Throughout the history of life on Earth, organisms have been constantly exposed to cosmic rays and radionuclides formed by them in the atmosphere, as well as radiation from substances commonly found in nature. Modern life has adapted to all the features and limitations of the environment, including the natural sources of x-ray radiation.
Despite the fact that a high level of radiation is certainly harmful to organisms, some types of radiation are important for life. For example, radiation background contributed to the fundamental processes of chemical and biological evolution. Also obvious is the fact that the heat of the Earth’s core is provided and maintained by the decay heat of primary, natural radionuclides.
Cosmic rays
Radiation of extraterrestrial origin that continuously bombards the Earth is called cosmic.
The fact that this penetrating radiation comes to our planet from outer space, and not of terrestrial origin, was discovered in experiments on measuring ionization at various altitudes, from sea level to 9000 m. It was found that the intensity of ionizing radiation decreased to a height of 700 m. and then with climb climbed rapidly. The initial decrease can be explained by a decrease in the intensity of terrestrial gamma rays, and an increase by the action of cosmic rays.
The sources of x-ray radiation in space are as follows:
- groups of galaxies;
- Seyfert galaxies;
- The sun;
- stars;
- quasars;
- black holes;
- supernova remnants;
- white dwarfs;
- dark stars, etc.
Evidence of such radiation, for example, is the increase in the intensity of cosmic rays observed on Earth after flares in the Sun. But our luminary does not make the main contribution to the total flow, since its diurnal variations are very small.
Two types of rays
Cosmic rays are divided into primary and secondary. Radiation that does not interact with matter in the atmosphere, lithosphere or hydrosphere of the Earth is called primary. It consists of protons (≈ 85%) and alpha particles (≈ 14%), with much smaller fluxes (<1%) of heavier nuclei. Secondary cosmic x-rays, the radiation sources of which are primary radiation and the atmosphere, are composed of subatomic particles such as pions, muons and electrons. At sea level, almost all of the observed radiation consists of secondary cosmic rays, 68% of which are muons and 30% are electrons. Less than 1% of the flow at sea level consists of protons.
Primary cosmic rays, as a rule, have tremendous kinetic energy. They are positively charged and receive energy due to acceleration in magnetic fields. In a vacuum in outer space, charged particles can exist for a long time and travel millions of light years. During this flight, they acquire high kinetic energy, of the order of 2–30 GeV (1 GeV = 10 9 eV). Individual particles have energies up to 10 10 GeV.
The high energies of the primary cosmic rays allow them to literally split atoms in an earthly atmosphere in a collision. Along with neutrons, protons and subatomic particles , light elements such as hydrogen, helium and beryllium can form. Muons are always charged, and also quickly decay into electrons or positrons.
Magnetic shield
The intensity of cosmic rays with a rise sharply increases to a maximum at an altitude of about 20 km. From 20 km to the atmospheric boundary (up to 50 km), the intensity decreases.
This pattern is explained by an increase in the production of secondary radiation as a result of an increase in air density. At an altitude of 20 km, most of the primary radiation has already entered into interaction, and a decrease in intensity from 20 km to sea level reflects the absorption of secondary rays by the atmosphere, which is equivalent to about a 10-meter layer of water.
Radiation intensity is also related to latitude. At one height, the cosmic flow increases from the equator to a latitude of 50-60 ° and remains constant up to the poles. This is due to the shape of the Earth’s magnetic field and the distribution of primary radiation energy. Magnetic lines of force extending beyond the atmosphere are generally parallel to the earth's surface at the equator and perpendicular to the poles. Charged particles move easily along the lines of the magnetic field, but hardly overcome it in the transverse direction. From poles to 60 °, almost all primary radiation reaches the Earth’s atmosphere, and at the equator only particles with energies exceeding 15 GeV can penetrate through the magnetic screen.
Secondary X-ray sources
As a result of the interaction of cosmic rays with matter, a significant amount of radionuclides is continuously produced. Most of them are fragments, but some of them are formed by the activation of stable atoms by neutrons or muons. The natural production of radionuclides in the atmosphere corresponds to the intensity of cosmic radiation in height and latitude. About 70% of them occur in the stratosphere, and 30% in the troposphere.
With the exception of H-3 and C-14, radionuclides are usually found in very low concentrations. Tritium is diluted and mixed with water and H-2, and C-14 combines with oxygen to form CO 2 , which mixes with atmospheric carbon dioxide. Carbon-14 penetrates plants during photosynthesis.
Earth Radiation
Of the many radionuclides that formed with the Earth, only a few have a half-life long enough to explain their current existence. If our planet formed about 6 billion years ago, then it would take at least 100 million years to have a half-life of them in order to remain in measurable quantities. Of the primary radionuclides that are still being detected, three are of the greatest importance. The x-ray source is K-40, U-238 and Th-232. Uranium and thorium each form a chain of decay products, which are almost always in the presence of the initial isotope. Although many of the daughter radionuclides are short-lived, they are common in the environment because they are constantly formed from long-lived starting materials.
Other initial long-lived X-ray sources are, in short, very low concentrations. These are Rb-87, La-138, Ce-142, Sm-147, Lu-176, etc. Naturally occurring neutrons form many other radionuclides, but their concentration is usually very low. In Oklo's quarry in Gabon, Africa, evidence of the existence of a "natural reactor" in which nuclear reactions occurred is located. The depletion of U-235 and the presence of fission products within a rich uranium deposit indicate that about 2 billion years ago a spontaneous chain reaction took place here.
Although the original radionuclides are omnipresent, their concentration depends on the location. The main reservoir of natural radioactivity is the lithosphere. In addition, within the lithosphere, it varies significantly. Sometimes it is associated with certain types of compounds and minerals, sometimes it is purely regional, with a slight correlation with the types of rocks and minerals.
The distribution of primary radionuclides and their daughter decay products in natural ecosystems depends on many factors, including the chemical properties of the nuclides, the physical factors of the ecosystem, as well as the physiological and environmental attributes of the flora and fauna. The weathering of rocks, their main reservoir, supplies U, Th, and K to the soil. The decomposition products of Th and U also take part in this transfer. From soil K, Ra, a little U and very little Th are absorbed by plants. They use potassium-40 in the same way as stable K. Radium, the decay product of U-238, is used by the plant, not because it is an isotope, but because it is chemically close to calcium. The uptake of uranium and thorium by plants is generally negligible, since these radionuclides are usually insoluble.
Radon
The most important of all sources of natural radiation is an element without taste and odor, an invisible gas that is 8 times heavier than air, radon. It consists of two main isotopes - radon-222, one of the decay products of U-238, and radon-220, formed during the decay of Th-232.
Rocks, soil, plants, animals emit radon into the atmosphere. Gas is a decay product of radium and is produced in any material that contains it. Since radon is an inert gas, it can be released by surfaces in contact with the atmosphere. The amount of radon that comes from a given rock mass depends on the amount of radium and the surface area. The finer the breed, the more radon it can release. The concentration of Rn in the air next to the radium-containing materials also depends on the speed of air movement. In basements, caves and mines that have poor air circulation, radon concentrations can reach significant levels.
Rn decays rather quickly and forms a number of daughter radionuclides. After the formation in the atmosphere, the decay products of radon combine with small particles of dust, which settles on soil and plants, and is also inhaled by animals. Rains are especially effective at clearing the air of radioactive elements, but the collision and sedimentation of aerosol particles also contributes to their deposition.
In a temperate climate, the concentration of radon in the room is on average about 5–10 times higher than in the open air.
Over the past few decades, humans have “artificially” produced several hundred radionuclides, concomitant x-rays, sources, properties that are used in medicine, military affairs, energy production, instrument making and mineral exploration.
The individual effect of anthropogenic radiation sources varies greatly. Most people receive a relatively small dose of artificial radiation, but some receive many thousand times the radiation of natural sources. Man-made sources are better controlled than natural sources.
X-ray sources in medicine
In industry and medicine, as a rule, only pure radionuclides are used, which simplifies the identification of leak paths from storage sites and the disposal process.
The use of radiation in medicine is widespread and could potentially have significant effects. It includes X-ray sources used in medicine for:
- diagnostics;
- therapy
- analytical procedures;
- pacing
For the diagnosis, both sealed sources and a wide variety of radioactive indicators are used. Medical facilities typically distinguish between these applications as radiology and nuclear medicine.
Is an x-ray tube a source of ionizing radiation? Computed tomography and fluorography are well-known diagnostic procedures that are performed with its help. In addition, there are many uses of isotope sources in medical radiography, including gamma and beta, and experimental neutron sources for cases where x-ray machines are uncomfortable, inappropriate, or can be dangerous. From an environmental point of view, X-ray radiation is not dangerous as long as its sources remain accountable and disposed of properly. In this regard, the history of radium elements, radon needles and radium-containing luminescent compounds is not encouraging.
Typically, X-ray sources based on 90 Sr or 147 Pm are used. The advent of 252 Cf as a portable neutron generator made neutron radiography widely available, although in general this method is still highly dependent on the availability of nuclear reactors.
Nuclear medicine
The main environmental hazards are radioisotope tags in nuclear medicine and x-ray sources. Examples of undesirable effects are as follows:
- patient exposure;
- exposure of hospital staff;
- radiation during transportation of radioactive pharmaceuticals;
- impact in the manufacturing process;
- exposure to radioactive waste.
In recent years, there has been a tendency to reduce patient exposure due to the introduction of short-lived isotopes of a more focused action and the use of more highly localized drugs.
A shorter half-life reduces the effect of radioactive waste, since most of the long-lived elements are excreted through the kidneys.
Apparently, the environmental impact through the sewage system does not depend on whether the patient is in the hospital or is treated on an outpatient basis. Although most of the released radioactive elements are likely to be short-lived, the cumulative effect far exceeds the pollution level of all nuclear power plants combined.
The most commonly used radionuclides in medicine are x-ray sources:
- 99m Tc - scan of the skull and brain, cerebral blood scan, scan of the heart, liver, lungs, thyroid gland, placental localization;
- 131 I - blood, liver scan, placental localization, scanning and treatment of the thyroid gland;
- 51 Cr - determination of the duration of red blood cells or sequestration, blood volume;
- 57 Co - Schilling test;
- 32 P - metastases in bone tissue.
The widespread use of radioimmunoassay procedures, radiation analysis of urine and other research methods using labeled organic compounds significantly increased the use of liquid scintillation preparations. Organic phosphorus solutions, usually based on toluene or xylene, make up a rather large volume of liquid organic waste that must be disposed of. Processing in liquid form is potentially hazardous and environmentally unacceptable. For this reason, waste incineration is preferred.
Since long-lived 3 N or 14 C easily dissolve in the environment, their effects are within normal limits. But the cumulative effect can be significant.
Another medical use of radionuclides is the use of plutonium batteries to power pacemakers. Thousands of people are alive today because these devices help their hearts function. Sealed 238 Pu sources (150 GBq) are surgically implanted in patients.
Industrial X-ray radiation: sources, properties, applications
Medicine is not the only area in which this part of the electromagnetic spectrum has found application. A significant component of the technogenic radiation environment is the radioisotopes and X-ray sources used in industry. Examples of this application:
- industrial radiography;
- radiation measurement;
- smoke detectors;
- self-luminous materials;
- X-ray crystallography;
- scanners for baggage and hand luggage;
- x-ray lasers;
- synchrotrons;
- cyclotrons.
Since most of these applications entail the use of encapsulated isotopes, radiation exposure occurs during transportation, transmission, maintenance and disposal.
Is an x-ray tube a source of ionizing radiation in industry? Yes, it is used in non-destructive control systems of airports, in the study of crystals, materials and structures, industrial control. Over the past decades, radiation doses in science and industry have reached half the value of this indicator in medicine; therefore, the contribution is substantial.
Encapsulated x-ray sources themselves have little effect. But their transportation and disposal are alarming when they are lost or mistakenly thrown into a landfill. Such X-ray sources are typically supplied and installed in the form of double-sealed discs or cylinders. Capsules are made of stainless steel and require periodic leak checks. Their disposal can be a problem.Short-lived sources can be stored and decomposed, but even then they must be properly accounted for and residual active material must be disposed of in a licensed institution. Otherwise, the capsules should be sent to specialized institutions. Their power determines the material and the size of the active part of the x-ray source.
Storage locations for X-ray sources
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Nuclear explosions
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In 1963, three countries (the USSR, the USA, and Great Britain) signed an agreement to partially ban nuclear tests in the atmosphere, ocean, and outer space. Over the next two decades, France and China conducted a series of much smaller tests, which ended in 1980. Underground tests are still in progress, but they generally do not cause precipitation.
After atmospheric testing, radioactive contamination falls near the site of the explosion. Partially, they remain in the troposphere and are spread by the wind all over the world at the same latitude. As they move, they fall to the ground, remaining in the air for about a month. But most are pushed into the stratosphere, where pollution remains for many months, and slowly descend across the planet.
Radioactive fallout includes several hundred different radionuclides, but only a few of them are able to affect the human body, for example, their size is very small and decay occurs quickly. The most significant are C-14, Cs-137, Zr-95 and Sr-90.
Zr-95 has a half-life of 64 days, while Cs-137 and Sr-90 have about 30 years. Only carbon-14 with a half-life of 5730 will remain active in the distant future.
Atomic Energy
Nuclear energy is the most controversial of all anthropogenic radiation sources, but it has very little contribution to human health. During normal operation, nuclear facilities emit a small amount of radiation into the environment. As of February 2016, there were 442 civilian nuclear reactors in 31 countries and another 66 were under construction. This is only part of the nuclear fuel production cycle. It begins with the extraction and grinding of uranium ore and continues with the production of nuclear fuel. After being used in power plants, fuel cells are sometimes reprocessed to reduce uranium and plutonium. In the end, the cycle ends with the disposal of nuclear waste. At every stage of this cycle, radioactive material may leak .
About half of the world's uranium ore mining comes from open pits, the other half from mines. Then it is crushed in nearby crushers, which produce a large amount of waste - hundreds of millions of tons. This waste remains radioactive for millions of years after the plant ceases to operate, although radiation makes up a very small fraction of the natural background.
After that, uranium is converted into fuel by further processing and purification at the enrichment plants. These processes lead to air and water pollution, but they are much smaller than at other stages of the fuel cycle.