Throughout the history of life on Earth, organisms have been constantly exposed to cosmic rays and the radionuclides formed by them in the atmosphere, as well as radiation from substances ubiquitous in nature. Modern life has adapted to all the features and limitations of the environment, including natural sources of X-rays.
Although high levels of radiation are certainly harmful to organisms, certain types of radiation are essential to life. For example, the 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
The radiation of extraterrestrial origin that continuously bombards the Earth is calledspace.
The fact that this penetrating radiation reaches our planet from outer space, and not from Earth, was discovered in experiments to measure ionization at various altitudes, from sea level to 9000 m. It was found that the intensity of ionizing radiation decreased up to a height of 700 m, and then rapidly increased with climb. 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.
X-ray sources in space are as follows:
- groups of galaxies;
- Seyfert galaxies;
- 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 solar flares. But our star does not make the main contribution to the total flux, since its daily 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, whose sources of radiation are primary radiation and the atmosphere, are composed of subatomic particles such as pions, muons, andelectrons. At sea level, almost all observed radiation consists of secondary cosmic rays, 68% of which are muons and 30% are electrons. Less than 1% of the flux at sea level is made up of protons.
Primary cosmic rays, as a rule, have a huge kinetic energy. They are positively charged and gain energy by accelerating in magnetic fields. In the vacuum of outer space, charged particles can exist for a long time and travel millions of light years. During this flight, they acquire high kinetic energy, on the order of 2–30 GeV (1 GeV=109 eV). Individual particles have energies up to 1010 GeV.
The high energies of primary cosmic rays allow them to literally split atoms in the earth's atmosphere when they collide. Along with neutrons, protons, and subatomic particles, light elements such as hydrogen, helium, and beryllium can be formed. Muons are always charged and also quickly decay into electrons or positrons.
Magnetic shield
The intensity of cosmic rays increases sharply with ascent until reaching a maximum at an altitude of about 20 km. From 20 km to the boundary of the atmosphere (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 interacted, and the decrease in intensity from 20 km to sea level reflects the absorption of secondary rays.atmosphere, equivalent to about 10 meters of water.
The intensity of radiation is also related to latitude. At the same height, the cosmic flow increases from the equator to a latitude of 50–60° and remains constant up to the poles. This is explained by the shape of the Earth's magnetic field and the distribution of the energy of the primary radiation. Magnetic field lines that extend beyond the atmosphere are usually parallel to the earth's surface at the equator and perpendicular at the poles. Charged particles easily move along the lines of the magnetic field, but hardly overcome it in the transverse direction. From the poles to 60°, virtually all primary radiation reaches the Earth's atmosphere, and at the equator only particles with energies exceeding 15 GeV can penetrate the magnetic shield.
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 originate 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 CO2, which mixes with atmospheric carbon dioxide. Carbon-14 enters plants through photosynthesis.
Earth Radiation
Of the many radionuclides that have formed with the Earth, only a few have half-lives long enough to explain their current existence. If our planet formed about 6 billion years ago, they would need a half-life of at least 100 million years to remain in measurable quantities. Of the primary radionuclides so far discovered, 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 that are almost always in the presence of the original isotope. Although many of the daughter radionuclides are short-lived, they are common in the environment as they are constantly formed from long-lived parent materials.
Other primordial long-lived X-ray sources, in short, are in 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. The Oklo quarry in Gabon, Africa, contains evidence of a "natural reactor" in which nuclear reactions took place. The depletion of U-235 and the presence of fission products within a rich uranium deposit indicate that a spontaneously induced chain reaction took place here about 2 billion years ago.
Although primordial radionuclides are ubiquitous, their concentration varies by location. MainThe reservoir of natural radioactivity is the lithosphere. In addition, it changes significantly within the lithosphere. Sometimes it is associated with certain types of compounds and minerals, sometimes it is purely regional, with little correlation with types of rocks and minerals.
The distribution of primary radionuclides and their progeny decay products in natural ecosystems depends on many factors, including the chemical properties of the nuclides, the physical factors of the ecosystem, and the physiological and ecological attributes of flora and fauna. The weathering of rocks, their main reservoir, supplies U, Th and K to the soil. The decay products of Th and U also take part in this transfer. From the 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, a decay product of U-238, is used by the plant, not because it is an isotope, but because it is chemically close to calcium. Uptake of uranium and thorium by plants is generally negligible as these radionuclides are usually insoluble.
Radon
The most important of all sources of natural radiation is the tasteless, odorless element, 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. The gas is a decay product of radium and is produced in any materialwhich contains it. Because radon is an inert gas, it can be released from surfaces that come into contact with the atmosphere. The amount of radon that comes out of a given mass of rock depends on the amount of radium and the surface area. The smaller the rock, the more radon it can release. The concentration of Rn in the air next to radium-containing materials also depends on the air velocity. In basements, caves and mines that have poor air circulation, radon concentrations can reach significant levels.
Rn decays quite quickly and forms a number of daughter radionuclides. Once formed in the atmosphere, radon decay products combine with fine dust particles that settle on the soil and plants, and are also inhaled by animals. Rainfall is particularly effective in clearing radioactive elements from the air, but the impact and settling of aerosol particles also contribute to their deposition.
In temperate climates indoor radon concentrations are on average about 5 to 10 times higher than outdoors.
Over the past few decades, man has "artificially" produced several hundred radionuclides, associated X-rays, sources, properties that have applications in medicine, military, power generation, instrumentation and mineral exploration.
Individual effects of man-made sources of radiation vary greatly. Most people receive a relatively small dose of artificial radiation, but some receive many thousands of times the radiation from natural sources. Man-made sources are bettercontrolled than natural.
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 has the potential to have a significant impact. It includes X-ray sources used in medicine for:
- diagnostics;
- therapy;
- analytical procedures;
- pacing.
For diagnostics, both sealed sources and a wide variety of radioactive tracers are used. Medical institutions generally 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 applications of isotope sources in medical radiography, including gamma and beta sources, and experimental neutron sources for cases where x-ray machines are inconvenient, inappropriate, or may be dangerous. From an environmental point of view, radiographic radiation does not pose a hazard as long as its sources remain accountable and properly disposed of. In this regard, the history of radium elements, radon needles and radium-containing luminescent compounds is not encouraging.
Commonly used X-ray sources based on 90Sror 147 Pm. The advent of 252Cf as a portable neutron generator has made neutron radiography widely available, although in general the technique is still highly dependent on the availability of nuclear reactors.
Nuclear Medicine
The main environmental hazards are radioisotope labels in nuclear medicine and X-ray sources. Examples of unwanted influences are as follows:
- irradiation of the patient;
- irradiation of hospital staff;
- exposure during transportation of radioactive pharmaceuticals;
- impact during production;
- exposure to radioactive waste.
In recent years, there has been a trend towards reducing patient exposure through the introduction of shorter-lived isotopes with a more narrow effect and the use of more highly localized drugs.
Shorter half-life reduces the impact of radioactive waste, as most of the long-lived elements are excreted through the kidneys.
Apparently, the environmental impact through sewerage does not depend on whether the patient is in a hospital or treated as an outpatient. While most of the released radioactive elements are likely to be short-lived, the cumulative effect far exceeds the pollution levels of all nuclear power plants combined.
The most commonly used radionuclides in medicine are X-ray sources:
- 99mTc – skull and brain scan, cerebral blood scan, heart, liver, lung, thyroid scan, placental localization;
- 131I - blood, liver scan, placental localization, thyroid scan and treatment;
- 51Cr - determination of the duration of the existence of red blood cells or sequestration, blood volume;
- 57Co - Schilling test;
- 32P – bone metastases.
The widespread use of radioimmunoassay procedures, urinalysis and other research methods using labeled organic compounds has significantly increased the use of liquid scintillation preparations. Organic phosphorus solutions, usually based on toluene or xylene, constitute a fairly 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 the long-lived 3H or 14C easily dissolve in the environment, their exposure is within the normal range. 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 sources of 238Pu (150 GBq) are surgically implanted in patients.
Industrial X-rays: sources, properties, applications
Medicine is not the only area in which this part of the electromagnetic spectrum has found application. Radioisotopes and X-ray sources used in industry are a significant part of the technogenic radiation situation. Application examples:
- industrial radiography;
- radiation measurement;
- smoke detectors;
- self-luminous materials;
- X-ray crystallography;
- scanners for screening luggage and hand luggage;
- x-ray lasers;
- synchrotrons;
- cyclotrons.
Because most of these applications involve the use of encapsulated isotopes, radiation exposure occurs during transport, transfer, maintenance and disposal.
Is an X-ray tube a source of ionizing radiation in industry? Yes, it is used in airport non-destructive testing systems, in the study of crystals, materials and structures, and in industrial control. Over the past decades, doses of radiation exposure in science and industry have reached half the value of this indicator in medicine; hence the contribution is substantial.
Encapsulated X-ray sources by themselves have little effect. But their transportation and disposal are worrisome when they are lost or mistakenly dumped in a landfill. Such sourcesX-rays are usually supplied and installed as doubly sealed discs or cylinders. The capsules are made of stainless steel and require periodic checking for leakage. Their disposal can be a problem. Short-lived sources may be stored and degraded, but even then they must be properly accounted for and residual active material must be disposed of at a licensed facility. Otherwise, the capsules should be sent to specialized institutions. Their power determines the material and size of the active part of the X-ray source.
X-ray source storage locations
A growing problem is the safe decommissioning and decontamination of industrial sites where radioactive materials have been stored in the past. These are mostly older nuclear reprocessing facilities, but other industries need to be involved, such as plants for the production of self-luminous tritium signs.
Long-lived low-level sources, which are widespread, are a particular problem. For example, 241Am is used in smoke detectors. In addition to radon, these are the main sources of X-ray radiation in everyday life. Individually, they do not pose any danger, but a significant number of them may present a problem in the future.
Nuclear explosions
During the last 50 years, everyone has been exposed to radiation from fallout caused by nuclear weapons testing. Their peak was at1954-1958 and 1961-1962.
In 1963, three countries (USSR, USA and Great Britain) signed an agreement on a partial ban on 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 ceased in 1980. Underground tests are still underway, but they generally do not produce precipitation.
Radioactive contamination from atmospheric tests fall near the explosion site. Some of them remain in the troposphere and are carried by the wind around the world at the same latitude. As they move, they fall to the ground, remaining about a month in the air. But most are pushed into the stratosphere, where pollution remains for many months, and slowly sinks across the planet.
Radioactive fallout includes several hundred different radionuclides, but only a few of them are able to affect the human body, so, their size is very small, and decay is fast. 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 far into the future.
Nuclear Energy
Nuclear power is the most controversial of all anthropogenic radiation sources, but it contributes very little to human he alth impacts. During normal operation, nuclear facilities release negligible amounts of radiation into the environment. February 2016There were 442 civil operating nuclear reactors in 31 countries and 66 more were under construction. This is only part of the nuclear fuel production cycle. It begins with the mining and grinding of uranium ore and continues with the manufacture of nuclear fuel. After being used in power plants, fuel cells are sometimes reprocessed to recover uranium and plutonium. In the end, the cycle ends with the disposal of nuclear waste. At every stage of this cycle, radioactive materials can be released.
About half of the world's uranium ore production comes from open pits, the other half from mines. It is then crushed at 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 operations, although radiation is a very small fraction of the natural background.
Uranium is then converted into fuel through further processing and purification at enrichment plants. These processes lead to air and water pollution, but they are much less than at other stages of the fuel cycle.
