For Russia, the energy of the Earth's heat can become a constant, reliable source of providing cheap and affordable electricity and heat using new high, environmentally friendly technologies for its extraction and supply to the consumer. This is especially true at the moment
Limited resources of fossil energy raw materials
The demand for organic energy raw materials is great in industrialized and developing countries (USA, Japan, states of united Europe, China, India, etc.). At the same time, their own hydrocarbon resources in these countries are either insufficient or reserved, and a country, for example, the United States, buys energy raw materials abroad or develops deposits in other countries.
In Russia, one of the richest countries in terms of energy resources, the economic needs for energy are still satisfied by the possibilities of using natural resources. However, the extraction of fossil hydrocarbons from the subsoil occurs at a very fast pace. If in the 1940s-1960s. The main oil-producing regions were the "Second Baku" in the Volga and Cis-Urals, then, starting from the 1970s, and to the present, Western Siberia has been such an area. But even here there is a significant decline in the production of fossil hydrocarbons. The era of "dry" Cenomanian gas is passing away. The previous stage of extensive development of natural gas production has come to an end. Its extraction from such giant deposits as Medvezhye, Urengoyskoye and Yamburgskoye amounted to 84, 65 and 50%, respectively. The proportion of oil reserves favorable for development also decreases over time.
Due to the active consumption of hydrocarbon fuels, onshore reserves of oil and natural gas have been significantly reduced. Now their main reserves are concentrated on the continental shelf. And although the raw material base of the oil and gas industry is still sufficient for the extraction of oil and gas in Russia in the required volumes, in the near future it will be provided to an increasing extent through the development of fields with complex mining and geological conditions. At the same time, the cost of hydrocarbon production will grow.
Most of the non-renewable resources extracted from the subsoil are used as fuel for power plants. First of all, this is the share of which in the fuel structure is 64%.
In Russia, 70% of electricity is generated at thermal power plants. Energy enterprises of the country annually burn about 500 million tons of c.e. tons for the purpose of generating electricity and heat, while the production of heat consumes 3-4 times more hydrocarbon fuel than the generation of electricity.
The amount of heat obtained from the combustion of these volumes of hydrocarbon raw materials is equivalent to the use of hundreds of tons of nuclear fuel - the difference is huge. However, nuclear power requires ensuring environmental safety (to prevent a repeat of Chernobyl) and protecting it from possible terrorist attacks, as well as the safe and costly decommissioning of obsolete and spent nuclear power units. The proven recoverable reserves of uranium in the world are about 3 million 400 thousand tons. For the entire previous period (until 2007), about 2 million tons were mined.
RES as the future of global energy
The increased interest in the world in recent decades in alternative renewable energy sources (RES) is caused not only by the depletion of hydrocarbon fuel reserves, but also by the need to solve environmental problems. Objective factors (fossil fuel and uranium reserves, as well as environmental changes associated with the use of traditional fire and nuclear energy) and energy development trends suggest that the transition to new methods and forms of energy production is inevitable. Already in the first half of the XXI century. there will be a complete or almost complete transition to non-traditional energy sources.
The sooner a breakthrough is made in this direction, the less painful it will be for the whole society and the more beneficial for the country, where decisive steps will be taken in this direction.
The world economy has already set a course for the transition to a rational combination of traditional and new energy sources. Energy consumption in the world by 2000 amounted to more than 18 billion tons of fuel equivalent. tons, and energy consumption by 2025 may increase to 30–38 billion tons of fuel equivalent. tons, according to forecast data, by 2050 consumption at the level of 60 billion tons of fuel equivalent is possible. t. A characteristic trend in the development of the world economy in the period under review is a systematic decrease in the consumption of fossil fuels and a corresponding increase in the use of non-traditional energy resources. The thermal energy of the Earth occupies one of the first places among them.
Currently, the Ministry of Energy of the Russian Federation has adopted a program for the development of non-traditional energy, including 30 large projects for the use of heat pump units (HPU), the principle of operation of which is based on the consumption of low-potential thermal energy of the Earth.
Low-potential energy of the Earth's heat and heat pumps
The sources of low-potential energy of the Earth's heat are solar radiation and thermal radiation of the heated bowels of our planet. At present, the use of such energy is one of the most dynamically developing areas of energy based on renewable energy sources.
The heat of the Earth can be used in various types of buildings and structures for heating, hot water supply, air conditioning (cooling), as well as for heating tracks in the winter season, preventing icing, heating fields in open stadiums, etc. In the English-language technical literature of the system utilizing the Earth's heat in heating and air conditioning systems are referred to as GHP - "geothermal heat pumps" (geothermal heat pumps). The climatic characteristics of the countries of Central and Northern Europe, which, together with the United States and Canada, are the main areas for the use of low-grade heat of the Earth, determine this mainly for heating purposes; cooling of the air, even in summer, is relatively rarely required. Therefore, unlike in the USA, heat pumps in European countries operate mainly in heating mode. In the US, they are more often used in air heating systems combined with ventilation, which allows both heating and cooling of the outside air. In European countries, heat pumps are usually used in water heating systems. Since their efficiency increases as the temperature difference between the evaporator and condenser decreases, floor heating systems are often used for heating buildings, in which a coolant of a relatively low temperature (35–40 ° C) circulates.
Types of systems for the use of low-potential energy of the Earth's heat
In the general case, two types of systems for using the low-potential energy of the Earth's heat can be distinguished:
- open systems: as a source of low-potential thermal energy, groundwater is used, which is supplied directly to heat pumps;
- closed systems: heat exchangers are located in the soil massif; when a coolant with a temperature lower than the ground circulates through them, thermal energy is “taken off” from the ground and transferred to the heat pump evaporator (or when a coolant with a higher temperature relative to the ground is used, it is cooled).
The disadvantages of open systems are that wells require maintenance. In addition, the use of such systems is not possible in all areas. The main requirements for soil and groundwater are as follows:
- sufficient water permeability of the soil, allowing replenishment of water reserves;
– good groundwater chemistry (e.g. low iron content) to avoid pipe scale and corrosion problems.
Closed systems for the use of low-potential energy of the Earth's heat
Closed systems are horizontal and vertical (Figure 1).
Rice. 1. Scheme of a geothermal heat pump installation with: a - horizontal
and b - vertical ground heat exchangers.
Horizontal ground heat exchanger
In the countries of Western and Central Europe, horizontal ground heat exchangers are usually separate pipes laid relatively tightly and connected to each other in series or in parallel (Fig. 2).
Rice. 2. Horizontal ground heat exchangers with: a - sequential and
b - parallel connection.
To save the area of the site where the heat is removed, improved types of heat exchangers have been developed, for example, heat exchangers in the form of a spiral (Fig. 3), located horizontally or vertically. This form of heat exchangers is common in the USA.
2. Thermal regime of the Earth
The earth is a cold cosmic body. The surface temperature depends mainly on the heat supplied from outside. 95% of the heat of the Earth's upper layer is external (solar) heat and only 5% heat internal , which comes from the bowels of the Earth and includes several sources of energy. In the bowels of the Earth, the temperature increases with depth from 1300 o C (in the upper mantle) to 3700 o C (in the center of the core).
external heat. Heat comes to the Earth's surface mainly from the Sun. Each square centimeter of the surface receives about 2 calories of heat within one minute. This value is called solar constant and determines the total amount of heat coming to the Earth from the Sun. For a year, it amounts to 2.26 10 21 calories. The depth of penetration of solar heat into the bowels of the Earth depends mainly on the amount of heat that falls per unit surface area, and on the thermal conductivity of rocks. The maximum depth to which external heat penetrates is 200 m in the oceans and about 40 m on land.
internal warmth. With depth, there is an increase in temperature, which occurs very unevenly in different territories. The increase in temperature follows an adiabatic law and depends on the compression of the substance under pressure when heat exchange with the environment is impossible.
The main sources of heat inside the Earth:
Heat released during the radioactive decay of elements.
Residual heat left over from the formation of the Earth.
Gravitational heat released during the compression of the Earth and the distribution of matter in density.
Heat generated by chemical reactions occurring in the depths of the earth's crust.
Heat released by the tidal friction of the Earth.
There are 3 temperature zones:
I- variable temperature zone . The change in temperature is determined by the climate of the area. Daily fluctuations practically die out at a depth of about 1.5 m, and annual fluctuations at depths of 20 ... 30 m. Ia - freezing zone.
II - constant temperature zone located at depths of 15…40 m, depending on the region.
III - hot zone .
The temperature regime of rocks in the bowels of the earth's crust is usually expressed by a geothermal gradient and a geothermal step.
The amount of temperature rise for every 100 m of depth is called geothermal gradient. In Africa, at the Witwatersrand field, it is 1.5 °С, in Japan (Echigo) - 2.9 °С, in South Australia - 10.9 °С, in Kazakhstan (Samarinda) - 6.3 °С, on the Kola Peninsula - 0.65 °С.
Rice. 3. Temperature zones in the earth's crust: I - zone of variable temperatures, Ia - freezing zone; II - zone of constant temperatures; III - zone of temperature increase.
The depth at which the temperature rises by 1 degree is called geothermal step. The numerical values of the geothermal step are not constant not only at different latitudes, but also at different depths of the same point in the region. The value of the geothermal step varies from 1.5 to 250 m. In Arkhangelsk it is 10 m, in Moscow - 38.4 m, and in Pyatigorsk - 1.5 m. Theoretically, the average value of this step is 33 m.
In a well drilled in Moscow to a depth of 1,630 m, the bottomhole temperature was 41 °C, and in a mine drilled in the Donbass to a depth of 1,545 m, the temperature was 56.3 °C. The highest temperature was recorded in the USA in a well with a depth of 7136 m, where it is equal to 224 °C. The increase in temperature with depth should be taken into account when designing deep structures. According to calculations, at a depth of 400 km, the temperature should reach 1400...1700 °C. The highest temperatures (about 5000 °C) were obtained for the Earth's core.
The warmth of the earth. Possible sources of internal heat
Geothermy- science that studies the thermal field of the Earth. The average surface temperature of the Earth has a general tendency to decrease. Three billion years ago, the average temperature on the Earth's surface was 71 o, now it is 17 o. Sources of heat (thermal ) Earth's fields are internal and external processes. The heat of the Earth is caused by solar radiation and originates in the bowels of the planet. The values of heat influx from both sources are quantitatively extremely different and their roles in the life of the planet are different. Solar heating of the Earth is 99.5% of the total amount of heat received by its surface, and internal heating accounts for 0.5%. In addition, the influx of internal heat is very unevenly distributed on the Earth and is concentrated mainly in places of manifestation of volcanism.
External source is solar radiation . Half of the solar energy is absorbed by the surface, vegetation and near-surface layer of the earth's crust. The other half is reflected into world space. Solar radiation maintains the temperature of the Earth's surface at an average of about 0 0 C. The Sun warms the near-surface layer of the Earth to an average depth of 8 - 30 m, with an average depth of 25 m, the effect of solar heat ceases and the temperature becomes constant (neutral layer). This depth is minimal in areas with a maritime climate and maximal in the Subpolar region. Below this boundary there is a belt of constant temperature corresponding to the average annual temperature of the area. So, for example, in Moscow on the territory of agricultural. academy. Timiryazev, at a depth of 20 m, the temperature has invariably remained equal to 4.2 ° C since 1882. In Paris, at a depth of 28 m, a thermometer has consistently shown 11.83 ° C for more than 100 years. The layer with a constant temperature is the deepest where perennial ( eternal Frost. Below the belt of constant temperature is the geothermal zone, which is characterized by heat generated by the Earth itself.
Internal sources are the bowels of the Earth. The Earth radiates more heat into space than it receives from the Sun. Internal sources include residual heat from the time when the planet was melted, the heat of thermonuclear reactions occurring in the bowels of the Earth, the heat of the gravitational compression of the Earth under the action of gravity, the heat of chemical reactions and crystallization processes, etc. (for example, tidal friction). The heat from the bowels comes mainly from the moving zones. The increase in temperature with depth is associated with the existence of internal heat sources - the decay of radioactive isotopes - U, Th, K, gravitational differentiation of matter, tidal friction, exothermic redox chemical reactions, metamorphism and phase transitions. The rate of temperature increase with depth is determined by a number of factors - thermal conductivity, permeability of rocks, proximity to volcanic chambers, etc.
Below the belt of constant temperatures there is an increase in temperature, on average 1 o per 33 m ( geothermal stage) or 3 o every 100 m ( geothermal gradient). These values are indicators of the thermal field of the Earth. It is clear that these values are average and different in magnitude in different areas or zones of the Earth. The geothermal step is different at different points on the Earth. For example, in Moscow - 38.4 m, in Leningrad 19.6, in Arkhangelsk - 10. So, when drilling a deep well on the Kola Peninsula at a depth of 12 km, a temperature of 150 ° was assumed, in reality it turned out to be about 220 degrees. When drilling wells in the northern Caspian at a depth of 3000 m, the temperature was assumed to be 150 degrees, but it turned out to be 108 degrees.
It should be noted that the climatic features of the area and the average annual temperature do not affect the change in the value of the geothermal step, the reasons lie in the following:
1) in the different thermal conductivity of the rocks that make up a particular area. Under the measure of thermal conductivity is understood the amount of heat in calories transferred in 1 second. Through a section of 1 cm 2 with a temperature gradient of 1 o C;
2) in the radioactivity of rocks, the greater the thermal conductivity and radioactivity, the lower the geothermal step;
3) in different conditions of occurrence of rocks and the age of their occurrence; observations have shown that the temperature rises faster in the layers collected in folds, they often have violations (cracks), through which the access of heat from the depths is facilitated;
4) the nature of groundwater: hot groundwater flows warm rocks, cold ones cool;
5) remoteness from the ocean: near the ocean due to the cooling of rocks by a mass of water, the geothermal step is larger, and at the contact it is smaller.
Knowing the specific value of the geothermal step is of great practical importance.
1. This is important when designing mines. In some cases, it will be necessary to take measures to artificially lower the temperature in deep workings (temperature - 50 ° C is the limit for a person in dry air and 40 ° C in wet air); in others, it will be possible to work at great depths.
2. The assessment of temperature conditions during tunneling in mountainous areas is of great importance.
3. The study of the geothermal conditions of the Earth's interior makes it possible to use steam and hot springs emerging on the Earth's surface. Underground heat is used, for example, in Italy, Iceland; in Russia, an experimental industrial power plant was built on natural heat in Kamchatka.
Using data on the size of the geothermal step, one can make some assumptions about the temperature conditions of the deep zones of the Earth. If we take the average value of the geothermal step as 33 m and assume that the increase in temperature with depth occurs evenly, then at a depth of 100 km there will be a temperature of 3000 ° C. This temperature exceeds the melting points of all substances known on Earth, therefore, at this depth there should be molten masses . But due to the huge pressure of 31,000 atm. Superheated masses do not have the characteristics of liquids, but are endowed with the characteristics of a solid body.
With depth, the geothermal step must apparently increase significantly. If we assume that the step does not change with depth, then the temperature in the center of the Earth should be about 200,000 degrees, and according to calculations, it cannot exceed 5000 - 10,000 degrees.
The main sources of thermal energy of the Earth are [ , ]:
To date, only the first four sources have been quantified. In our country, the main merit in this belongs to O.G. Sorokhtin and S.A. Ushakov. The following data is mainly based on the calculations of these scientists.
Heat of the Earth's gravitational differentiation
One of the most important regularities in the development of the Earth is differentiation its substance, which continues at the present time. This differentiation resulted in the formation core and crust, change in the composition of the primary robes, while the separation of an initially homogeneous substance into fractions of different densities is accompanied by the release thermal energy, and the maximum heat release occurs when the terrestrial matter is divided into dense and heavy core and residual lighter silicate shell earth mantle. At present, most of this heat is generated at the border mantle - core.
Earth's Gravitational Differentiation Energies for the entire time of its existence stood out - 1.46 * 10 38 erg (1.46 * 10 31 J). Given energy for the most part first goes into kinetic energy convective currents of the mantle substance, and then in warmly; another part of it is spent on additional compression of the earth's interior, arising due to the concentration of dense phases in the central part of the Earth. From 1.46*10 38 erg energy of the Earth's gravitational differentiation went to its additional compression 0.23*10 38 erg (0.23*10 31 J), and in the form of heat released 1.23*10 38 erg (1.23*10 31 J). The magnitude of this thermal component significantly exceeds the total release in the Earth of all other types of energy. The time distribution of the total value and rate of release of the thermal component of gravitational energy is shown in Fig. 3.6 .
Rice. 3.6.
The current level of heat generation during the gravitational differentiation of the Earth - 3*10 20 erg/s (3*10 13W), which depends on the value of the modern heat flux passing through the surface of the planet in ( 4.2-4.3) * 10 20 erg / s ((4.2-4.3)*10 13W), is ~ 70%
.
radiogenic heat
Caused by the radioactive decay of unstable isotopes. The most energy-intensive and long-lived ( with a half-life commensurate with the age of the Earth) are isotopes 238 U, 235 U, 232Th and 40K. Most of them are concentrated in continental crust. Modern level of generation radiogenic heat:
- by American geophysicist V.Vakye - 1.14*10 20 erg/s (1.14*10 13W) ,
- according to Russian geophysicists O.G. Sorokhtin and S.A. Ushakov - 1.26*10 20 erg/s(1.26*10 13W) .
From the value of the modern heat flow, this is ~ 27-30%.
Of the total heat of radioactive decay in 1.26*10 20 erg/s (1.26*10 13W) in the earth's crust stands out - 0.91*10 20 erg/s, and in the mantle - 0.35*10 20 erg/s. It follows from this that the proportion of mantle radiogenic heat does not exceed 10% of the total modern heat loss of the Earth, and it cannot be the main source of energy for active tectono-magmatic processes, the depth of which can reach 2900 km; and the radiogenic heat released in the crust is relatively quickly lost through the earth's surface and practically does not participate in the heating of the deep interior of the planet.
In past geological epochs, the amount of radiogenic heat released in the mantle must have been higher. Its estimates at the time of the formation of the Earth ( 4.6 billion years ago) give - 6.95*10 20 erg/s. Since that time, there has been a steady decrease in the rate of release of radiogenic energy (Fig. 3.7 ).
For all the time in the Earth stood out ~4.27*10 37 erg(4.27*10 30 J) the thermal energy of radioactive decay, which is almost three times lower than the total value of the heat of gravitational differentiation.
Heat of tidal friction
It stands out during the gravitational interaction of the Earth, primarily with the Moon, as the nearest large cosmic body. Due to mutual gravitational attraction, tidal deformations occur in their bodies - swelling or humps. The tidal humps of the planets, with their additional attraction, influence their movement. Thus, the attraction of both tidal humps of the Earth creates a pair of forces acting both on the Earth itself and on the Moon. However, the influence of the near, moon-facing swelling is somewhat stronger than that of the far one. Due to the fact that the angular velocity of rotation of the modern Earth ( 7.27*10 -5 s -1) exceeds the orbital velocity of the Moon ( 2.66*10 -6 s -1), and the substance of the planets is not ideally elastic, then the tidal humps of the Earth are, as it were, carried away by its forward rotation and are noticeably ahead of the movement of the Moon. This leads to the fact that the maximum tides of the Earth always occur on its surface somewhat later than the moment climax Moon, and an additional moment of forces acts on the Earth and the Moon (Fig. 3.8 ) .
The absolute values of the forces of tidal interaction in the Earth-Moon system are now relatively small and the tidal deformations of the lithosphere caused by them can reach only a few tens of centimeters, but they lead to a gradual deceleration of the Earth's rotation and, conversely, to the acceleration of the orbital motion of the Moon and its removal from the Earth. The kinetic energy of the movement of the earth's tidal humps is converted into thermal energy due to the internal friction of matter in the tidal humps.
At present, the rate of release of tidal energy by G. McDonald is ~0.25*10 20 erg/s (0.25*10 13W), while its main part (about 2/3) is presumably dissipates(dispersed) in the hydrosphere. Consequently, the fraction of tidal energy caused by the interaction of the Earth with the Moon and dissipated in the solid Earth (primarily in the asthenosphere) does not exceed 2 %
total thermal energy generated in its depths; and the fraction of solar tides does not exceed 20 %
from the influence of the lunar tides. Therefore, solid tides now play practically no role in feeding tectonic processes with energy, but in some cases they can act as "triggers", for example, earthquakes.
The magnitude of tidal energy is directly related to the distance between space objects. And if the distance between the Earth and the Sun does not assume any significant changes in the geological time scale, then in the Earth-Moon system this parameter is a variable. Regardless of ideas about, almost all researchers admit that in the early stages of the development of the Earth, the distance to the Moon was significantly less than the modern one, while in the process of planetary development, according to most scientists, it gradually increases, and according to Yu.N. Avsyuku this distance experiences long-term changes in the form of cycles "arrival - departure" of the moon. This implies that in past geological epochs the role of tidal heat in the overall heat balance of the Earth was more significant. In general, for the entire time of the development of the Earth, it has stood out ~3.3*10 37 erg (3.3*10 30 J) tidal heat energy (this is subject to the successive removal of the Moon from the Earth). The change in time of the rate of release of this heat is shown in Fig. 3.10 .
More than half of the total tidal energy was released in katarchee (hellea)) - 4.6-4.0 billion years ago, and at that time, only due to this energy, the Earth could additionally warm up by ~ 500 0 С. energy-intensive endogenous processes .
accretion heat
This is the heat stored by the Earth since its formation. During accretions, which lasted for several tens of millions of years, due to the collision planetesimals The earth has experienced significant heating. At the same time, there is no consensus on the magnitude of this heating. Currently, researchers are inclined to believe that in the process of accretion, the Earth experienced, if not complete, then significant partial melting, which led to the initial differentiation of the Proto-Earth into a heavy iron core and a light silicate mantle, and to the formation "magma ocean" on its surface or at shallow depths. Although even before the 1990s, the model of a relatively cold primary Earth was considered practically universally recognized, which gradually warmed up due to the above processes, accompanied by the release of a significant amount of thermal energy.
An accurate estimate of the primary accretionary heat and its share that has survived to the present time is associated with significant difficulties. By O.G. Sorokhtin and S.A. Ushakov, who are supporters of a relatively cold primary Earth, the value of the accretion energy converted into heat is - 20.13*10 38 erg (20.13*10 31 J). This energy in the absence of heat loss would be enough for complete evaporation terrestrial matter, because temperature could rise to 30 000 0 С. But the accretion process was relatively long, and the energy of planetesimal impacts was released only in the near-surface layers of the growing Earth and was quickly lost with thermal radiation, so the initial heating of the planet was not large. The magnitude of this thermal radiation, which goes in parallel with the formation (accretion) of the Earth, is estimated by the indicated authors as 19.4*10 38 erg (19.4*10 31 J)
.
In the modern energy balance of the Earth, accretion heat most likely plays an insignificant role.
THEM. Kapitonov
Earth's nuclear heat
Earth heat
The earth is a rather strongly heated body and is a source of heat. It heats up primarily due to the solar radiation it absorbs. But the Earth also has its own thermal resource comparable to the heat received from the Sun. It is believed that this own energy of the Earth has the following origin. The Earth arose about 4.5 billion years ago following the formation of the Sun from a protoplanetary gas-dust disk rotating around it and condensing. At an early stage of its formation, the earth's substance was heated up due to relatively slow gravitational compression. An important role in the heat balance of the Earth was also played by the energy released during the fall of small cosmic bodies on it. Therefore, the young Earth was molten. Cooling down, it gradually came to its current state with a solid surface, a significant part of which is covered by ocean and sea waters. This hard outer layer is called the earth's crust and on average, on land, its thickness is about 40 km, and under oceanic waters - 5-10 km. The deeper layer of the earth, called mantle also consists of a solid. It extends to a depth of almost 3000 km and contains the bulk of the Earth's matter. Finally, the innermost part of the Earth is its core. It consists of two layers - external and internal. outer core this is a layer of molten iron and nickel at a temperature of 4500-6500 K with a thickness of 2000-2500 km. inner core with a radius of 1000-1500 km is a solid iron-nickel alloy heated to a temperature of 4000-5000 K with a density of about 14 g / cm 3, which arose at a huge (almost 4 million bar) pressure.
In addition to the internal heat of the Earth, inherited from the earliest hot stage of its formation, and the amount of which should decrease with time, there is another, long-term, associated with the radioactive decay of nuclei with a long half-life - first of all, 232 Th, 235 U , 238 U and 40 K. The energy released in these decays - they account for almost 99% of the earth's radioactive energy - constantly replenishes the thermal reserves of the Earth. The above nuclei are contained in the crust and mantle. Their decay leads to heating of both the outer and inner layers of the Earth.
Part of the huge heat contained inside the Earth constantly comes out to its surface, often in very large-scale volcanic processes. The heat flow flowing from the depths of the Earth through its surface is known. It is (47±2)·10 12 watts, which is equivalent to the heat that can be generated by 50 thousand nuclear power plants (the average power of one nuclear power plant is about 10 9 watts). The question arises whether radioactive energy plays any significant role in the total thermal budget of the Earth, and if so, what role? The answer to these questions remained unknown for a long time. Now there are opportunities to answer these questions. The key role here belongs to neutrinos (antineutrinos), which are born in the processes of radioactive decay of nuclei that make up the Earth's matter and which are called geo-neutrino.
Geo-neutrino
Geo-neutrino is the combined name for neutrinos or antineutrinos, which are emitted as a result of the beta decay of nuclei located under the earth's surface. Obviously, due to the unprecedented penetrating ability, registration of these (and only them) by ground-based neutrino detectors can provide objective information about the processes of radioactive decay occurring deep inside the Earth. An example of such a decay is the β - decay of the 228 Ra nucleus, which is the product of the α decay of the long-lived 232 Th nucleus (see table):
The half-life (T 1/2) of the 228 Ra nucleus is 5.75 years, and the released energy is about 46 keV. The energy spectrum of antineutrinos is continuous with an upper limit close to the released energy.
The decays of 232 Th, 235 U, 238 U nuclei are chains of successive decays that form the so-called radioactive series. In such chains, α-decays are interspersed with β − -decays, since in α-decays the final nuclei turn out to be shifted from the β-stability line to the region of nuclei overloaded with neutrons. After a chain of successive decays at the end of each row, stable nuclei are formed with the number of protons and neutrons close to or equal to magic numbers (Z
=
82,N= 126). Such final nuclei are stable isotopes of lead or bismuth. Thus, the decay of T 1/2 ends with the formation of the doubly magic nucleus 208 Pb, and on the path 232 Th → 208 Pb, six α-decays occur, alternating with four β - decays (in the chain 238 U → 206 Pb, eight α- and six β - - decays; there are seven α- and four β − decays in the 235 U → 207 Pb chain). Thus, the energy spectrum of antineutrinos from each radioactive series is a superposition of partial spectra from individual β − decays that make up this series. The spectra of antineutrinos produced in 232 Th, 235 U, 238 U, 40 K decays are shown in Figs. 1. The 40 K decay is a single β − decay (see table). Antineutrinos reach their highest energy (up to 3.26 MeV) in the decay
214 Bi → 214 Po, which is a link in the 238 U radioactive series. The total energy released during the passage of all decay links in the 232 Th → 208 Pb series is 42.65 MeV. For the radioactive series 235 U and 238 U, these energies are 46.39 and 51.69 MeV, respectively. Energy released in decay
40 K → 40 Ca is 1.31 MeV.
Characteristics of 232 Th, 235 U, 238 U, 40 K nuclei
| Core | Share in % in a mixture isotopes |
Number of cores relates. Si nuclei |
T 1/2 billion years |
First links decay |
|---|---|---|---|---|
| 232Th | 100 | 0.0335 | 14.0 | |
| 235 U | 0.7204 | 6.48 10 -5 | 0.704 | |
| 238 U | 99.2742 | 0.00893 | 4.47 | |
| 40K | 0.0117 | 0.440 | 1.25 |
 |
The estimate of the geo-neutrino flux, made on the basis of the decay of the 232 Th, 235 U, 238 U, 40 K nuclei contained in the composition of the Earth's matter, leads to a value of the order of 10 6 cm -2 sec -1 . By registering these geo-neutrinos, one can obtain information about the role of radioactive heat in the total heat balance of the Earth and test our ideas about the content of long-lived radioisotopes in the composition of terrestrial matter.
Rice. 1. Energy spectra of antineutrinos from nuclear decay
232 Th, 235 U, 238 U, 40 K normalized to one decay of the parent nucleus
The reaction is used to register electron antineutrinos
P → e + + n, (1)
in which this particle was actually discovered. The threshold for this reaction is 1.8 MeV. Therefore, only geo-neutrinos formed in decay chains starting from 232 Th and 238 U nuclei can be registered in the above reaction. The effective cross section of the reaction under discussion is extremely small: σ ≈ 10 -43 cm 2. Hence it follows that a neutrino detector with a sensitive volume of 1 m 3 will register no more than a few events per year. Obviously, for reliable fixation of geo-neutrino fluxes, large-volume neutrino detectors are needed, located in underground laboratories for maximum protection from the background. The idea to use detectors designed to study solar and reactor neutrinos for registration of geo-neutrinos arose in 1998. Currently, there are two large volume neutrino detectors using a liquid scintillator and suitable for solving the problem. These are the neutrino detectors of the KamLAND experiments (Japan, ) and Borexino (Italy, ). Below we consider the device of the Borexino detector and the results obtained on this detector on the registration of geo-neutrinos.
Borexino detector and registration of geo-neutrinos
The Borexino neutrino detector is located in central Italy in an underground laboratory under the Gran Sasso mountain range, whose mountain peaks reach 2.9 km (Fig. 2).
Rice. Fig. 2. Location diagram of the neutrino laboratory under the Gran Sasso mountain range (central Italy)
Borexino is a non-segmented massive detector whose active medium is
280 tons of organic liquid scintillator. It filled a nylon spherical vessel 8.5 m in diameter (Fig. 3). The scintillator was pseudocumene (C 9 H 12) with a spectrum-shifting PPO additive (1.5 g/L). The light from the scintillator is collected by 2212 eight-inch photomultipliers (PMTs) placed on a stainless steel sphere (SSS).
Rice. 3. Scheme of the device of the Borexino detector
A nylon vessel with pseudocumene is an internal detector whose task is to register neutrinos (antineutrinos). The inner detector is surrounded by two concentric buffer zones that protect it from external gamma rays and neutrons. The inner zone is filled with a non-scintillating medium consisting of 900 tons of pseudocumene with dimethyl phthalate additives to quench scintillations. The outer zone is located on top of the SNS and is a water Cherenkov detector containing 2000 tons of ultrapure water and cutting off signals from muons entering the facility from outside. For each interaction occurring in the internal detector, energy and time are determined. Calibration of the detector using various radioactive sources made it possible to very accurately determine its energy scale and the degree of reproducibility of the light signal.
Borexino is a very high radiation purity detector. All materials were rigorously selected, and the scintillator was cleaned to minimize the internal background. Because of its high radiation purity, Borexino is an excellent detector for detecting antineutrinos.
In reaction (1), the positron gives an instantaneous signal, which after some time is followed by the capture of a neutron by a hydrogen nucleus, which leads to the appearance of a γ-quantum with an energy of 2.22 MeV, which creates a signal delayed relative to the first one. In Borexino, the neutron capture time is about 260 μs. The instantaneous and delayed signals are correlated in space and time, providing accurate recognition of the event caused by e .
The threshold for reaction (1) is 1.806 MeV and, as can be seen from Fig. 1, all geo-neutrinos from the decays of 40 K and 235 U are below this threshold, and only a part of the geo-neutrinos that originated in the decays of 232 Th and 238 U can be detected.
The Borexino detector first detected signals from geo-neutrinos in 2010 and recently published new results based on observations over 2056 days from December 2007 to March 2015. Below we present the obtained data and the results of their discussion, based on article.
As a result of the analysis of experimental data, 77 candidates for electron antineutrinos that passed all the selection criteria were identified. The background from events simulating e was estimated by . Thus, the signal/background ratio was ≈100.
The main background source was reactor antineutrinos. For Borexino, the situation was quite favorable, since there are no nuclear reactors near the Gran Sasso laboratory. In addition, reactor antineutrinos are more energetic than geo-neutrinos, which made it possible to separate these antineutrinos from the positron by signal strength. The results of the analysis of the contributions of geo-neutrinos and reactor antineutrinos to the total number of recorded events from e are shown in Figs. 4. The number of registered geo-neutrinos given by this analysis (the shaded area corresponds to them in Fig. 4) is equal to 
. In the spectrum of geo-neutrinos extracted as a result of the analysis, two groups are visible - less energetic, more intense and more energetic, less intense. The authors of the described study associate these groups with the decays of thorium and uranium, respectively.
In the analysis under discussion, we used the ratio of the masses of thorium and uranium in the matter of the Earth
m(Th)/m(U) = 3.9 (in the table this value is ≈3.8). This figure reflects the relative content of these chemical elements in chondrites - the most common group of meteorites (more than 90% of meteorites that fell to Earth belong to this group). It is believed that the composition of chondrites, with the exception of light gases (hydrogen and helium), repeats the composition of the solar system and the protoplanetary disk from which the Earth was formed.
Rice. Fig. 4. Spectrum of the light output from positrons in units of the number of photoelectrons for antineutrino candidate events (experimental points). The shaded area is the contribution of geo-neutrinos. The solid line is the contribution of reactor antineutrinos.
