Nuclear fission is the splitting of a heavy atom into two fragments of approximately equal mass, accompanied by the release large quantity energy.
The discovery of nuclear fission began a new era - the “atomic age.” The potential of its possible use and the risk-to-benefit ratio of its use have not only generated many sociological, political, economic and scientific advances, but also serious problems. Even from a purely scientific point of view, the process of nuclear fission has created large number puzzles and complications, and its full theoretical explanation is a matter for the future.
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Binding energies (per nucleon) differ for different nuclei. Heavier ones have lower binding energy than those located in the middle of the periodic table.
This means that heavy nuclei with an atomic number greater than 100 benefit from splitting into two smaller fragments, thereby releasing energy that is converted into kinetic energy of the fragments. This process is called splitting
According to the stability curve, which shows the number of protons versus the number of neutrons for stable nuclides, heavier nuclei prefer a higher number of neutrons (relative to the number of protons) than lighter nuclei. This suggests that some "spare" neutrons will be emitted along with the fission process. In addition, they will also absorb part of the released energy. A study of the fission of the nucleus of a uranium atom showed that 3-4 neutrons are released: 238 U → 145 La + 90 Br + 3n.
The atomic number (and atomic mass) of the fragment is not equal to half the atomic mass of the parent. The difference between the masses of atoms formed as a result of splitting is usually about 50. However, the reason for this is not yet entirely clear.
The binding energies of 238 U, 145 La and 90 Br are 1803, 1198 and 763 MeV, respectively. This means that as a result of this reaction, the fission energy of the uranium nucleus is released, equal to 1198 + 763-1803 = 158 MeV.
Spontaneous fission
Spontaneous fission processes are known in nature, but they are very rare. The average lifetime of this process is about 10 17 years, and, for example, the average lifetime of the alpha decay of the same radionuclide is about 10 11 years.
The reason for this is that in order to split into two parts, the core must first undergo deformation (stretch) into an ellipsoidal shape, and then, before finally splitting into two fragments, form a “neck” in the middle.
Potential barrier
In a deformed state, two forces act on the core. One is increased surface energy (the surface tension of a liquid drop explains its spherical shape), and the other is Coulomb repulsion between fission fragments. Together they produce a potential barrier.
As in the case of alpha decay, for spontaneous fission of the nucleus of a uranium atom to occur, the fragments must overcome this barrier using quantum tunneling. The barrier magnitude is about 6 MeV, as in the case of alpha decay, but the probability of an alpha particle tunneling is much greater than that of the much heavier atomic fission product.
Forced splitting
Much more likely is the induced fission of the uranium nucleus. In this case, the mother nucleus is irradiated with neutrons. If the parent absorbs it, they bond, releasing binding energy in the form of vibrational energy that can exceed the 6 MeV required to overcome the potential barrier.
Where the energy of the additional neutron is not sufficient to overcome the potential barrier, the incident neutron must have a minimum kinetic energy in order to be able to induce atomic fission. In the case of 238 U, the binding energy of additional neutrons is missing by about 1 MeV. This means that the fission of a uranium nucleus is induced only by a neutron with a kinetic energy greater than 1 MeV. On the other hand, the 235 U isotope has one unpaired neutron. When a nucleus absorbs an additional one, it pairs with it, and this pairing results in additional binding energy. This is enough to release the amount of energy necessary for the nucleus to overcome the potential barrier and the isotope fission occurs upon collision with any neutron.
Beta decay
Even though the fission reaction produces three or four neutrons, the fragments still contain more neutrons than their stable isobars. This means that cleavage fragments tend to be unstable to beta decay.
For example, when the fission of the uranium nucleus 238 U occurs, the stable isobar with A = 145 is neodymium 145 Nd, which means that the lanthanum 145 La fragment decays in three stages, each time emitting an electron and an antineutrino, until a stable nuclide is formed. A stable isobar with A = 90 is zirconium 90 Zr, so the cleavage fragment of bromine 90 Br decays in five stages of the β-decay chain.
These β-decay chains release additional energy, almost all of which is carried away by electrons and antineutrinos.
Nuclear reactions: fission of uranium nuclei
Direct neutron emission from a nuclide with too many neutrons to ensure nuclear stability is unlikely. The point here is that there is no Coulomb repulsion and so the surface energy tends to keep the neutron bound to the parent. However, this happens sometimes. For example, the fission fragment of 90 Br in the first stage of beta decay produces krypton-90, which can be in an excited state with enough energy to overcome the surface energy. In this case, neutron emission can occur directly with the formation of krypton-89. is still unstable to β decay until it becomes stable yttrium-89, so krypton-89 decays in three steps.
Fission of uranium nuclei: chain reaction
Neutrons emitted in the fission reaction can be absorbed by another parent nucleus, which then itself undergoes induced fission. In the case of uranium-238, the three neutrons that are produced come out with an energy of less than 1 MeV (the energy released during the fission of the uranium nucleus - 158 MeV - is mainly converted into the kinetic energy of the fission fragments), so they cannot cause further fission of this nuclide. However, at a significant concentration of the rare isotope 235 U, these free neutrons can be captured by 235 U nuclei, which can actually cause fission, since in this case there is no energy threshold below which fission is not induced.
This is the principle of a chain reaction.
Types of nuclear reactions
Let k be the number of neutrons produced in a sample of fissile material at stage n of this chain, divided by the number of neutrons produced at stage n - 1. This number will depend on how many neutrons produced at stage n - 1 are absorbed by the nucleus that may undergo forced division.
If k< 1, то цепная реакция просто выдохнется и процесс остановится очень быстро. Именно это и происходит в природной в которой концентрация 235 U настолько мала, что вероятность поглощения одного из нейтронов этим изотопом крайне ничтожна.
If k > 1, then the chain reaction will grow until all the fissile material has been used up. This is achieved by enriching natural ore to obtain a sufficiently large concentration of uranium-235. For a spherical sample, the value of k increases with increasing probability of neutron absorption, which depends on the radius of the sphere. Therefore, the mass U must exceed a certain amount so that the fission of uranium nuclei (chain reaction) can occur.
If k = 1, then a controlled reaction takes place. This is used in a process controlled by the distribution among the uranium of cadmium or boron rods that absorb most of neutrons (these elements have the ability to capture neutrons). The fission of the uranium nucleus is controlled automatically by moving the rods so that the value of k remains equal to unity.
It is often said that there are two types of sciences - big sciences and small ones. Splitting the atom is a big science. It has gigantic experimental facilities, colossal budgets and receives the lion's share of Nobel Prizes.
Why did physicists need to split the atom? The simple answer - to understand how the atom works - contains only part of the truth, but there is a more general reason. It is not entirely correct to speak literally about the splitting of the atom. In reality, we are talking about the collision of high-energy particles. In a collision subatomic particles moving at high speeds, a new world of interactions and fields is being born. The fragments of matter carrying enormous anergy, scattering after collisions, conceal the secrets of nature, which from the “creation of the world” remained buried in the depths of the atom.
The installations where high-energy particles collide - particle accelerators - are striking in their size and cost. They reach several kilometers across, making even laboratories that study particle collisions seem tiny in comparison. In other areas of scientific research, the equipment is located in a laboratory; in high-energy physics, laboratories are attached to an accelerator. Recently, the European Center for Nuclear Research (CERN), located near Geneva, allocated several hundred million dollars for the construction of a ring accelerator. The circumference of the tunnel being built for this purpose reaches 27 km. The accelerator, called LEP (Large Electron-Positron ring), is designed to accelerate electrons and their antiparticles (positrons) to speeds that are only “a hair’s breadth” different from the speed of light. To get an idea of the scale of energy, imagine that instead of electrons, a penny coin is accelerated to such speeds. At the end of the acceleration cycle, it would have enough energy to produce $1,000 million worth of electricity! It is not surprising that such experiments are usually classified as “high energy” physics. Moving towards each other inside the ring, beams of electrons and positrons experience head-on collisions, in which the electrons and positrons annihilate, releasing energy sufficient to produce dozens of other particles.
What are these particles? Some of them are the very “building blocks” from which we are built: protons and neutrons that make up atomic nuclei, and electrons orbiting around the nuclei. Other particles are usually not found in the matter around us: their lifespan is extremely short, and after it expires they disintegrate into ordinary particles. The number of varieties of such unstable short-lived particles is amazing: several hundred of them are already known. Like stars, unstable particles are too numerous to be identified by name. Many of them are indicated only by Greek letters, and some are simply numbers.
It is important to keep in mind that all these numerous and varied unstable particles are not literally components of protons, neutrons or electrons. When colliding, high-energy electrons and positrons do not scatter into many subatomic fragments. Even during collisions of high-energy protons, which obviously consist of other objects (quarks), they, as a rule, are not split into their component parts in the usual sense. What happens in such collisions is better viewed as the direct creation of new particles from the energy of the collision.
About twenty years ago, physicists were completely baffled by the number and variety of new subatomic particles, which seemed to have no end. It was impossible to understand why there were so many particles. Perhaps elementary particles are like the inhabitants of a zoo, with their implicit family affiliation, but without any clear taxonomy. Or perhaps, as some optimists have believed, elementary particles hold the key to the universe? What are the particles observed by physicists: insignificant and random fragments of matter or the outlines of a vaguely perceived order emerging before our eyes, indicating the existence of a rich and complex structure of the subnuclear world? Now there is no doubt about the existence of such a structure. There is a deep and rational order to the microworld, and we begin to understand the meaning of all these particles.
It is often said that there are two types of sciences - big sciences and small ones. Splitting the atom is a big science. It has gigantic experimental facilities, colossal budgets and receives the lion's share of Nobel Prizes.
Why did physicists need to split the atom? The simple answer - to understand how the atom works - contains only part of the truth, but there is a more general reason. It is not entirely correct to speak literally about the splitting of the atom. In reality, we are talking about the collision of high-energy particles. When subatomic particles moving at high speeds collide, a new world of interactions and fields is born. The fragments of matter carrying enormous anergy, scattering after collisions, conceal the secrets of nature, which from the “creation of the world” remained buried in the depths of the atom.
The installations where high-energy particles collide - particle accelerators - are striking in their size and cost. They reach several kilometers across, making even laboratories that study particle collisions seem tiny in comparison. In other areas of scientific research, the equipment is located in a laboratory; in high-energy physics, laboratories are attached to an accelerator. Recently, the European Center for Nuclear Research (CERN), located near Geneva, allocated several hundred million dollars for the construction of a ring accelerator. The circumference of the tunnel being built for this purpose reaches 27 km. The accelerator, called LEP (Large Electron-Positron ring), is designed to accelerate electrons and their antiparticles (positrons) to speeds that are only “a hair’s breadth” different from the speed of light. To get an idea of the scale of energy, imagine that instead of electrons, a penny coin is accelerated to such speeds. At the end of the acceleration cycle, it would have enough energy to produce $1,000 million worth of electricity! It is not surprising that such experiments are usually classified as “high energy” physics. Moving towards each other inside the ring, beams of electrons and positrons experience head-on collisions, in which the electrons and positrons annihilate, releasing energy sufficient to produce dozens of other particles.
What are these particles? Some of them are the very “building blocks” from which we are built: protons and neutrons that make up atomic nuclei, and electrons orbiting around the nuclei. Other particles are usually not found in the matter around us: their lifespan is extremely short, and after it expires they disintegrate into ordinary particles. The number of varieties of such unstable short-lived particles is amazing: several hundred of them are already known. Like stars, unstable particles are too numerous to be identified by name. Many of them are indicated only by Greek letters, and some are simply numbers.
It is important to keep in mind that all these numerous and varied unstable particles are by no means literally components protons, neutrons or electrons. When colliding, high-energy electrons and positrons do not scatter into many subatomic fragments. Even during collisions of high-energy protons, which obviously consist of other objects (quarks), they, as a rule, are not split into their component parts in the usual sense. What happens in such collisions is better viewed as the direct creation of new particles from the energy of the collision.
About twenty years ago, physicists were completely baffled by the number and variety of new subatomic particles, which seemed to have no end. It was impossible to understand For what so many particles. Perhaps elementary particles are like the inhabitants of a zoo, with their implicit family affiliation, but without any clear taxonomy. Or perhaps, as some optimists have believed, elementary particles hold the key to the universe? What are the particles observed by physicists: insignificant and random fragments of matter or the outlines of a vaguely perceived order emerging before our eyes, indicating the existence of a rich and complex structure of the subnuclear world? Now there is no doubt about the existence of such a structure. There is a deep and rational order to the microworld, and we begin to understand the meaning of all these particles.
The first step towards understanding the microworld was made as a result of the systematization of all known particles, just as in the 18th century. biologists compiled detailed catalogs of plant and animal species. The most important characteristics of subatomic particles include mass, electric charge, and spin.
Because mass and weight are related, particles with high mass are often called “heavy.” Einstein's relation E =mc^ 2 indicates that the mass of a particle depends on its energy and, therefore, on its speed. A particle in motion is heavier than a particle at rest. When they talk about the mass of a particle, they mean it rest mass, since this mass does not depend on the state of motion. A particle with zero rest mass moves at the speed of light. The most obvious example of a particle with zero rest mass is the photon. It is believed that the electron is the lightest particle with a non-zero rest mass. The proton and neutron are nearly 2,000 times heavier, while the heaviest particle created in the laboratory (the Z particle) is about 200,000 times the mass of the electron.
The electric charge of particles varies in a rather narrow range, but, as we noted, it is always a multiple of the fundamental unit of charge. Some particles, such as photons and neutrinos, have no electrical charge. If the charge of a positively charged proton is taken to be +1, then the charge of the electron is -1.
In ch. 2 we introduced another characteristic of particles – spin. It also always takes values that are multiples of some fundamental unit, which for historical reasons is chosen to be 1 /2. Thus, a proton, neutron and electron have a spin 1/2, and the photon spin is 1. Particles with spin 0, 3/2 and 2 are also known. Fundamental particles with a spin greater than 2 have not been found, and theorists believe that particles with such spins do not exist.
The spin of a particle is an important characteristic, and depending on its value, all particles are divided into two classes. Particles with spins 0, 1 and 2 are called “bosons” - after the Indian physicist Chatyendranath Bose, and particles with half-integer spin (i.e. with spin 1/2 or 3/2 - “fermions” in honor of Enrico Fermi. Belonging to one of these two classes is probably the most important in the list of characteristics of a particle.
Another important characteristic of a particle is its lifetime. Until recently, it was believed that electrons, protons, photons and neutrinos were absolutely stable, i.e. have an infinitely long lifetime. A neutron remains stable while it is “locked” in the nucleus, but a free neutron decays in about 15 minutes. All other known particles are highly unstable, with lifetimes ranging from a few microseconds to 10-23 seconds. Such time intervals seem incomprehensible small, but we should not forget that a particle flying at a speed close to the speed of light (and most particles born at accelerators move at precisely such speeds) manages to fly a distance of 300 m in a microsecond.
Unstable particles undergo decay, which is a quantum process, and therefore there is always an element of unpredictability in the decay. The lifespan of a particular particle cannot be predicted in advance. Based on statistical considerations, only the average lifetime can be predicted. Usually they talk about the half-life of a particle - the time during which the population of identical particles is reduced by half. The experiment shows that the decrease in population size occurs exponentially (see Fig. 6) and the half-life is 0.693 of the average life time.
It is not enough for physicists to know that this or that particle exists; they strive to understand what its role is. The answer to this question depends on the properties of particles listed above, as well as on the nature of the forces acting on the particle from outside and inside it. First of all, the properties of a particle are determined by its ability (or inability) to participate in strong interactions. Particles participating in strong interactions form a special class and are called androns. Particles that participate in weak interactions and do not participate in strong interactions are called leptons, which means “lungs”. Let's take a brief look at each of these families.
Splitting the nuclei of atoms of various elements is currently used quite widely. All nuclear power plants operate on the fission reaction; the operating principle of all nuclear weapons is based on this reaction. In the case of a controlled or chain reaction, the atom, having split into parts, can no longer join back and return to its original state. But, using principles and laws quantum mechanics Scientists managed to split an atom into two halves and connect them again without violating the integrity of the atom itself.
Scientists from the University of Bonn used the principle of quantum uncertainty, which allows objects to exist in several states at once. In the experiment, with the help of some physical tricks, scientists forced a single atom to exist in two places at once, the distance between them was a little more than one hundredth of a millimeter, which on the atomic scale is simply a huge distance.
Such quantum effects can only appear at extremely low temperatures. A cesium atom was cooled by laser light to a temperature of one-tenth of one millionth of a degree above absolute zero. The cooled atom was then optically trapped by a beam of light from another laser.
It is known that the nucleus of an atom can rotate in one of two directions; depending on the direction of rotation, the laser light pushes the nucleus to the right or to the left. “But an atom, in a certain quantum state, can have a “split personality”, one half of it rotates in one direction, the other in the opposite direction. But, at the same time, the atom is still a whole object,” says physicist Andreas Steffen. Thus, the nucleus of an atom, parts of which rotate in opposite directions, can be split into two parts by a laser beam, and these parts of the atom can be separated over a considerable distance, which is what scientists managed to achieve during their experiment.
Scientists claim that using a similar method, it is possible to create so-called “quantum bridges”, which are conductors of quantum information. An atom of a substance is divided into halves, which are moved apart until they come into contact with adjacent atoms. Something like a roadbed is formed, a span connecting two pillars of a bridge, along which information can be transmitted. This is possible due to the fact that an atom divided in this way continues to remain a single whole at the quantum level due to the fact that the parts of the atom are entangled at the quantum level.
Scientists at the University of Bonn intend to use such technology to simulate and create complex quantum systems. “For us, the atom is like a well-oiled gear,” says Dr Andrea Alberti, the team leader. “Using many of these gears, you can create a quantum computing device with characteristics that far exceed those of the most advanced computers. You just need to be able to correctly position and connect these gears.”
November 26, 1894. The wedding of Russian Tsar Nicholas II and German Princess Alice of Hesse-Darmstadt took place in St. Petersburg. After the wedding, the emperor's wife accepted the Orthodox faith and received the name Alexandra Feodorovna.
November 27, 1967. The Moscow cinema "Mir" hosted the premiere of the first Soviet thriller "Viy". The main roles were played by Leonid Kuravlev and Natalia Varley. Filming took place in the Ivano-Frankivsk region and the village of Sednev in the Chernihiv region.
November 28, 1942 Soviet Union concluded an agreement with France on a joint fight against Nazi Germany in the skies. The first French aviation squadron "Normandie-Niemen" consisted of 14 pilots and 17 technical workers.
November 29, 1812 Napoleon's army was defeated while crossing the Berezina River. Napoleon lost about 35 thousand people. Losses of Russian troops, according to the inscription on the 25th wall of the gallery military glory Cathedral of Christ the Savior, amounted to 4 thousand soldiers. Almost 10 thousand French were captured by the Russian general Peter Wittgenstein.
December 1, 1877 In the village of Markovka, Vinnytsia region, Nikolai Leontovich, a Ukrainian composer, choral conductor, author of the songs “Dudarik”, “The Cossack is Carrying”, “Little Mother of One Daughter”, “Shchedrik” (the song is known in the West as the Christmas carol of the bells (“Carol of the Bells").
December 1, 1991. An all-Ukrainian referendum took place on the issue of state independence of Ukraine. Leonid Kravchuk was elected the first president of the country.
December 2, 1942. Physicist Enrico Fermi and a group of American scientists from the University of Chicago carried out a controlled nuclear reaction, splitting an atom for the first time.
On December 1, 1992, the Ukrainian domain UA was registered in the international database
Among the former Soviet republics, Ukraine became the first country to receive a national Internet domain on December 1, 1992. Russia was registered later: the RU domain appeared on April 7, 1994. In the same year, the Republic of Belarus - BY, Armenia - AM and Kazakhstan - KZ received their domains. And the first national domain in the history of the Internet was the American US, it was registered in March 1985. At the same time, the domains of Great Britain - UK and Israel - IL appeared. The creation of a domain system made it possible to immediately understand where it was located by the name of the site.
In January 1993, at a conference of Ukrainian Internet specialists in the village of Slavskoye, Lviv region, 27 domains were proposed, created on a geographical basis, selected by telephone numbering code. Ukrainian cities and enterprises have the opportunity to create their own websites on the Internet, for example, kiev.ua, crimea.ua, dnepropetrovsk.ua. All responsibilities for their administration continued to be performed by individuals on a voluntary basis. In some public domains this practice continues to this day. Now each national or geographic domain has its own administrator - a company or individual who determines the registration rules. Over time, the Internet gave birth to its own version of the language. Domain name, which ends with the abbreviation COM, NET, EDU, stands for the abbreviation general concept. For example, COM is commercial, NET is network, EDU is educational. In our country, the most popular domain is COM. In the spring of 2001, in order to restore order, it was finally created legal entity Hostmaster LLC, which included administrators of UA and other Ukrainian domains. Individuals, the former owners of the Ukrainian domain UA, officially transferred part of the powers to “Hostmaster”.
Nowadays anyone can create their own website and get a domain. The first stage, during which only trademark owners could register domains in the UA zone, has already ended. Since 2010, free domain registration is available for anyone for a period of ten years; the price of using a domain for one year is 90 hryvnia. By the way, the writer, philosopher and public figure of the 19th century, Vladimir Odoevsky, was the first to predict the Internet. In the novel “Year 4338,” published in 1837, Odoevsky wrote: “ Magnetic telegraphs are installed between familiar houses, through which those living at a great distance communicate with each other." Now, by opening a website on the Internet without leaving home, each of us can buy an air and train ticket, make purchases at an electronics supermarket, publish our works without intermediaries, and even find a life partner on a dating site. Twenty-year-olds can hardly imagine an era when they went to the library to buy books, letters were written by hand, and news was learned only from television programs or printed publications.
