Uranium nuclei fission occurs in the following way: First, a neutron hits the nucleus, like a bullet hitting an apple. In the case of an apple, a bullet would either make a hole in it or blow it into pieces. When a neutron hits a nucleus, it is captured nuclear forces. The neutron is known to be neutral, so it is not repelled by electrostatic forces.
How does a uranium nucleus fission occur?
So, having entered the nucleus, the neutron disturbs the equilibrium, and the nucleus is excited. It stretches out to the sides like a dumbbell or an infinity sign: ∞ . Nuclear forces, as is known, act at a distance commensurate with the size of the particles. When the core is stretched, the action of nuclear forces becomes insignificant for the outer particles of the “dumbbell”, while electrical forces They act very powerfully at such a distance, and the core simply breaks into two parts. In this case, two or three more neutrons are emitted.
Fragments of the nucleus and released neutrons scatter at great speed in different directions. The fragments slow down quite quickly environment, however, their kinetic energy is enormous. It transforms into internal energy environment that heats up. In this case, the amount of energy released is enormous. The energy obtained from the complete fission of one gram of uranium is approximately equal to the energy obtained from burning 2.5 tons of oil.
Chain reaction of fission of several nuclei
We looked at the fission of one uranium nucleus. During fission, several (usually two or three) neutrons are released. They fly apart at great speed and can easily get into the nuclei of other atoms, causing a fission reaction in them. This is a chain reaction.
That is, the neutrons obtained as a result of nuclear fission excite and force other nuclei to fission, which in turn themselves emit neutrons, which continue to stimulate further fission. And so on until fission of all uranium nuclei in the immediate vicinity occurs.
In this case, a chain reaction can occur avalanche-like, for example, in the event of an atomic bomb explosion. The number of nuclear fissions increases in geometric progression in a short period of time. However, a chain reaction can also occur with attenuation.
The fact is that not all neutrons meet nuclei on their way, which they induce to fission. As we remember, inside a substance the main volume is occupied by the void between the particles. Therefore, some neutrons fly through all matter without colliding with anything along the way. And if the number of nuclear fissions decreases over time, then the reaction gradually fades.
Nuclear reactions and critical mass of uranium
What determines the type of reaction? From the mass of uranium. How more mass- the more particles the flying neutron encounters on its path and the more chances it has to get into the nucleus. Therefore, they distinguish “ critical mass"Uranium is the minimum mass at which a chain reaction can occur.
The number of neutrons produced will be equal to the number of neutrons that fly out. And the reaction will proceed with approximately same speed until the entire volume of the substance is exhausted. This is used in practice nuclear power plants and is called a controlled nuclear reaction.
In 1934, E. Fermi decided to obtain transuranium elements by irradiating 238 U with neutrons. E. Fermi's idea was that as a result of the β - decay of the isotope 239 U, chemical element with serial number Z = 93. However, it was not possible to identify the formation of the 93rd element. Instead, as a result of radiochemical analysis of radioactive elements carried out by O. Gan and F. Strassmann, it was shown that one of the products of irradiation of uranium with neutrons is barium (Z = 56) - a chemical element of medium atomic weight, while according to the assumption of Fermi's theory, transuranium elements should have been obtained.
L. Meitner and O. Frisch suggested that as a result of the capture of a neutron by a uranium nucleus, the compound nucleus collapses into two parts
92 U + n → 56 Ba + 36 Kr + xn.
The fission process of uranium is accompanied by the appearance of secondary neutrons (x > 1), capable of causing the fission of other uranium nuclei, which opens up the potential for a fission chain reaction to occur - one neutron can give rise to a branched chain of fission of uranium nuclei. In this case, the number of fissioned nuclei should increase exponentially. N. Bohr and J. Wheeler calculated the critical energy required for the 236 U nucleus, formed as a result of neutron capture by the 235 U isotope, to split. This value is 6.2 MeV, which is less than the excitation energy of the 236 U isotope formed during the capture of a thermal neutron by 235 U. Therefore, when thermal neutrons are captured, a fission chain reaction of 235 U is possible. For the most common isotope 238 U, the critical energy is 5.9 MeV, while when a thermal neutron is captured, the excitation energy of the resulting 239 U nucleus is only 5.2 MeV. Therefore, the chain reaction of fission of the most common isotope in nature, 238 U, under the influence of thermal neutrons turns out to be impossible. In one fission event, energy is released ≈ 200 MeV (for comparison in chemical reactions combustion in one reaction event releases energy ≈ 10 eV). The possibility of creating conditions for a fission chain reaction has opened up prospects for using the energy of the chain reaction to create atomic reactors and atomic weapons. The first nuclear reactor was built by E. Fermi in the USA in 1942. In the USSR, the first nuclear reactor was launched under the leadership of I. Kurchatov in 1946. In 1954, the world's first nuclear power plant began operating in Obninsk. Currently, electrical energy is generated in approximately 440 nuclear reactors in 30 countries.
In 1940, G. Flerov and K. Petrzhak discovered the spontaneous fission of uranium. The complexity of the experiment is evidenced by the following figures. The partial half-life in relation to the spontaneous fission of the 238 U isotope is 10 16 –10 17 years, while the decay period of the 238 U isotope is 4.5∙10 9 years. The main decay channel of the 238 U isotope is α decay. In order to observe the spontaneous fission of the 238 U isotope, it was necessary to register one fission event against a background of 10 7 –10 8 α-decay events.
The probability of spontaneous fission is mainly determined by the permeability of the fission barrier. The probability of spontaneous fission increases with increasing nuclear charge, because in this case, the division parameter Z 2 /A increases. In isotopes Z< 92-95
деление происходит преимущественно с образованием двух осколков деления с
отношением масс тяжёлого и лёгкого осколков 3:2. В изотопах
Z >100, symmetrical fission predominates with the formation of fragments of equal mass. As the nuclear charge increases, the proportion of spontaneous fission compared to α-decay increases.
| Isotope | Half life | Decay channels |
|---|---|---|
| 235 U | 7.04·10 8 years | α (100%), SF (7·10 -9%) |
| 238 U | 4.47 10 9 years | α (100%), SF (5.5·10 -5%) |
| 240 Pu | 6.56·10 3 years | α (100%), SF (5.7·10 -6%) |
| 242 Pu | 3.75 10 5 years | α (100%), SF (5.5·10 -4%) |
| 246 cm | 4.76·10 3 years | α (99.97%), SF (0.03%) |
| 252 Cf | 2.64 years | α (96.91%), SF (3.09%) |
| 254 Cf | 60.5 years | α (0.31%), SF (99.69%) |
| 256 Cf | 12.3 years | α (7.04·10 -8%), SF (100%) |
Nuclear fission. Story
1934- E. Fermi, irradiating uranium with thermal neutrons, discovered radioactive nuclei among the reaction products, the nature of which could not be determined.
L. Szilard put forward the idea of a nuclear chain reaction.
1939− O. Hahn and F. Strassmann discovered barium among the reaction products.
L. Meitner and O. Frisch were the first to announce that under the influence of neutrons, uranium was divided into two fragments of comparable mass.
N. Bohr and J. Wheeler gave a quantitative interpretation of nuclear fission by introducing the fission parameter.
Ya. Frenkel developed the drop theory of nuclear fission by slow neutrons.
L. Szilard, E. Wigner, E. Fermi, J. Wheeler, F. Joliot-Curie, Y. Zeldovich, Y. Khariton substantiated the possibility of a nuclear fission chain reaction occurring in uranium.
1940− G. Flerov and K. Pietrzak discovered the phenomenon of spontaneous fission of uranium U nuclei.
1942− E. Fermi carried out a controlled fission chain reaction in the first atomic reactor.
1945− First test nuclear weapons(Nevada, USA). On Japanese cities Atomic bombs were dropped by American troops in Hiroshima (August 6) and Nagasaki (August 9).
1946− Under the leadership of I.V. Kurchatov, the first reactor in Europe was launched.
1954− The world's first nuclear power plant was launched (Obninsk, USSR).
Nuclear fission.Since 1934, E. Fermi began to use neutrons to bombard atoms. Since then, the number of stable or radioactive nuclei obtained by artificial transformation has increased to many hundreds, and almost all places in the periodic table have been filled with isotopes.
The atoms arising in all these nuclear reactions occupied the same place in the periodic table as the bombarded atom, or neighboring places. Therefore, the proof by Hahn and Strassmann in 1938 that when bombarded with neutrons at the last element of the periodic table created a great sensation−uranium−decomposition occurs into elements that are in the middle parts of the periodic table. They perform here different kinds decay. The resulting atoms are mostly unstable and immediately decay further; some have half-lives measured in seconds, so Gan had to use analytical method Curie to prolong such a rapid process. It is important to note that the upstream elements of uranium, protactinium and thorium, also exhibit similar decay when exposed to neutrons, although it takes longer for the decay to begin. high energy neutrons than in the case of uranium. Along with this, in 1940, G. N. Flerov and K. A. Petrzhak discovered the spontaneous fission of a uranium nucleus with the largest half-life known until then: about 2· 10 15 years; this fact becomes clear due to the neutrons released during this process. This made it possible to understand why the “natural” periodic system ends with the three named elements. Transuranic elements have now become known, but they are so unstable that they decay quickly.
The fission of uranium by means of neutrons now makes it possible to use atomic energy, which many have already imagined as “the dream of Jules Verne.”
M. Laue, “History of Physics”
1939 O. Hahn and F. Strassmann, irradiating uranium salts with thermal neutrons, discovered barium (Z = 56) among the reaction products
 Otto Gann (1879 – 1968) |
Nuclear fission is the splitting of a nucleus into two (less often three) nuclei with similar masses, which are called fission fragments. During fission, other particles also appear - neutrons, electrons, α-particles. As a result of fission, energy of ~200 MeV is released. Fission can be spontaneous or forced under the influence of other particles, most often neutrons.
Characteristic feature fission is that fission fragments, as a rule, differ significantly in mass, i.e., asymmetric fission predominates. Thus, in the case of the most probable fission of the uranium isotope 236 U, the ratio of the masses of the fragments is 1.46. The heavy fragment has a mass number of 139 (xenon), and the light fragment has a mass number of 95 (strontium). Taking into account the emission of two prompt neutrons, the fission reaction under consideration has the form
Nobel Prize in Chemistry
1944 – O. Gan.
For the discovery of the fission reaction of uranium nuclei by neutrons.
Fission fragments
Dependence of averages masses light and heavy groups of fragments from the mass of the fissile nucleus.
Discovery of nuclear fission. 1939
I arrived in Sweden, where Lise Meitner was suffering from loneliness, and I, like a devoted nephew, decided to visit her for Christmas. She lived in the small hotel Kungälv near Gothenburg. I found her at breakfast. She thought about the letter she had just received from Gan. I was very skeptical about the contents of the letter, which reported the formation of barium when uranium was irradiated with neutrons. However, she was attracted by the opportunity. We walked in the snow, she on foot, I on skis (she said that she could make it this way without falling behind me, and she proved it). By the end of the walk we could already formulate some conclusions; the core did not split, and pieces did not fly off from it, but this was a process that was more reminiscent of Bohr’s droplet model of the nucleus; like a drop, the nucleus could elongate and divide. I then researched how electric charge decreases nucleons surface tension, which, as I was able to establish, drops to zero at Z = 100 and is probably quite small for uranium. Lise Meitner worked to determine the energy released during each decay due to a mass defect. She was very clear about the mass defect curve. It turned out that due to electrostatic repulsion, the fission elements would acquire an energy of about 200 MeV, and this exactly corresponded to the energy associated with the mass defect. Therefore, the process could proceed purely classically without involving the concept of passing through a potential barrier, which, of course, would be useless here.
We spent two or three days together over Christmas. Then I returned to Copenhagen and barely had time to inform Bohr about our idea at the very moment when he was already boarding a ship departing for the USA. I remember how he slapped his forehead as soon as I began to speak and exclaimed: “Oh, what fools we were! We should have noticed this earlier." But he didn't notice, and no one noticed.
Lise Meitner and I wrote an article. At the same time, we constantly kept in touch by long-distance telephone from Copenhagen to Stockholm.
O. Frisch, Memoirs. UFN. 1968. T. 96, issue 4, p. 697.
Spontaneous nuclear fission
In the experiments described below, we used the method first proposed by Frisch for recording nuclear fission processes. An ionization chamber with plates coated with a layer of uranium oxide is connected to a linear amplifier configured in such a way that α particles emitted from the uranium are not detected by the system; impulses from fragments, much larger in magnitude than impulses from α-particles, unlock the output thyratron and are considered a mechanical relay.
An ionization chamber was specially designed in the form of a multilayer flat capacitor with with total area 15 plates in 1000 cm. The plates, located at a distance of 3 mm from each other, were coated with a layer of uranium oxide 10-20 mg/cm 2
.
In the very first experiments with an amplifier configured for counting fragments, it was possible to observe spontaneous (in the absence of a neutron source) pulses on a relay and an oscilloscope. The number of these pulses was small (6 in 1 hour), and it is therefore understandable that this phenomenon could not be observed with cameras of the usual type...
We tend to think that the effect we observed should be attributed to fragments resulting from the spontaneous fission of uranium...
Spontaneous fission should be attributed to one of the unexcited U isotopes with half-lives obtained from an evaluation of our results:
U 238
– 10
16
~ 10
17
years,
U 235
– 10
14
~ 10
15
years,
U 234
– 10
12
~ 10
13
years.
Isotope decay 238 U
Spontaneous nuclear fission
Half-lives of spontaneously fissile isotopes Z = 92 - 100
First experimental system with a uranium-graphite lattice was built in 1941 under the leadership of E. Fermi. It was a graphite cube with an edge 2.5 m long, containing about 7 tons of uranium oxide, enclosed in iron vessels, which were placed in the cube at equal distances from each other. A RaBe neutron source was placed at the bottom of the uranium-graphite lattice. The reproduction coefficient in such a system was ≈ 0.7. Uranium oxide contained from 2 to 5% impurities. Further efforts were aimed at obtaining purer materials and by May 1942, uranium oxide was obtained, in which the impurity was less than 1%. To ensure a fission chain reaction, it was necessary to use a large number of graphite and uranium - about several tons. The impurities had to be less than a few parts per million. The reactor, assembled by the end of 1942 by Fermi at the University of Chicago, had the shape of an incomplete spheroid cut off from above. It contained 40 tons of uranium and 385 tons of graphite. On the evening of December 2, 1942, after the neutron absorber rods were removed, it was discovered that a nuclear chain reaction was occurring inside the reactor. The measured coefficient was 1.0006. Initially, the reactor operated at a power level of 0.5 W. By December 12, its power was increased to 200 watts. Subsequently, the reactor was moved to a more safe place, and its power was increased to several kW. At the same time, the reactor consumed 0.002 g of uranium-235 per day.
The first nuclear reactor in the USSR
The building for the first nuclear research reactor in the USSR, F-1, was ready by June 1946.
After all the necessary experiments have been carried out, a control and protection system for the reactor has been developed, the dimensions of the reactor have been established, all the necessary experiments have been carried out with reactor models, the neutron density has been determined on several models, graphite blocks have been obtained (the so-called nuclear purity) and (after neutron-physical checks) uranium blocks, in November 1946 they began construction of the F-1 reactor.
Total radius The reactor was 3.8 m long. It required 400 tons of graphite and 45 tons of uranium. The reactor was assembled in layers and at 15:00 on December 25, 1946, the last, 62nd layer was assembled. After removing the so-called emergency rods, the control rod was raised, the neutron density count began, and at 18:00 on December 25, 1946, the first reactor in the USSR came to life and started working. It was an exciting victory for the scientists - the creators of the nuclear reactor and everything Soviet people. And a year and a half later, on June 10, 1948, the industrial reactor with water in the channels reached a critical state and soon the industrial production of a new type of nuclear fuel, plutonium, began.
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.
Opening nuclear fission the beginning of a new era - the “atomic age”. The potential of its possible use and the risk-to-benefit ratio of its use not only gave rise to many sociological, political, economic and scientific achievements, but also serious problems. Even with clean scientific point view the process of nuclear fission created big number puzzles and complications, and its full theoretical explanation is a matter for the future.
Sharing is profitable
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 that have atomic number more than 100, it is advantageous to divide into two smaller fragments, thereby releasing energy, which turns into 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 are preferred larger number neutrons (compared to the number of protons) than lighter ones. 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 atomic mass 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. Average life time the specified 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 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 with the help of quantum tunneling. The barrier value 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 a 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 until enough high concentration 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 nuclear reactors. The process is controlled by the distribution of 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 controlled automatically by moving the rods so that the value of k remains equal to unity.
The energy E released during fission increases with increasing Z 2 /A. The value of Z 2 /A = 17 for 89 Y (yttrium). Those. fission is energetically favorable for all nuclei heavier than yttrium. Why are most nuclei resistant to spontaneous fission? To answer this question, it is necessary to consider the division mechanism.
During the process of fission, the shape of the nucleus changes. The core sequentially passes through next stages(Fig. 7.1): a ball, an ellipsoid, a dumbbell, two pear-shaped fragments, two spherical fragments. How does the potential energy of the nucleus change by various stages divisions?
Initial core with magnification r takes the form of an increasingly elongated ellipsoid of revolution. In this case, due to the evolution of the shape of the nucleus, the change in its potential energy is determined by the change in the sum of the surface and Coulomb energies E p + E k. In this case, the surface energy increases as the surface area of the nucleus increases. The Coulomb energy decreases as the average distance between protons increases. If, under slight deformation, characterized by a small parameter , the original core has taken the shape of an axially symmetrical ellipsoid, the surface energy E" p and the Coulomb energy E" k as functions of the deformation parameter change as follows:
In ratios (7.4–7.5) E n and E k are the surface and Coulomb energies of the initial spherically symmetric nucleus.
In the region of heavy nuclei 2E p > E k and the sum of the surface and Coulomb energies increases with increasing . From (7.4) and (7.5) it follows that at small deformations the growth of surface energy is prevented further change the shape of the nucleus, and therefore division.
Relationship (7.5) is valid for small deformations. If the deformation is so great that the core takes the shape of a dumbbell, then the surface and Coulomb forces tend to separate the core and give the fragments a spherical shape. Thus, with a gradual increase in the deformation of the nucleus, its potential energy passes through a maximum. A graph of changes in the surface and Coulomb energies of the nucleus depending on r is shown in Fig. 7.2.
The presence of a potential barrier prevents the instantaneous spontaneous fission of nuclei. In order for a nucleus to split, it needs to impart an energy Q that exceeds the height of the fission barrier H. The maximum potential energy of a fissioning nucleus E + H (for example gold) into two identical fragments is ≈ 173 MeV, and the amount of energy E released during fission is 132 MeV . Thus, when a gold nucleus fissions, it is necessary to overcome a potential barrier with a height of about 40 MeV.
The height of the fission barrier H is greater, the higher less attitude Coulomb and surface energy E k /E p in the initial nucleus. This ratio, in turn, increases with increasing division parameter Z 2 /A (7.3). The heavier the nucleus, the less height fission barrier H, since the fission parameter, assuming that Z is proportional to A, increases with increasing mass number:
| E k /E p = (a 3 Z 2)/(a 2 A) ~ A. | (7.6) |
Therefore, heavier nuclei generally need to impart less energy to cause nuclear fission.
The height of the fission barrier vanishes at 2E p – E k = 0 (7.5). In this case
2E p /E k = 2(a 2 A)/(a 3 Z 2),
Z 2 /A = 2a 2 /(a 3 Z 2) ≈ 49.
Thus, according to the droplet model, nuclei with Z 2 /A > 49 cannot exist in nature, since they must almost instantly, within a characteristic nuclear time of the order of 10–22 s, spontaneously split into two fragments. The dependences of the shape and height of the potential barrier H, as well as the fission energy on the value of the parameter Z 2 /A are shown in Fig. 7.3.
Rice. 7.3. Radial dependence of the shape and height of the potential barrier and fission energy E at various values of the parameter Z 2 /A. On vertical axis the value E p + E k is plotted.
Spontaneous fission of nuclei with Z 2 /A< 49,
для которых высота барьера H
не равна нулю, с точки зрения classical physics impossible. However, in quantum mechanics such fission is possible due to the tunnel effect - the passage of fission fragments through a potential barrier. It is called spontaneous fission. The probability of spontaneous fission increases with increasing fission parameter Z 2 /A, i.e., with decreasing height of the fission barrier. In general, the period of spontaneous fission decreases when moving from lighter to heavier nuclei from T 1/2 > 10 21 years for 232 Th to 0.3 s for 260 Rf.
Forced fission of nuclei with Z 2 /A< 49
может быть вызвано их возбуждением фотонами, нейтронами, протонами, дейтронами,
a частицами и другими частицами, если вносимая в
ядро энергия достаточна для преодоления барьера деления.
The minimum value of the excitation energy of a compound nucleus E* formed during neutron capture is equal to the neutron binding energy in this nucleus ε n. Table 7.1 compares the barrier height H and the neutron binding energy ε n for the Th, U, and Pu isotopes formed after neutron capture. The binding energy of a neutron depends on the number of neutrons in the nucleus. Due to the pairing energy, the binding energy of an even neutron is greater than the binding energy of an odd neutron.
Table 7.1
Fission barrier height H, neutron binding energy ε n
| Isotope | Fission barrier height H, MeV | Isotope | Neutron binding energy ε n |
|---|---|---|---|
| 232 Th | 5.9 | 233Th | 4.79 |
| 233 U | 5.5 | 234 U | 6.84 |
| 235 U | 5.75 | 236U | 6.55 |
| 238 U | 5.85 | 239U | 4.80 |
| 239 Pu | 5.5 | 240 Pu | 6.53 |
A characteristic feature of fission is that the fragments, as a rule, have various masses. In the case of the most probable fission of 235 U, the mass ratio of the fragments is on average ~ 1.5. The mass distribution of fragments from the fission of 235 U by thermal neutrons is shown in Fig. 7.4. For the most probable fission, the heavy fragment has a mass number of 139, the light one - 95. Among the fission products there are fragments with A = 72 - 161 and Z = 30 - 65. The probability of fission into two fragments of equal mass is not zero. When 235 U is fissioned by thermal neutrons, the probability of symmetric fission is approximately three orders of magnitude less than in the case of the most probable fission into fragments with A = 139 and 95.
Asymmetric division is explained by the shell structure of the nucleus. The nucleus tends to split in such a way that the main part of the nucleons of each fragment forms the most stable magical skeleton.
The ratio of the number of neutrons to the number of protons in the 235 U nucleus N/Z = 1.55, while for stable isotopes with a mass number close to the mass number of fragments, this ratio is 1.25 − 1.45. Consequently, fission fragments turn out to be heavily overloaded with neutrons and must be
β - radioactive. Therefore, fission fragments experience successive β - decays, and the charge of the primary fragment can change by 4 − 6 units. Below is a typical chain of radioactive decays of 97 Kr, one of the fragments formed during the fission of 235 U:
The excitation of fragments, caused by a violation of the ratio of the number of protons and neutrons, characteristic of stable nuclei, is also removed due to the emission of prompt fission neutrons. These neutrons are emitted by moving fragments in a time less than ~ 10 -14 s. On average, 2–3 prompt neutrons are emitted in each fission event. Their energy spectrum is continuous with a maximum of about 1 MeV. The average energy of a prompt neutron is close to 2 MeV. The emission of more than one neutron in each fission event makes possible to receive energy due to a nuclear fission chain reaction.
With the most probable fission of 235 U by thermal neutrons, a light fragment (A = 95) acquires a kinetic energy of ≈ 100 MeV, and a heavy fragment (A = 139) acquires a kinetic energy of about 67 MeV. Thus, the total kinetic energy of the fragments is ≈ 167 MeV. Total Energy divisions in in this case is 200 MeV. Thus, the remaining energy (33 MeV) is distributed among other fission products (neutrons, electrons and antineutrinos from β-decay fragments, γ-radiation from fragments and their decay products). The distribution of fission energy between the various products during the fission of 235 U by thermal neutrons is given in Table 7.2.
Table 7.2
Fission energy distribution 235 U thermal neutrons
Nuclear fission products (NFP) are a complex mixture of more than 200 radioactive isotopes 36 elements (from zinc to gadolinium). Most of the activity comes from short-lived radionuclides. Thus, 7, 49 and 343 days after the explosion, PYD activity decreases by 10, 100 and 1000 times, respectively, compared to the activity one hour after the explosion. The yield of the most biologically significant radionuclides is given in Table 7.3. In addition to PYN, radioactive contamination is caused by radionuclides of induced activity (3 H, 14 C, 28 Al, 24 Na, 56 Mn, 59 Fe, 60 Co, etc.) and the undivided part of uranium and plutonium. The role of induced activity during thermo nuclear explosions.
Table 7.3
The release of some fission products from a nuclear explosion
| Radionuclide | Half life | Output per division, % | Activity per 1 Mt, 10 15 Bq |
|---|---|---|---|
| 89 Sr | 50.5 days. | 2.56 | 590 |
| 90 Sr | 29.12 years | 3.5 | 3.9 |
| 95 Zr | 65 days | 5.07 | 920 |
| 103 Ru | 41 days | 5.2 | 1500 |
| 106 Ru | 365 days | 2.44 | 78 |
| 131 I | 8.05 days | 2.9 | 4200 |
| 136 Cs | 13.2 days | 0.036 | 32 |
| 137 Cs | 30 years | 5.57 | 5.9 |
| 140 Ba | 12.8 days | 5.18 | 4700 |
| 141 Cs | 32.5 days. | 4.58 | 1600 |
| 144 Cs | 288 days | 4.69 | 190 |
| 3 H | 12.3 years | 0.01 | 2.6·10 -2 |
During nuclear explosions in the atmosphere, a significant part of the precipitation (up to 50% for ground explosions) falls near the test area. Some radioactive substances are retained in the lower part of the atmosphere and, under the influence of wind, move to long distances, remaining approximately at the same latitude. Staying in the air for about a month, radioactive substances gradually fall to Earth during this movement. Most of the radionuclides are emitted into the stratosphere (to a height of 10–15 km), where they are globally dissipated and largely disintegrated.
Various structural elements of nuclear reactors have been highly active for decades (Table 7.4)
Table 7.4
Specific activity values (Bq/t uranium) of the main fission products in fuel elements removed from the reactor after three years of operation
| Radionuclide | 0 | 1 day | 120 days | 1 year | 10 years | |
|---|---|---|---|---|---|---|
| 85 Kr | 5. 78· 10 14 | 5. 78· 10 14 | 5. 66· 10 14 | 5. 42· 10 14 |
4. 7· 10 14 |
3. 03· 10 14 |
| 89 Sr | 4. 04· 10 16 | 3. 98· 10 16 | 5. 78· 10 15 | 2. 7· 10 14 |
1. 2· 10 10 |
|
| 90 Sr | 3. 51· 10 15 | 3. 51· 10 15 | 3. 48· 10 15 | 3. 43· 10 15 |
3. 26· 10 15 |
2. 75· 10 15 |
| 95 Zr | 7. 29· 10 16 | 7. 21· 10 16 | 1. 99· 10 16 | 1. 4· 10 15 | 5. 14· 10 11 | |
| 95 Nb | 7. 23· 10 16 | 7. 23· 10 16 | 3. 57· 10 16 | 3. 03· 10 15 | 1. 14· 10 12 | |
| 103 Ru | 7. 08· 10 16 | 6. 95· 10 16 | 8. 55· 10 15 | 1. 14· 10 14 | 2. 97· 10 8 | |
| 106 Ru | 2. 37· 10 16 | 2. 37· 10 16 | 1. 89· 10 16 | 1. 19· 10 16 | 3. 02· 10 15 | 2. 46· 10 13 |
| 131 I | 4. 49· 10 16 | 4. 19· 10 16 | 1. 5· 10 12 | 1. 01· 10 3 | ||
| 134 Cs | 7. 50· 10 15 | 7. 50· 10 15 | 6. 71· 10 15 | 5. 36· 10 15 | 2. 73· 10 15 | 2. 6· 10 14 |
| 137 Cs | 4. 69· 10 15 | 4. 69· 10 15 | 4. 65· 10 15 | 4. 58· 10 15 | 4. 38· 10 15 | 3. 73· 10 15 |
| 140 Ba | 7. 93· 10 16 | 7. 51· 10 16 | 1. 19· 10 14 | 2. 03· 10 8 | ||
| 140 La | 8. 19· 10 16 | 8. 05· 10 16 | 1. 37· 10 14 | 2. 34· 10 8 | ||
| 141 Ce | 7. 36· 10 16 | 7. 25· 10 16 | 5. 73· 10 15 | 3. 08· 10 13 | 5. 33· 10 6 | |
| 144 Ce | 5. 44· 10 16 | 5. 44· 10 16 | 4. 06· 10 16 | 2. 24· 10 16 | 3. 77· 10 15 | 7. 43· 10 12 |
| 143 PM | 6. 77· 10 16 | 6. 70· 10 16 | 1. 65· 10 14 | 6. 11· 10 8 | ||
| 147 PM | 7. 05·10 15 | 7. 05· 10 15 | 6. 78· 10 15 | 5. 68· 10 15 |
3. 35· 10 14 |
Class
Lesson No. 42-43
Chain reaction fission of uranium nuclei. Nuclear energy and ecology. Radioactivity. Half life.
A nuclear reaction is a process of interaction atomic nucleus with another kernel or elementary particle, accompanied by a change in the composition and structure of the nucleus and the release of secondary particles or γ quanta.
As a result of nuclear reactions, new radioactive isotopes can be formed that are not found on Earth under natural conditions.
The first nuclear reaction was carried out by E. Rutherford in 1919 in experiments to detect protons in nuclear decay products (see § 9.5). Rutherford bombarded nitrogen atoms with alpha particles. When the particles collided, a nuclear reaction occurred, proceeding according to the following scheme:
During nuclear reactions several conservation laws: impulse, energy, angular momentum, charge. In addition to these classical laws conservation in nuclear reactions the law of conservation of the so-called baryon charge(that is, the number of nucleons - protons and neutrons). A number of other conservation laws specific to nuclear and particle physics also hold.
Nuclear reactions can occur when atoms are bombarded with fast charged particles (protons, neutrons, α-particles, ions). The first reaction of this kind was carried out using high-energy protons produced at an accelerator in 1932:
where M A and M B are the masses of the initial products, M C and M D are the masses of the final reaction products. The quantity ΔM is called mass defect. Nuclear reactions can occur with the release (Q > 0) or with the absorption of energy (Q< 0). Во втором случае первоначальная кинетическая энергия исходных продуктов должна превышать величину |Q|, которая называется порогом реакции.
In order for a nuclear reaction to have a positive energy output, specific binding energy nucleons in the nuclei of the initial products must be less than the specific binding energy of nucleons in the nuclei of the final products. This means that the ΔM value must be positive.
There are basically two possible different ways liberation nuclear energy.
1. Fission of heavy nuclei. Unlike the radioactive decay of nuclei, which is accompanied by the emission of α- or β-particles, fission reactions are a process in which an unstable nucleus is divided into two large fragments of comparable masses.
In 1939, German scientists O. Hahn and F. Strassmann discovered the fission of uranium nuclei. Continuing the research begun by Fermi, they established that when uranium is bombarded with neutrons, elements of the middle part of the periodic table arise - radioactive isotopes of barium (Z = 56), krypton (Z = 36), etc.
Uranium occurs in nature in the form of two isotopes: (99.3%) and (0.7%). When bombarded by neutrons, the nuclei of both isotopes can split into two fragments. In this case, the fission reaction occurs most intensely with slow (thermal) neutrons, while nuclei enter into a fission reaction only with fast neutrons with an energy of the order of 1 MeV.
The main interest for nuclear energy is the fission reaction of a nucleus. Currently, about 100 different isotopes with mass numbers from approximately 90 to 145 are known, resulting from the fission of this nucleus. Two typical fission reactions of this nucleus are:
Note that nuclear fission initiated by a neutron produces new neutrons that can cause fission reactions in other nuclei. The fission products of uranium-235 nuclei can also be other isotopes of barium, xenon, strontium, rubidium, etc.
The kinetic energy released during the fission of one uranium nucleus is enormous - about 200 MeV. An estimate of the energy released during nuclear fission can be made using specific binding energy nucleons in the nucleus. The specific binding energy of nucleons in nuclei with mass number A ≈ 240 is about 7.6 MeV/nucleon, while in nuclei with mass numbers A = 90–145 the specific energy is approximately 8.5 MeV/nucleon. Consequently, the fission of a uranium nucleus releases energy of the order of 0.9 MeV/nucleon, or approximately 210 MeV per uranium atom. The complete fission of all nuclei contained in 1 g of uranium releases the same energy as the combustion of 3 tons of coal or 2.5 tons of oil.
The fission products of the uranium nucleus are unstable because they contain a significant excess number of neutrons. Indeed, the N / Z ratio for the heaviest nuclei is of the order of 1.6 (Fig. 9.6.2), for nuclei with mass numbers from 90 to 145 this ratio is of the order of 1.3–1.4. Therefore, fragment nuclei undergo a series of successive β – -decays, as a result of which the number of protons in the nucleus increases and the number of neutrons decreases until a stable nucleus is formed.
When a uranium-235 nucleus fissions, which is caused by a collision with a neutron, 2 or 3 neutrons are released. Under favorable conditions, these neutrons can hit other uranium nuclei and cause them to fission. At this stage, from 4 to 9 neutrons will appear, capable of causing new decays of uranium nuclei, etc. Such an avalanche-like process is called a chain reaction. Development scheme chain reaction fission of uranium nuclei is shown in Fig. 9.8.1.
 |
| Figure 9.8.1. Diagram of the development of a chain reaction. |
For a chain reaction to occur, it is necessary that the so-called neutron multiplication factor was greater than one. In other words, in each subsequent generation there should be more neutrons than in the previous one. The multiplication coefficient is determined not only by the number of neutrons produced in each elementary act, but also by the conditions under which the reaction occurs - some of the neutrons can be absorbed by other nuclei or leave the reaction zone. Neutrons released during the fission of uranium-235 nuclei are capable of causing the fission of only the nuclei of the same uranium, which accounts for only 0.7% of natural uranium. This concentration is insufficient to start a chain reaction. The isotope can also absorb neutrons, but this does not cause a chain reaction.
A chain reaction in uranium with an increased content of uranium-235 can develop only when the mass of uranium exceeds the so-called critical mass. In small pieces of uranium, most neutrons fly out without hitting any nucleus. For pure uranium-235, the critical mass is about 50 kg. The critical mass of uranium can be reduced many times by using so-called retarders neutrons. The fact is that neutrons produced during the decay of uranium nuclei have too high speeds, and the probability of capturing slow neutrons by uranium-235 nuclei is hundreds of times greater than fast ones. The best neutron moderator is heavy water D 2 O. When interacting with neutrons, ordinary water itself turns into heavy water.
Graphite, whose nuclei do not absorb neutrons, is also a good moderator. During elastic interaction with deuterium or carbon nuclei, neutrons are slowed down to thermal speeds.
The use of neutron moderators and a special beryllium shell, which reflects neutrons, makes it possible to reduce the critical mass to 250 g.
IN atomic bombs An uncontrolled nuclear chain reaction occurs when two pieces of uranium-235, each of which has a mass slightly below critical, quickly combine.
A device that supports a controlled nuclear fission reaction is called nuclear(or atomic) reactor. The diagram of a nuclear reactor using slow neutrons is shown in Fig. 9.8.2.
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| Figure 9.8.2. Diagram of a nuclear reactor. |
The nuclear reaction takes place in the reactor core, which is filled with a moderator and penetrated by rods containing an enriched mixture of uranium isotopes with a high content of uranium-235 (up to 3%). Control rods containing cadmium or boron are introduced into the core, which intensively absorb neutrons. Inserting rods into the core allows you to control the speed of the chain reaction.
The core is cooled using a pumped coolant, which can be water or a metal with a low melting point (for example, sodium, which has a melting point of 98 °C). In the steam generator, the coolant transfers thermal energy water, turning it into steam high pressure. The steam is sent to a turbine connected to an electric generator. From the turbine, steam enters the condenser. To avoid radiation leakage, the coolant I and steam generator II circuits operate in closed cycles.
The turbine of a nuclear power plant is a heat engine that determines the overall efficiency of the plant in accordance with the second law of thermodynamics. Modern nuclear power plants have a coefficient useful action approximately equal Therefore, to produce 1000 MW of electrical power, the thermal power of the reactor must reach 3000 MW. 2000 MW must be carried away by the water cooling the condenser. This leads to local overheating of natural reservoirs and the subsequent emergence of environmental problems.
However, the main problem is to ensure complete radiation safety people working at nuclear power plants and preventing accidental releases of radioactive substances that accumulate in large quantities in the reactor core. When developing nuclear reactors, much attention is paid to this problem. However, after accidents at some nuclear power plants, in particular at the Pennsylvania nuclear power plant (USA, 1979) and at Chernobyl nuclear power plant(1986), the problem of nuclear energy safety has become particularly acute.
Along with the nuclear reactor operating on slow neutrons described above, reactors operating without a moderator on fast neutrons. In such reactors, the nuclear fuel is an enriched mixture containing at least 15% of the isotope. The advantage of fast neutron reactors is that during their operation, uranium-238 nuclei, absorbing neutrons, are transformed into plutonium nuclei through two successive β - decays, which then can be used as nuclear fuel:
The breeding factor of such reactors reaches 1.5, that is, for 1 kg of uranium-235 up to 1.5 kg of plutonium is obtained. Conventional reactors also produce plutonium, but in much smaller quantities.
The first nuclear reactor was built in 1942 in the USA under the leadership of E. Fermi. In our country, the first reactor was built in 1946 under the leadership of I.V. Kurchatov.
2. Thermonuclear reactions. The second way to release nuclear energy is associated with fusion reactions. When light nuclei fuse and form a new nucleus, a large amount of energy must be released. This can be seen from the curve of specific binding energy versus mass number A (Fig. 9.6.1). Up to nuclei with a mass number of about 60, the specific binding energy of nucleons increases with increasing A. Therefore, the synthesis of any nucleus with A< 60 из более легких ядер должен сопровождаться выделением энергии. total weight The products of the synthesis reaction will in this case be less than the mass of the original particles.
Fusion reactions of light nuclei are called thermonuclear reactions, since they can only occur at very high temperatures. In order for two nuclei to enter into a fusion reaction, they must approach to a distance of nuclear forces of the order of 2·10 –15 m, overcoming their electrical repulsion positive charges. For this purpose, the average kinetic energy thermal movement molecules must exceed the potential energy Coulomb interaction. Calculation of the temperature T required for this leads to a value of the order of 10 8 –10 9 K. This is an extremely high temperature. At this temperature, the substance is in a completely ionized state, which is called plasma.
The energy released during thermonuclear reactions per nucleon is several times higher than the specific energy released in chain reactions of nuclear fission. For example, in the fusion reaction of deuterium and tritium nuclei
| |
3.5 MeV/nucleon is released. Overall, this reaction releases 17.6 MeV. This is one of the most promising thermonuclear reactions.
Implementation controlled thermonuclear reactions will give humanity a new environmentally friendly and practically inexhaustible source of energy. However, obtaining ultra-high temperatures and confining plasma heated to a billion degrees represents the most difficult scientific and technical task on the path to implementing controlled thermal nuclear fusion.
On at this stage development of science and technology was achieved only uncontrolled fusion reaction V hydrogen bomb. Heat, necessary for nuclear fusion, is achieved here using the explosion of a conventional uranium or plutonium bomb.
Thermonuclear reactions play extremely important role in the evolution of the Universe. The radiation energy of the Sun and stars is of thermonuclear origin.
Radioactivity
Almost 90% of the known 2500 atomic nuclei are unstable. An unstable nucleus spontaneously transforms into other nuclei, emitting particles. This property of nuclei is called radioactivity. U large kernels instability arises due to competition between the attraction of nucleons by nuclear forces and the Coulomb repulsion of protons. There are no stable nuclei with a charge number Z > 83 and a mass number A > 209. But atomic nuclei with significantly lower values of the Z and A numbers can also be radioactive. If the nucleus contains significantly more protons than neutrons, then the instability is caused by an excess of Coulomb interaction energy . Nuclei that would contain a large excess of neutrons over the number of protons turn out to be unstable due to the fact that the mass of the neutron exceeds the mass of the proton. An increase in the mass of the nucleus leads to an increase in its energy.
The phenomenon of radioactivity was discovered in 1896 by the French physicist A. Becquerel, who discovered that uranium salts emit unknown radiation that can penetrate barriers opaque to light and cause blackening of the photographic emulsion. Two years later, French physicists M. and P. Curie discovered the radioactivity of thorium and discovered two new radioactive elements - polonium and radium
In subsequent years, many physicists, including E. Rutherford and his students, studied the nature of radioactive radiation. It was found that radioactive nuclei can emit particles of three types: positively and negatively charged and neutral. These three types of radiation were called α-, β- and γ-radiation. In Fig. 9.7.1 shows an experimental diagram that allows you to detect complex composition radioactive radiation. In a magnetic field, α- and β-rays experience deviations of opposite sides, and β-rays are deviated much more. γ-rays in a magnetic field are not deflected at all.
These three types of radioactive radiation differ greatly from each other in their ability to ionize atoms of matter and, therefore, in their penetrating ability. α-radiation has the least penetrating ability. In the air at normal conditionsα-rays travel a distance of several centimeters. β-rays are much less absorbed by matter. They are able to pass through a layer of aluminum several millimeters thick. γ-rays have the greatest penetrating ability, capable of passing through a layer of lead 5–10 cm thick.
In the second decade of the 20th century, after E. Rutherford’s discovery of the nuclear structure of atoms, it was firmly established that radioactivity is property of atomic nuclei. Studies have shown that α-rays represent a flow of α-particles - helium nuclei, β-rays are a flow of electrons, γ-rays represent short-wavelength electromagnetic radiation with extremely short wavelength λ< 10 –10 м и вследствие этого – ярко выраженными corpuscular properties, that is, it is a flow of particles - γ-quanta.
Alpha decay. Alpha decay is the spontaneous transformation of an atomic nucleus with the number of protons Z and neutrons N into another (daughter) nucleus containing the number of protons Z – 2 and neutrons N – 2. In this case, an α particle is emitted - the nucleus of a helium atom. An example of such a process is the α-decay of radium:
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Alpha particles emitted by the nuclei of radium atoms were used by Rutherford in experiments on scattering by the nuclei of heavy elements. The speed of α-particles emitted during the α-decay of radium nuclei, measured from the curvature of the trajectory in a magnetic field, is approximately 1.5 10 7 m/s, and the corresponding kinetic energy is about 7.5 10 –13 J (approximately 4. 8 MeV). This value can be easily determined by known values masses of the mother and daughter nuclei and the helium nucleus. Although the speed of the escaping α-particle is enormous, it is still only 5% of the speed of light, so when calculating, you can use a non-relativistic expression for kinetic energy.
Research has shown that a radioactive substance can emit alpha particles with several discrete energies. This is explained by the fact that nuclei can be, like atoms, in different excited states. The daughter nucleus may end up in one of these excited states during α decay. During the subsequent transition of this nucleus to the ground state, a γ-quantum is emitted. Scheme of α-decay of radium with emission of α-particles with two values kinetic energies shown in Fig. 9.7.2.
Thus, α-decay of nuclei is in many cases accompanied by γ-radiation.
In the theory of α-decay, it is assumed that groups consisting of two protons and two neutrons, that is, an α-particle, can be formed inside nuclei. The mother nucleus is for α-particles potential hole, which is limited potential barrier . The energy of the α particle in the nucleus is not sufficient to overcome this barrier (Fig. 9.7.3). The departure of an alpha particle from the nucleus is possible only due to a quantum mechanical phenomenon called tunnel effect . According to quantum mechanics, there is a non-zero probability of a particle passing under a potential barrier. The phenomenon of tunneling is probabilistic in nature.
Beta decay. During beta decay, an electron is ejected from the nucleus. Electrons cannot exist inside nuclei (see § 9.5); they arise during beta decay as a result of the transformation of a neutron into a proton. This process can occur not only inside the nucleus, but also with free neutrons. The average lifetime of a free neutron is about 15 minutes. During decay, a neutron turns into a proton and an electron
Measurements have shown that in this process there is an apparent violation of the law of conservation of energy, since the total energy of the proton and electron resulting from the decay of a neutron is less than the energy of the neutron. In 1931, W. Pauli suggested that during the decay of a neutron, another particle with zero mass and charge is released, which takes away part of the energy. New particle got the name neutrino(small neutron). Due to the lack of charge and mass of a neutrino, this particle interacts very weakly with the atoms of matter, so it is extremely difficult to detect in experiment. The ionizing ability of neutrinos is so small that one ionization event in the air occurs approximately 500 km of the way. This particle was discovered only in 1953. It is now known that there are several types of neutrinos. During the decay of a neutron, a particle is produced, which is called electron antineutrino. It is denoted by the symbol Therefore, the neutron decay reaction is written as
|
A similar process occurs inside nuclei during β-decay. An electron produced by the decay of one of the nuclear neutrons, is immediately thrown out of the “parental home” (core) with enormous speed, which can differ from the speed of light by only a fraction of a percent. Since the distribution of energy released during β-decay between the electron, neutrino and daughter nucleus is random, β-electrons can have different velocities over a wide range.
During β-decay, the charge number Z increases by one, but the mass number A remains unchanged. The daughter nucleus turns out to be the nucleus of one of the isotopes of the element, serial number which in the periodic table is one higher than the ordinal number of the original nucleus. A typical example of β-decay is the transformation of thorium isotone resulting from the α-decay of uranium into palladium
Gamma decay. Unlike α- and β-radioactivity, γ-radioactivity of nuclei is not associated with a change in the internal structure of the nucleus and is not accompanied by a change in charge or mass numbers. Both during α- and β-decay, the daughter nucleus may find itself in some excited state and have an excess of energy. The transition of a nucleus from an excited state to a ground state is accompanied by the emission of one or more γ quanta, the energy of which can reach several MeV.
Law of Radioactive Decay. In any sample radioactive substance contains a huge number of radioactive atoms. Since radioactive decay is random in nature and does not depend on external conditions, the law of decrease in the number N(t) of undecayed at this moment time t nuclei can serve as an important statistical characteristic process of radioactive decay.
Let the number of undecayed nuclei N(t) change by ΔN over a short period of time Δt< 0. Так как вероятность распада каждого ядра неизменна во времени, что число распадов будет пропорционально количеству ядер N(t) и промежутку времени Δt:
The proportionality coefficient λ is the probability of nuclear decay in time Δt = 1 s. This formula means that the rate of change of the function N(t) is directly proportional to the function itself.
where N 0 is the initial number of radioactive nuclei at t = 0. During the time τ = 1 / λ, the number of undecayed nuclei will decrease by e ≈ 2.7 times. The quantity τ is called average life time radioactive nucleus.
For practical use It is convenient to write the law of radioactive decay in a different form, using the number 2 rather than e as the base:
The quantity T is called half-life. During time T, half of the original number of radioactive nuclei decays. The quantities T and τ are related by the relation
Half-life is the main quantity characterizing the rate of radioactive decay. How less period half-life, the more intense the decay occurs. Thus, for uranium T ≈ 4.5 billion years, and for radium T ≈ 1600 years. Therefore, the activity of radium is much higher than that of uranium. There are radioactive elements with half-lives of a fraction of a second.
Not found naturally, and ends in bismuth. This series of radioactive decays occurs in nuclear reactors.
Interesting application radioactivity is a method of dating archaeological and geological finds by the concentration of radioactive isotopes. The most commonly used method of dating is radiocarbon dating. An unstable isotope of carbon appears in the atmosphere due to nuclear reactions caused by cosmic rays. A small percentage of this isotope is found in the air along with the usual stable isotope Plants and other organisms take up carbon from the air and accumulate both isotopes in the same proportions as in the air. After the plants die, they stop consuming carbon and the unstable isotope gradually turns into nitrogen as a result of β-decay with a half-life of 5730 years. By precise measurement The relative concentration of radioactive carbon in the remains of ancient organisms can determine the time of their death.
Radioactive radiation all types (alpha, beta, gamma, neutrons), as well as electromagnetic radiation ( x-ray radiation) have a very strong biological effect on living organisms, which consists in the processes of excitation and ionization of atoms and molecules that make up living cells. Under the influence ionizing radiation are destroyed complex molecules And cellular structures, that leads to radiation injury body. Therefore, when working with any source of radiation, it is necessary to take all measures to radiation protection people who may be exposed to radiation.
However, a person can be exposed to ionizing radiation at home. Inert, colorless, radioactive gas radon As can be seen from the diagram shown in Fig. 9.7.5, radon is a product of the α-decay of radium and has a half-life T = 3.82 days. Radium is found in small quantities in soil, stones, and various building structures. Despite the relatively little time life, the radon concentration is continuously replenished due to new decays of radium nuclei, so radon can accumulate in enclosed spaces. Once in the lungs, radon emits α-particles and turns into polonium, which is not a chemically inert substance. What follows is a chain of radioactive transformations of the uranium series (Fig. 9.7.5). According to the American Radiation Safety and Control Commission, the average person receives 55% of their ionizing radiation from radon and only 11% from medical care. Contribution cosmic rays is approximately 8%. The total radiation dose a person receives during his life is many times less maximum permissible dose(SDA), which is established for people in certain professions who are subject to additional exposure to ionizing radiation.