Splitting the nucleus. Great-grandfather of the atomic bomb How to split the atom at home

The splitting of the nuclei of atoms of various elements is currently used quite widely. All nuclear power plants work on the fission reaction; the principle of operation of all nuclear weapons is based on this reaction. In the case of a controlled or chain reaction, the atom, having been divided into parts, can no longer connect back and return to its original state. But, using the principles and laws of quantum mechanics, scientists managed to split the 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 made a single atom exist in two places at once, the distance between which was a little more than one hundredth of a millimeter, which on an atomic scale is just a huge distance.

Such quantum effects can manifest themselves only at extremely low temperatures. The 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 held in an optical trap of 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 by a considerable distance, which scientists managed to realize 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 parted to the sides until they come into contact with adjacent atoms. A kind of roadbed is formed, a span connecting the two pillars of the bridge, through which information can be transmitted. This is possible due to the fact that the atom divided in this way continues to be 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 are going to use this technology to model and create complex quantum systems. "The atom is like a well-oiled gear for us," says Dr Andrea Alberti, leader of the team. “Using many of these gears, you can create a quantum calculator with characteristics that far exceed those of the most advanced computers. You just need to be able to correctly position and connect these gears.”

Nuclear fission is the splitting of a heavy atom into two fragments of approximately equal mass, accompanied by the release of a large amount of energy.

The discovery of nuclear fission began a new era - the "atomic age". The potential of its possible use and the ratio of risk to benefit from its use have not only generated many sociological, political, economic and scientific achievements, but also serious problems. Even from a purely scientific point of view, the process of nuclear fission has created a large number of puzzles and complications, and its full theoretical explanation is a matter of the future.

Sharing is profitable

The binding energies (per nucleon) differ for different nuclei. Heavier ones have lower binding energies than those located in the middle of the periodic table.

This means that for heavy nuclei with an atomic number greater than 100, it is advantageous to divide into two smaller fragments, thereby releasing energy, which is converted into the kinetic energy of the fragments. This process is called splitting

According to the stability curve, which shows the dependence of the number of protons on the number of neutrons for stable nuclides, heavier nuclei prefer more neutrons (compared to the number of protons) than lighter ones. This suggests that along with the splitting process, some "spare" neutrons will be emitted. In addition, they will also take on some of the released energy. The study of nuclear fission of the 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. True, 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 division

The processes of spontaneous splitting 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 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 nucleus must first be deformed (stretched) into an ellipsoidal shape, and then, before finally splitting into two fragments, form a “neck” in the middle.

Potential Barrier

In the deformed state, two forces act on the core. One is the increased surface energy (the surface tension of a liquid drop explains its spherical shape), and the other is the Coulomb repulsion between fission fragments. Together they produce a potential barrier.

As in the case of alpha decay, in order for the spontaneous fission of the uranium atom nucleus to occur, the fragments must overcome this barrier using quantum tunneling. The barrier is about 6 MeV, as in the case of alpha decay, but the probability of tunneling an alpha particle is much greater than that of a much heavier atom fission product.

forced splitting

Much more likely is the induced fission of the uranium nucleus. In this case, the parent nucleus is irradiated with neutrons. If the parent absorbs it, they bind, 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 insufficient to overcome the potential barrier, the incident neutron must have a minimum kinetic energy in order to be able to induce the splitting of an atom. In the case of 238 U, the binding energy of additional neutrons is about 1 MeV short. This means that fission of the 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 the nucleus absorbs an additional one, it forms a pair with it, and as a result of this pairing, additional binding energy appears. 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 emits three or four neutrons, the fragments still contain more neutrons than their stable isobars. This means that cleavage fragments are generally unstable against beta decay.

For example, when uranium 238U fission occurs, the stable isobar with A = 145 is neodymium 145Nd, which means that the lanthanum 145La fragment decays in three steps, each time emitting an electron and an antineutrino, until a stable nuclide is formed. The stable isobar with A = 90 is zirconium 90 Zr; therefore, the bromine 90 Br splitting fragment decomposes in five stages of the β-decay chain.

These β-decay chains release additional energy, which is almost all carried away by electrons and antineutrinos.

Nuclear reactions: fission of uranium nuclei

Direct emission of a neutron from a nuclide with too many of them to ensure the stability of the nucleus is unlikely. The point here is that there is no Coulomb repulsion, and so the surface energy tends to keep the neutron in bond with the parent. However, this sometimes happens. For example, a 90 Br fission fragment in the first beta decay stage produces krypton-90, which can be in an excited state with enough energy to overcome the surface energy. In this case, the emission of neutrons can occur directly with the formation of krypton-89. still unstable with respect to β decay until converted to stable yttrium-89, so that krypton-89 decays in three steps.

Fission of uranium nuclei: a chain reaction

The 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. Nevertheless, at a significant concentration of the rare isotope 235 U, these free neutrons can be captured by 235 U nuclei, which can indeed 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 in stage n of this chain, divided by the number of neutrons produced in stage n - 1. This number will depend on how many neutrons produced in stage n - 1 are absorbed by the nucleus, which may be forced to divide.

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. 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 an increase in the neutron absorption probability, which depends on the radius of the sphere. Therefore, the mass U must exceed a certain amount in order for the fission of uranium nuclei (chain reaction) to occur.

If k = 1, then a controlled reaction takes place. This is used in a process controlled by distributing cadmium or boron rods among the uranium, which absorb most of the neutrons (these elements have the ability to capture neutrons). The fission of the uranium nucleus is automatically controlled by moving the rods in such a way that the value of k remains equal to one.

Choose the appropriate isotope. Some elements or isotopes undergo radioactive decay, and different isotopes may behave differently. The most common isotope of uranium has an atomic weight of 238 and consists of 92 protons and 146 neutrons, but its nuclei usually absorb neutrons without splitting into nuclei of lighter elements. The isotope of uranium, whose nucleus contains three fewer neutrons, 235 U, fissions much more easily than 238 U, and is called a fissile isotope.

• The fission of uranium releases three neutrons that collide with other uranium atoms, resulting in a chain reaction.
• Some isotopes fission so easily and quickly that it is impossible to maintain a constant nuclear reaction. This phenomenon is called spontaneous, or spontaneous, decay. For example, the plutonium isotope 240 Pu is subject to such decay, in contrast to 239 Pu with a lower fission rate.

In order for the reaction to continue after the decay of the first atom, enough isotope must be collected. To do this, it is necessary to have a certain minimum amount of fissile isotope that will support the reaction. This quantity is called the critical mass. Enough starting material is required to reach critical mass and increase the probability of decay.

In 1939Albert Einsteinturned to President Roosevelt with a proposal to make every effort to master the energy of atomic decay before the Nazis. By that time, having emigrated from fascist ItalyEnrico Fermialready worked on this problem at Columbia University.

(In the accelerator chamber of the European Laboratory of Particle Physics (CERN), the largest center of its kind in Europe. Paradoxically, gigantic structures are needed to study the smallest particles.)

Introduction

In 1854 a German Heinrich Geisler. (1814-79) invented a vacuum glass tube with electrodes, called the Heusler tube, and a mercury pump, which made it possible to obtain a high vacuum. By connecting a high-voltage induction coil to the electrodes of the tube, he received a green glow on the glass opposite the negative electrode. In 1876 a German physicist Evgeny Goldstein(1850-1931) suggested that this glow was caused by rays emitted by the cathode, and called these rays cathode.

(New Zealand physicist Ernest Rutherford (1871-1937) at the Cavendish Laboratory at the University of Cambridge, which he took over in 1919.)

Electrons

English scientist William Crooks(1832-1919) improved the Geisler tube and showed the possibility of deflecting cathode rays by a magnetic field. In 1897, another English researcher, Joseph Thomson, suggested that the rays are negatively charged particles, and determined their mass, which turned out to be about 2000 times less than the mass of a hydrogen atom. He called these particles electrons, taking a name suggested years earlier by an Irish physicist. George Stoney(1826-1911), who theoretically calculated the magnitude of their charge. Thus the divisibility of the atom became apparent. Thomson proposed a model in which electrons were embedded in an atom like raisins in a cake. And soon other particles that make up the atom were discovered. Since 1895, he began to work at the Cavendish Laboratory Ernest Rutherford(1871-1937), who, together with Thomson, began to study the radioactivity of uranium and discovered two types of particles emitted by the atoms of this element. He called particles with the charge and mass of an electron beta particles, and others, positively charged, with a mass equal to the mass of 4 hydrogen atoms, alpha particles. In addition, uranium atoms were a source of high-frequency electromagnetic radiation - gamma rays.

(Otto Hahn and Lise Meitner. In 1945, Gan waswas interned by the Allies in England and only there did he learn about the award of the Nobel Prize in Chemistry for 1944 "for the discovery of the fission of heavy nuclei".)

Protons

In 1886, Goldstein discovered another radiation propagating in the direction opposite to the cathode rays, and he called them cathode rays. Later it was proved that they are composed of ions of atoms. Rutherford proposed to name the positive hydrogen ion protone (from the Greekproton- the first), since he considered the hydrogen nucleus to be an integral part of the nuclei of atoms of all other elements. Thus, at the beginning of the XX century. The existence of three subatomic particles was established: the electron, the proton, and the alpha particle. AT1907 Mr. Rutherford became a professor at the University of Manchester. Here, trying to figure out the structure of the atom, he conducted his famous experiments on the scattering of alpha particles. Investigating the passage of these particles through a thin metal foil, he came to the conclusion that in the center of the atom there is a small dense nucleus capable of reflecting alpha particles. Rutherford's assistant at the time was a young Dane physicistNiels Bohr(1885-1962), which in1913 BC, in accordance with the newly created quantum theory, proposed a model of the structure of the atom, known asRutherford-Bohr model. In it, electrons revolve around the nucleus like planets around the sun.

( Enrico Fermi (1901-54) received the Nobel Prize in 1938 for his work on the irradiation of matter with neutrons. In 1942, for the first time, he carried out a self-sustaining chain reaction of the decay of atomic nuclei.)

Atom Models

In this first model, the nucleus consisted of positively charged protons and a number of electrons that partially neutralized their charge; in addition, additional electrons moved around the nucleus, the total charge of which was equal to the positive charge of the nucleus.alpha particles, like the nuclei of helium atoms, should have consisted of4 protons and2 electrons.It's been over10 years before this model was revised. AT1930 German Walter Bothe(1891-1957) announced the discovery of a new type of radioactive radiation arising from the irradiation of beryllium with alpha particles. EnglishmanJames Chadwick(1891-1974) repeated these experiments and came to the conclusion that this radiation consists of particles equal in mass to protons, but without an electric charge. They were called neutrons. Then the GermanWerner Heisenberg(1901-76) proposed a model of an atom, the nucleus of which consisted only of protons and neutrons.Research team with one of the first subatomic particle accelerators -cyclotron(1932). This device is designed to accelerate particles and then bombard them with special targets.

(A group of researchers with one of the first subatomic particle accelerators, the cyclotron (1932). This device is designed to accelerate particles and then bombard them with special targets.)

Atom splitting

Physicists around the world immediately saw in neutrons an ideal tool for influencing atoms - these heavy, chargeless particles easily penetrated into atomic nuclei. AT1934-36 Italy Enrico Fermi(1901-54) received their help37 radioactive isotopes of various elements. Absorbing a neutron, the atomic nucleus became unstable and emitted energy in the form of gamma rays. Fermi irradiated uranium with neutrons, hopingpreturn it into a new element - "uranium" In the same direction of work in Berlin, the German Otto Hahn(1879-1 Sand AustrianLise Meitner(1878 - 1968). AT1938 Meitner, fleeing the Nazis, went to Stockholm, and continued to work withFriedrich Strassmann(1902-80). Soon Hahn and Meitner, continuing the experiment and comparing the results by correspondence, discovered the formation of radioactive barium in uranium irradiated with neutrons. Meitner suggested that I am a uranium atom (atomic number92) racesplits into two nuclei: barium (the atomic number of the element with the number43 later namedtechnetium). Thus, the possibility of splitting the atomic nucleus was discovered. It was also found that during the destruction of the nucleus of the uranium atom,2-3 neutrons, each of which, in turn, is capable of initiating the decay of uranium atoms, causing a chain reaction with the release of a huge amount of energy ...