiation in relatively small doses, a temporary stop of mitosis is observed. Large doses can cause complete cessation of division or cell death. Disruption of the normal course of mitosis is accompanied by chromosomal restructuring, the emergence of mutations leading to shifts in the genetic apparatus of the cell, and, consequently, to changes in subsequent cell generations (cytogenetic effect.) organisms . Under irradiation in large doses, swelling and pycnosis of the nucleus (chromatin compaction) occurs, then the structure of the nucleus disappears. In the cytoplasm under irradiation at doses of 10,000--20,000 r changes in viscosity, swelling of protoplasmic structures, the formation of vacuoles, and an increase in permeability are observed. All this sharply disrupts the vital activity of the cell. A comparative study of the radiosensitivity of the nucleus and the cytoplasm showed that in most cases the nucleus is sensitive to radiation (for example, irradiation of the nuclei of the heart muscle of a newt with a dose of several protons per nucleus caused typical destructive changes; a dose several thousand times greater did not damage the cytoplasm). Numerous data show that cells are most radiosensitive during the period of division and differentiation: during irradiation, growing tissues are primarily affected. This makes the radiation most dangerous for children and pregnant women. Radiotherapy of tumors is also based on this - the growing tumor tissue dies when irradiated in doses that less damage the surrounding normal tissues.

Changes arising in the irradiated cells lead to disturbances in tissues, organs and vital functions of the whole organism. The reaction of tissues is especially pronounced, in which? individual cells live for a relatively short time. This is the mucous membrane of the stomach and intestines, which, after irradiation, becomes inflamed, becomes covered with ulcers, which leads to impaired digestion and absorption, and then to depletion of the body, poisoning it with cell decay products (toxemia) and the penetration of bacteria living in the intestine into the blood (bacteremia) . The hematopoietic system is severely damaged, which leads to a sharp decrease in the number of leukocytes in the peripheral blood and to a decrease in its protective properties. At the same time, the production of antibodies also falls, which further weakens the body's defenses. (A decrease in the ability of an irradiated organism to produce antibodies and thereby resist the introduction of a foreign protein is used in organ and tissue transplantation - before the patient is irradiated.) The number of erythrocytes also decreases, which is associated with a violation of the respiratory function of the blood. The biological effect of radioactive radiation causes a violation of sexual function and the formation of germ cells up to complete sterility (sterility) of irradiated organisms. The nervous system plays an important role in the development of radiation injury in animals and humans. So, in rabbits, a fatal outcome is due to irradiation at a dose of 1000 r often defined by disorders in the central nervous system that cause cardiac arrest and respiratory paralysis. Studies of the bioelectric potentials of the brain of irradiated animals and people undergoing radiation therapy have shown that the nervous system responds to radiation exposure earlier than other body systems. Irradiation of dogs at a dose of 5-20 r and chronic irradiation at a dose of 0.05 r upon reaching a dose of 3 r leads to a change in conditioned reflexes. Disturbances in the activity of the endocrine glands also play an important role in the development of radiation sickness.

The biological effect of radioactive radiation is characterized by aftereffect, which can be very long, because after the end of the irradiation, the chain of biochemical and physiological reactions, which began with the absorption of radiation energy, continues for a long time. The long-term effects of radiation include changes in the blood (decrease in the number of leukocytes and erythrocytes), nephrosclerosis, cirrhosis of the liver, changes in the muscular membranes of blood vessels, early aging, and the appearance of tumors. These processes are associated with metabolic and neuroendocrine system disorders, as well as damage to the genetic apparatus of body cells (somatic mutations) . Plants are more radio-resistant than animals. Irradiation in small doses can stimulate the vital activity of plants - seed germination, intensity of root growth, accumulation of green mass, etc. Large doses (20,000-40,000 r) cause a decrease in plant survival, the appearance of deformities, mutations, the appearance of tumors. Disturbances in the growth and development of plants under irradiation are largely associated with changes in metabolism and the appearance of primary radiotoxins, which stimulate vital activity in small amounts, and suppress and disrupt it in large amounts. For example, washing irradiated seeds within 24 hours after irradiation reduces the inhibitory effect by 50-70%. Radiation damage to the body is accompanied by a simultaneous ongoing recovery process, which is associated with the normalization of metabolism and cell regeneration. In this regard, exposure to fractional or low dose rates causes less damage than massive exposure. The study of recovery processes is important for the search for radio-protective substances, as well as means and methods of protecting the body from radiation. In small doses, all the inhabitants of the Earth are constantly exposed to the action of ionizing radiation - cosmic rays and radioactive isotopes that are part of the organisms themselves and the environment. Tests of atomic weapons and the peaceful use of atomic energy increase the radioactive background. This makes the study of the biological effect of radioactive radiation and the search for protective equipment all the more important.

The biological effect of radioactive radiation is used in biological research, in medical and agricultural research. practice. Radiation therapy, X-ray diagnostics, and radioisotope therapy are based on the biological effect of radioactive radiation. In agriculture, radiation effects are used to develop new forms of plants, for pre-sowing seed treatment, pest control (by breeding and releasing irradiated males on the affected plantations), for radiation conservation of fruits and vegetables, protection of plant products from pests (doses, harmful for insects, harmless to grain), etc. Individual sensitivity of a person depends on many factors; in the first place - from age. The formed organism is more resistant to the action of radiation than the forming one (for children, youth). In acute radiation injury, which is caused by general irradiation of the body in large doses (observed in nuclear explosions and in the event of accidents at nuclear installations), the biological effects of radiation - death or various forms of radiation sickness - appear within several hours or days after exposure. At doses exceeding 100 Sv (Sievert is an equivalent dose unit in the SI system. 1 Sv corresponds to an absorbed dose of 1 J / kg of gamma radiation), instant death occurs (? First hours) due to irreversible damage to nerve cells (cerebral syndrome) ... Doses of 50-100 Sv are fatal 5-6 days after exposure. The intestinal form of radiation injury (gastrointestinal syndrome) is observed in the range of 10-50 Sv and leads to death on the 10-14th day. A typical form of radiation sickness develops at a dose of 1-10 Sv. Moreover, if no medical measures are taken, a dose of 3-5 Sv leads to the death of 50% of exposed people within 30 days. Irradiated patients are placed in sterile conditions, blood is transfused, and bone marrow transplant is performed to restore the hematopoietic system. All this is accompanied by the introduction of fortifying and anti-inflammatory drugs. Typical long-term consequences of radiation sickness are asthenia (increased fatigue), cataracts, increased susceptibility to infectious diseases due to decreased immunity. Radiation exposure significantly increases the risk of cancer, genetic damage and shortens life expectancy. The first position in the group of radiation-induced cancers is occupied by leukemias, the peak of which, depending on age, falls on the period from 5 to 25 years after irradiation. Somewhat later, cancer of the breast and thyroid gland, lungs and other organs occurs. The risk of genetic damage in the first two generations, according to experts, is about 40% of the risk of cancer.

The problem of the impact on the human body of exposure to "small doses" has become especially acute for the socialists after the Chernobyl accident. To solve it, a constant widespread examination of the population, monitoring the health of participants in the liquidation of the consequences of the accident and people living in contaminated areas is required. Already today, there is an increase in the number of thyroid cancer, an increase in the number of anemias, heart and other diseases associated with a weakened immune system. Natural radiation is a common component of the biosphere, an abiotic factor that continuously acts on organisms and forms a natural radioactive background, which is formed due to cosmic radiation and radiation of radionuclides in the external environment and inside living organisms. Artificial sources of radiation appear as a result of human activity. The biological effect of radiation is determined by the dose load and can be observed at all levels of organization of living systems. Individual sensitivity of a person to radiation exposure depends on age, psycho-emotional state, etc. Radiation injury, depending on the dose, can lead to death, various forms of radiation sickness, asthenia, cataracts, decreased immunity, reduced life expectancy, increased risk of cancer, genetic damage.

Lesson 64. Biological effects of radiation. The law of radioactive decay (Fedosova O.A.)

Lesson text

• Abstract

  Subject name - physics Class - 9 UMK (textbook title, author, year of publication) - Physics. Grade 9: textbook / A.V. Peryshkin, E.M. Gutnik. - M .: Bustard, 2014. Level of education (basic, in-depth, profile) - basic Topic of the lesson - Biological effects of radiation. The law of radioactive decay. The total number of hours devoted to the study of the topic - 1 Place of the lesson in the system of lessons on the topic - 64/11 The purpose of the lesson is to acquaint students with the latest scientific data on radiation, and its effects on biological objects. Lesson Objectives - To form students' knowledge about radioactivity. Assess the positive and negative manifestations of this discovery in modern society, expand the horizons of students. To form worldview ideas related to the use of radioactivity, To develop the oral speech of students through the organization of dialogical communication in the lesson, to form the ability to express their thoughts in a grammatically correct form. Form a positive motivation for learning and an increase in interest in knowledge. Expected results - Explain the physical meaning of radioactivity. The technical support of the lesson - a computer, a multimedia projector, the periodic table of chemical elements of D. I. Mendeleev. Additional methodological and didactic support of the lesson (links to Internet resources are possible) - presentation for the lesson from the disk "Physics Grade 9" from VIDEOUROKI.NET https://videouroki.net/look/diski/fizika9/index.html Lesson content 1. Organizational stage Mutual greeting of the teacher and students; checking for missing logs. 2. Actualization of the subject experience of students To review the basic concepts on the topic "Discovery of radioactivity": radioactivity; composition of radioactive radiation; α-radiation; β-radiation; γ-radiation. Name the names of scientists who are relevant to the topic of the lesson (and why?). 3. Learning new knowledge and ways of acting (working with presentation slides) In 1896, the French physicist Antoine Henri Becquerel discovered that uranium salts spontaneously emit rays. The phenomenon he discovered was called radioactivity. Let us recall that radioactivity is a phenomenon of spontaneous transformation of an unstable isotope of one chemical element into an isotope of another element, accompanied by the emission of particles with a high penetrating ability. Rutherford and other researchers have experimentally proven that radioactive radiation can be divided into three types: alpha, beta and gamma radiation. These names of radiation received from the first letters of the Greek alphabet. As we already know, radioactive radiation causes the ionization of atoms and molecules of matter, therefore they are often called ionizing radiation. It is now known that radioactive radiation under certain conditions can pose a danger to the health of living organisms. The mechanism of the biological action of radioactive radiation is complex. It is based on the processes of ionization and excitation of atoms and molecules in living tissues, which occur when they absorb ionizing radiation. The degree and nature of the negative impact of radiation depends on several factors, in particular, on what energy is transferred by the flow of ionizing particles to a given body and what is the mass of this body. The more energy a person receives from the flow of particles acting on him and the less is the mass of a person (that is, the more energy falls on each unit of mass), the more serious disturbances in his body will be. The absorbed dose of radiation is called a value equal to the ratio of the energy of ionizing radiation absorbed by the irradiated substance to the mass of this substance. In SI, the unit of absorbed radiation dose is gray. 1 gray is equal to the absorbed dose of radiation at which the energy of ionizing radiation 1 J is transferred to the irradiated substance with a mass of 1 kg. The non-systemic unit of the absorbed dose of radiation is radians. To measure the absorbed dose, special devices are used - dosimeters. The most widespread are dosimeters, in which the sensors are ionization chambers. Some dosimeters use particle counters, photographic film or scintillators as sensors. It is known that the greater the absorbed dose of radiation, the greater the harm (other things being equal) this radiation can cause to the body. But for a reliable assessment of the severity of the consequences that the action of ionizing radiation can lead to, it is also necessary to take into account that at the same absorbed dose, different types of radiation cause biological effects of different magnitude. The biological effects caused by any ionizing radiation are usually evaluated in comparison with the effect of X-ray or gamma radiation. For example, at the same absorbed dose, the biological effect of the action of alpha radiation will be 20 times greater than that of gamma radiation, the effect of fast neutrons can be 10 times greater than that of gamma radiation, of the action of beta radiation - the same as from gamma radiation. In this regard, it is customary to say that the quality factor of alpha radiation is 20, the aforementioned fast neutrons - 10, while the quality factor of gamma radiation (as well as X-ray and beta radiation) is considered equal to one. Thus, the quality factor shows how many times the radiation hazard from exposure to a living organism of this type of radiation is greater than from exposure to gamma radiation (at the same absorbed doses). Due to the fact that at the same absorbed dose, different radiation causes different biological effects, a value called the equivalent radiation dose was introduced to evaluate these effects. The equivalent dose of radiation is a quantity that determines the effect of radiation on the body, and the equal product of the absorbed dose and the quality factor. The equivalent dose can be measured in the same units as the absorbed dose, but there are special units for its measurement. In the International System, the unit of the equivalent dose is sIvert. Fractional units such as millisievert, microsievert, etc. are also used. The non-systemic unit of measurement is BER (biological equivalent of X-ray). When assessing the effects of ionizing radiation on a living organism, it is also taken into account that some parts of the body (organs, tissues) are more sensitive than others. For example, at the same equivalent dose, lung cancer is more likely than thyroid cancer. In other words, each organ and tissue has a certain radiation risk coefficient (for the lungs, for example, it is 0.12, and for the thyroid gland - 0.03). The maximum permissible dose of radiation is considered to be such an absorbed dose, which coincides in order of magnitude with the natural radioactive background that exists on Earth and is mainly caused by cosmic radiation and radioactivity of the Earth. From this point of view, the maximum permissible dose for a person in the range of X-ray, beta and gamma radiation is about 10 Gy per year. For thermal neutrons, this dose is 5 times lower, and for fast neutrons, protons and alpha particles, it is 10 times lower. The International Commission on Radiation Protection for people constantly working with sources of radioactive radiation has established a maximum permissible dose of no more than one thousandth of the warming per week, i.e. about 0.05 Gy per year. A dose over 3 - 6 Gray, received in a short time, is fatal for a person. The absorbed and equivalent doses also depend on the exposure time (i.e., on the time of interaction of radiation with the medium). All other things being equal, these doses are the greater, the longer the exposure time, i.e., the doses accumulate over time. When assessing the degree of danger that radioactive isotopes pose to living things, it is important to take into account the fact that the number of radioactive (i.e. that is, not yet decayed) atoms in the substance decreases over time. In this case, the number of radioactive decays per unit of time and the radiated energy are proportionally reduced. Energy, as we already know, is one of the factors that determine the degree of negative impact of radiation on a person. Therefore, it is so important to find a quantitative dependence (i.e., a formula) by which it would be possible to calculate how many radioactive atoms remain in a substance at any given moment in time. To derive this dependence, it is necessary to know that the rate of decrease in the number of radioactive nuclei for different substances is different and depends on a physical quantity called the half-life. Half-life is the length of time during which half of the original number of nuclei decays. Let us derive the dependence of the number of radioactive atoms on time and half-life. The time will be counted from the moment of the start of observation, when the number of radioactive atoms in the radiation source was equal to EN ZERO. Then after a period of time equal to the half-life, the number of non-decayed nuclei will be halved. After another same period of time, the number of non-decayed nuclei will once again decrease by half, and, in comparison with the initial number, by four times. After the expiry of the TE time equal to EN SMALL MULTIPLIED BY TE LARGE radioactive nuclei will remain: EN EQUAL TO EN ZERO DIVISION BY TWO IN THE DEGREE EN SMALL. we get a formula that is an analytical expression of the law of radioactive decay, established by Frederick Soddy: Knowing the law of radioactive decay, you can determine the number of decayed nuclei for any period of time. From the law of radioactive decay it follows that the longer the half-life of an element, the longer it "lives" and radiates, posing a danger to living organisms. This is clearly demonstrated by the graphs of the dependence of the number of remaining nuclei on time, plotted for isotopes of iodine and selenium, presented in the figure. To quantitatively characterize the number of decays per unit time, a physical quantity called the activity of a radioactive element is introduced. In the SI system, the unit of activity is becquerel - this is the activity of a radioactive preparation in which one nucleus decays in one second. The off-system unit of activity is curie. Nuclei resulting from radioactive decay can, in turn, be radioactive. This leads to the emergence of a chain or a series of radioactive transformations, ending with a stable isotope. The set of nuclei forming such a chain is called the radioactive family. There are three known radioactive families: the uranium-238 family, the thorium family, and the actinium family. All families end in stable lead isotopes. 4. Fixing the material What is the radiation dose? What is the natural background radiation? What is the maximum permissible annual dose of radiation for persons working with radioactive drugs? What is affected by radioactive radiation in the first place? Where do we get radioactive radiation? 5. Generalization and systematization Different types of radiation have different penetrating ability and affect a person in different ways. A sheet of paper with a thickness of 0.1 mm completely absorbs α-rays. And from β-rays will be protected by a sheet of aluminum with a thickness of 5 mm. The most difficult thing is to protect yourself from γ-rays, since even a centimeter layer of lead is only able to halve the intensity of these electromagnetic waves. There are the following methods of protection against radiation: 1) removal from the radiation source; 2) using a barrier made of materials that absorb radiation. The physical effect of X-ray radioactive radiation is the ionization of the atoms of the substance. The resulting free electrons and positive ions take part in a complex chain of reactions, as a result of which new molecules are formed, including free radicals. These free radicals, through a chain of reactions not yet fully understood, can cause chemical modification of biologically important molecules necessary for the normal functioning of the cell. Biochemical changes can occur both in a few seconds and decades after radiation and cause immediate cell death or changes in them that can lead to cancer. Radiation sickness can develop from both an increase in external and an increase in internal radiation. At the stage of development of the embryo, radiation does not kill the embryo, but causes the birth of freaks. Moreover, a dose of radiation that is safe for the mother's body can cause brain damage in the embryo. Today, the dose of absorbed radiation up to 5 mSv per year is considered acceptable and safe. And an admissible single exposure is considered to be an emergency exposure dose of 100 mSv. A single exposure of 750 mSv causes radiation sickness. A single irradiation of 4.5 Sv causes a severe degree of radiation sickness, in which 50% of the irradiated die. 6. Homework §61

  It is known that radioactive radiation under certain conditions can pose a danger to the health of living organisms. What is the reason for the negative effects of radiation on living things?

  The fact is that α-, β- and γ-particles, passing through a substance, ionize it, knocking out electrons from molecules and atoms. Ionization of living tissue disrupts the vital functions of the cells that make up this tissue, which negatively affects the health of the whole organism.

  The more energy a person receives from the flow of particles acting on him and the less is the mass of a person (i.e., the more energy falls on each unit of mass), the more serious violations in his body will be.

  • The energy of ionizing radiation absorbed by the irradiated substance (in particular, the tissues of the body) and calculated per unit mass is called the absorbed dose of radiation

  The absorbed dose of radiation D is equal to the ratio of the energy E absorbed by the body to its mass m:

  In SI, the unit of absorbed radiation dose is gray (Gy).

  It follows from this formula that

  1 Gy \u003d 1 J / 1 kg

  This means that the absorbed dose of radiation will be equal to 1 Gy if the radiation energy of 1 J is transferred to a substance weighing 1 kg.

  In certain cases (for example, when the soft tissues of living beings are irradiated with X-rays or γ-radiation), the absorbed dose can be measured in X-rays (R): 1 Gy corresponds to approximately 100 R.

  The greater the absorbed dose of radiation, the more harm (other things being equal) this radiation can inflict on the body.

  But for a reliable assessment of the severity of the consequences to which the action of ionizing radiation can lead, it is also necessary to take into account that with the same absorbed dose, different types of radiation cause biological effects of different magnitude.

  The biological effects caused by any ionizing radiation are usually evaluated in comparison with the effect of X-ray or γ-radiation. For example, at the same absorbed dose, the biological effect of the action of α-radiation will be 20 times greater than that of γ-radiation, the effect of fast neutrons can be 10 times greater than that of γ-radiation, of the action of β- radiation - the same as from γ-radiation.

  In this regard, it is customary to say that the quality factor of α-radiation is 20, the aforementioned fast neutrons - 10, while the quality factor of γ-radiation (as well as X-rays and β-radiation) is considered equal to one. In this way,

  • the quality factor K shows how many times the radiation hazard from exposure to a living organism of this type of radiation is greater than from exposure to γ-radiation (at the same absorbed doses)

  To assess the biological effects, a value was introduced called equivalent dose.

  The equivalent dose H is defined as the product of the absorbed dose D and the quality factor K:

  The equivalent dose can be measured in the same units as the absorbed dose, but there are special units for its measurement.

  The SI unit of the equivalent dose is the sievert (Sv). Fractional units are also used: millisievert (mSv), microsievert (μSv), etc.

  It follows from this formula that for X-ray, γ- and β-radiation (for which K \u003d 1) 1 Sv corresponds to an absorbed dose of 1 Gy, and for all other types of radiation - to a dose of 1 Gy, multiplied by the quality factor corresponding to this radiation ...

  When assessing the effects of ionizing radiation on a living organism, it is also taken into account that some parts of the body (organs, tissues) are more sensitive than others. For example, at the same equivalent dose, cancer is more likely to occur in the lungs than in the thyroid gland. In other words, each organ and tissue has a certain radiation risk coefficient (for the lungs, for example, it is 0.12, and for the thyroid gland - 0.03).

  The absorbed and equivalent doses also depend on the irradiation time (i.e., on the time of interaction of radiation with the medium). All other things being equal, these doses are the greater, the longer the exposure time, i.e., the doses accumulate over time.

  When assessing the degree of danger that radioactive isotopes pose to living things, it is also important to take into account the fact that the number of radioactive (i.e., not yet decayed) atoms in a substance decreases over time. In this case, the number of radioactive decays per unit of time and the radiated energy are proportionally reduced.

  Energy, as you already know, is one of the factors that determine the degree of negative impact of radiation on a person. Therefore, it is so important to find a quantitative dependence (i.e., a formula) by which it would be possible to calculate how many radioactive atoms remain in a substance at any given moment in time.

  To derive this dependence, it is necessary to know that the rate of decrease in the number of radioactive nuclei for different substances is different and depends on a physical quantity called the half-life.

  • The half-life T is the period of time during which the initial number of radioactive nuclei, on average, is halved

  Let us derive the dependence of the number N of radioactive atoms on time t and half-life T. The time will be counted from the moment of the beginning of observation t 0 \u003d 0, when the number of radioactive atoms in the radiation source was equal to N 0. Then after a period of time

  The formula is called the law of radioactive decay. It can be written in another form, for example. From the last formula it follows that the more T, the less 2 t / T and the more N (for given values \u200b\u200bof N 0 and t). This means that the longer the half-life of an element, the longer it "lives" and emits, posing a danger to living organisms. This is also confirmed by the plots of N versus t presented in Figure 165, plotted for isotopes of iodine (T I \u003d 8 days) and selenium (T Se \u003d 120 days).

  

  Figure: 165. Graph of the dependence of the number of radioactive atoms on time for isotopes of iodine and selenium

  Know how to protect yourself from radiation. Radioactive preparations should never be handled - they are taken with special long-handled forceps.

  The easiest way to protect yourself from α-radiation, since it has a low penetrating ability and therefore is retained, for example, by a sheet of paper, clothing, human skin. At the same time, α-particles that enter the body (with food, air, through open wounds) pose a great danger.

  β-Radiation has a much higher penetrating power, therefore it is more difficult to protect against its effects. β-Radiation can travel up to 5 m in air; it is able to penetrate into the tissues of the body (about 1-2 cm). For example, a layer of aluminum several millimeters thick can serve as protection against β-radiation.

  Gamma radiation has an even greater penetrating power, it is retained by a thick layer of lead or concrete. Therefore, γ-radioactive preparations are stored in thick-walled lead containers. For the same reason, a thick concrete layer is used in nuclear reactors to protect people from γ-rays and various particles (α-particles, neutrons, nuclear fission fragments, etc.).

  Questions

  1. What is the reason for the negative effects of radiation on living things?
  2. What is called the absorbed dose of radiation? Is radiation more or less harmful to the body if all other conditions are the same?
  3. Do different types of ionizing radiation cause the same or different biological effect in a living organism? Give examples.
  4. What does the radiation quality factor show? What value is called the equivalent radiation dose?
  5. What other factor (apart from energy, type of radiation and body weight) should be considered when assessing the effects of ionizing radiation on a living organism?
  6. What percentage of the atoms of a radioactive substance will remain after 6 days if its half-life is 2 days?
  7. Tell us about ways to protect yourself from the effects of radioactive particles and radiation.