Artificial radioactivity was discovered by the couple Irène (1897–1956) and Frédéric (1900–1958) Joliot-Curie. On January 15, 1934, their note was presented by J. Perrin at a meeting of the Paris Academy of Sciences. Irene and Frederick were able to establish that after bombardment by alpha particles, some light elements - magnesium, boron, aluminum - emit positrons. Next, they tried to establish the mechanism of this emission, which differed in nature from all cases of nuclear transformations known at that time. Scientists placed a source of alpha particles (polonium) at a distance of one millimeter from aluminum foil. They then exposed her to radiation for about ten minutes. A Geiger-Muller counter showed that the foil emits radiation, the intensity of which decreases exponentially with time, with a half-life of 3 minutes 15 seconds. In experiments with boron and magnesium, the half-lives were 14 and 2.5 minutes, respectively. But in experiments with hydrogen, lithium, carbon, beryllium, nitrogen, oxygen, fluorine, sodium, calcium, nickel and silver, no such phenomena were found. However, the Joliot-Curies concluded that the radiation caused by the bombardment of aluminum, magnesium and boron atoms could not be explained by the presence of any impurity in the polonium preparation. “An analysis of the radiation of boron and aluminum in a cloud chamber showed,” K. Manolov and V. Tyutyunnik write in their book “Biography of the Atom,” that it is a stream of positrons. It became clear that scientists were dealing with a new phenomenon that was significantly different from all known cases of nuclear transformations. Nuclear reactions known up to that time were of an explosive nature, while the emission of positive electrons by some light elements irradiated with the alpha rays of polonium continued for some more or less long time after the source of the alpha rays had been removed. In the case of boron, for example, this time reaches half an hour.” The Joliot-Curies came to the conclusion that here we're talking about about real radioactivity, manifested in the emission of a positron. New evidence was needed, and, first of all, it was necessary to isolate the corresponding radioactive isotope. Based on the research of Rutherford and Cockroft, Irene and Frederic Joliot-Curie were able to establish what happens to aluminum atoms when they are bombarded with polonium alpha particles. First, alpha particles are captured by the nucleus of an aluminum atom, the positive charge of which increases by two units, as a result of which it turns into the nucleus of a radioactive phosphorus atom, called “radiophosphorus” by scientists. This process is accompanied by the emission of one neutron, which is why the mass of the resulting isotope increases not by four, but by three units and becomes equal to 30. A stable isotope of phosphorus has a mass of 31. “Radiophosphorus” with a charge of 15 and a mass of 30 decays with a half-life of 3 minutes 15 seconds , emitting one positron and turning into a stable isotope of silicon. The only and indisputable evidence that aluminum turns into phosphorus and then into silicon with a charge of 14 and a mass of 30 could only be the isolation of these elements and their identification using their characteristic qualitative chemical reactions. For any chemist working with stable compounds, this was a simple task, but for Irene and Frederic the situation was completely different: the phosphorus atoms they produced lasted just over three minutes. Chemists have many methods for detecting this element, but they all require lengthy determinations. Therefore, the opinion of chemists was unanimous: to identify phosphorus as such short time impossible. However, the Joliot-Curie spouses did not recognize the word “impossible.” And although this “impossible” task required backbreaking labor, tension, virtuoso dexterity and endless patience, it was solved. Despite the extremely low yield of products of nuclear transformations and the completely insignificant mass of the substance that underwent the transformation - only a few million atoms, it was possible to establish chemical properties the resulting radioactive phosphorus. The discovery of artificial radioactivity was immediately rated as one of the largest discoveries of the century. Before this, the radioactivity that was inherent in some elements could not be caused, destroyed, or changed in any way by man. The Joliot-Curie couple were the first to artificially cause radioactivity by obtaining new radioactive isotopes. Scientists have foreseen great theoretical value this discovery and its possibilities practical applications in biology and medicine. The very next year, the discoverers of artificial radioactivity, Irene and Frédéric Joliot-Curie, were awarded Nobel Prize in chemistry. Continuing these studies, the Italian scientist Fermi showed that bombardment with neutrons causes artificial radioactivity in heavy metals. Enrico Fermi (1901–1954) was born in Rome. Even as a child, Enrico showed great aptitude for mathematics and physics. His outstanding knowledge in these sciences, acquired mainly as a result of self-education, allowed him to receive a scholarship in 1918 and enter the Higher normal school at the University of Pisa. Enrico then received a temporary position as lecturer in mathematics for chemists at the University of Rome. In 1923, he went on a business trip to Germany, to Göttingen, to see Max Born. Upon returning to Italy, Fermi worked at the University of Florence from January 1925 until the fall of 1926. Here he receives his first academic degree as a “free associate professor” and, most importantly, creates his own famous work on quantum statistics. In December 1926, he took the position of professor of the newly established department theoretical physics at the University of Rome. Here he organized a team of young physicists: Rasetti, Amaldi, Segre, Pontecorvo and others, who made up Italian school modern physics. When the first department of theoretical physics was established at the University of Rome in 1927, Fermi, who had gained international authority, was elected its head. Here in the capital of Italy, Fermi rallied several outstanding scientists around him and founded the country's first school of modern physics. In international scientific circles it began to be called the Fermi group. Two years later Fermi was appointed Benito Mussolini to the honorary position of member of the newly created Royal Academy Italy. In 1938, Fermi was awarded the Nobel Prize in Physics. In the decision Nobel Committee it was said that the prize was awarded to Fermi “for demonstrating the existence of new radioactive elements obtained by irradiation with neutrons, and the related discovery of nuclear reactions caused by slow neutrons.” Enrico Fermi learned about artificial radioactivity immediately, in the spring of 1934, as soon as the Joliot-Curie spouses published their results. Fermi decided to repeat the Joliot-Curie experiments, but took a completely different path, using neutrons as bombarding particles. Fermi later explained the reasons for the distrust of neutrons on the part of other physicists and his own lucky guess: “The use of neutrons as bombarding particles suffers from the disadvantage that the number of neutrons that can be practically disposed of is immeasurable.” less number alpha particles received from radioactive sources, or the number of protons and deuterons accelerated in high-voltage devices. But this disadvantage is partially compensated by the greater efficiency of neutrons in carrying out artificial nuclear transformations. Neutrons also have another advantage. They're in to a large extent capable of causing nuclear transformations. The number of elements that can be activated by neutrons greatly exceeds the number of elements that can be activated by other types of particles." In the spring of 1934, Fermi began irradiating elements with neutrons. Fermi's "neutron guns" were small tubes several centimeters long. They were filled with a “mixture” of fine beryllium powder and radium emanation. This is how Fermi described one of these neutron sources: “It was a glass tube only 1.5 cm in size ... in which there were grains of beryllium; Before soldering the tube, it was necessary to introduce a certain amount of radium emanation into it. Alpha particles emitted by radon collide in large numbers with beryllium atoms and produce neutrons... The experiment is performed as follows. A plate of aluminum, or iron, or in general the element that it is desired to study, is placed in close proximity to the neutron source and left for several minutes, hours or days (depending on the specific case). Neutrons emitted from a source collide with the nuclei of matter. In this case, many nuclear reactions of various types occur...” What did all this look like in practice? The sample under study was specified time under intense neutron irradiation, then one of Fermi's employees literally ran the sample to a Geiger-Muller counter located in another laboratory and recorded the counter pulses. After all, many new artificial radioisotopes were short-lived. In the first message, dated March 25, 1934, Fermi reported that by bombarding aluminum and fluorine, he obtained isotopes of sodium and nitrogen that emitted electrons (and not positrons, as in Joliot-Curie). The neutron bombardment method proved to be very effective, and Fermi wrote that this high efficiency in producing fission "completely compensates for the weakness of existing neutron sources compared with sources of alpha particles and protons." In fact, much was known. Neutrons hit the nucleus of the fired atom, turning it into unstable isotope, which spontaneously decayed and emitted. In this radiation lay the unknown: some of the artificially produced isotopes emitted beta rays, others gamma rays, and still others alpha particles. Every day the number of artificially obtained radioactive isotopes increased. Each new nuclear reaction had to be comprehended in order to understand the complex transformations of atoms. For each reaction it was necessary to establish the nature of the radiation, because only by knowing it can one imagine a diagram radioactive decay and predict the element that will result in end result. Then it was the chemists' turn. They had to identify the resulting atoms. This also took time. Using his "neutron gun", Fermi bombarded fluorine, aluminum, silicon, phosphorus, chlorine, iron, cobalt, silver and iodine. All of these elements were activated, and in many cases Fermi could indicate the chemical nature of the resulting radioactive element. He managed to activate 47 of the 68 elements studied by this method. Inspired by the success, he, in collaboration with F. Rasetti and O. Dagostino, undertook neutron bombardment of heavy elements: thorium and uranium. “Experiments have shown that both elements, previously purified from ordinary active impurities, can be strongly activated when bombarded with neutrons.” On October 22, 1934, Fermi made a fundamental discovery. By placing a paraffin wedge between the neutron source and the activated silver cylinder, Fermi noticed that the wedge did not reduce neutron activity, but slightly increased it. Fermi concluded that this effect was probably due to the presence of hydrogen in the paraffin, and decided to test how a large number of hydrogen-containing elements would affect the fission activity. Having carried out the experiment first with paraffin, then with water, Fermi noted an increase in activity hundreds of times. Fermi's experiments revealed enormous efficiency slow neutrons. But, in addition to remarkable experimental results, in the same year Fermi achieved remarkable theoretical achievements. Already in the December 1933 issue in Italian scientific journal His preliminary thoughts on beta decay were published. In early 1934, his classic article “Towards the Theory of Beta Rays” was published. The author's summary of the article reads: “A quantitative theory of beta decay is proposed, based on the existence of neutrinos: in this case, the emission of electrons and neutrinos is considered by analogy with the emission light quantum excited atom in the theory of radiation. Formulas are derived from the lifetime of the nucleus and for the shape of the continuous spectrum of beta rays; the resulting formulas are compared with experiment.” Fermi in this theory gave birth to the neutrino hypothesis and the proton-neutron model of the nucleus, also accepting the isotonic spin hypothesis proposed by Heisenberg for this model. Based on the ideas expressed by Fermi, Hideki Yukawa predicted in 1935 the existence of a new elementary particle, now known as the pi meson, or pion. Commenting on Fermi’s theory, F Rasetti wrote: “The theory he built on this basis turned out to be able to withstand two and a half decades almost unchanged revolutionary development nuclear physics. It might be noted that physical theory rarely is it born in such a final form.”
Radioactivity is the ability of some chemical elements(uranium, thorium, radium, californium) spontaneously decay and emit invisible radiation.
Radioactive substances (RS) decay at a strictly defined rate, measured by half-life, i.e. the time during which half of all atoms decay. Radioactive decay cannot be stopped or accelerated by any means.
A beam of radiation in a magnetic field is divided into three types of radiation:
b-radiation is a stream of positively charged particles representing a helium nucleus, moving at a speed of about 20,000 km/s, i.e. 35,000 times faster than modern aircraft. An alpha particle is a heavy particle; it is 7300 times heavier than an electron. In animal tissues, its penetrating ability is even less and is measured in microns. Alpha particles are part of cosmic rays near the Earth (6%).
Alpha decay is a spontaneous transformation of nuclei, accompanied by the emission of two protons and two neutrons forming the He 4 2 nucleus.
As a result of alpha decay, the nuclear charge decreases by 2 and the mass number by 4 units. For example: the kinetic energy of an escaping b-particle is determined by the masses of the initial and final core of the b-particle. More than 200 b-active nuclei are known, located mainly at the end of the periodic table. About 20 b-radioactive isotopes of rare earth elements are also known. Here, b-decay is most typical for nuclei with the number of neutrons N=84, which, when emitting b-particles, turn into nuclei with a filled nuclear shell (N=82). The lifetime of b-active nuclei varies widely: from 3*10 -7 sec (for Po 212) to (2-5)*10 15 years (natural isotopes Ce 142, 144, 176) The energy of the observed b-decay lies in within 4-9 MeV (with the exception of long-range b-particles) for all heavy nuclei and 2-4.5 MeV for rare earth elements.
c- radiation - a flow of charged negatively charged particles (electrons). Their speed of 200,000-300,000 km/s approaches the speed of light. The mass of beta particles is 1/1840 of the mass of hydrogen. Beta particles are light particles.
g-radiation - is short-wave electromagnetic radiation. Its properties are similar to X-ray radiation, but it has much greater speed and energy, but it travels at the speed of light. On the spectrum electromagnetic waves these rays occupy almost the position on the far right. They are followed only by cosmic rays. The energy of gamma rays averages about 1.3 MeV (megaelectronvolts). This is very high energy. The oscillation frequency of gamma ray waves is 10 20 times/sec, that is, gamma rays are very hard rays, and their penetrating power is high. They pass through the human body unhindered.
For some nuclear reactions Strongly penetrating radiation arises, not deflected by electric and magnetic fields. These rays penetrate a layer of lead several meters thick. This radiation is a stream of neutrally charged particles. These particles are called neutrons.
The mass of a neutron is equal to the mass of a proton. Neutrons have at different speeds, on average less than the speed of light. Fast neutrons develop energies of the order of 0.5 MeV and higher, slow ones - from fractions to several thousand electron volts. Neutrons, being electrically neutral particles, have, like gamma rays, high penetrating power. The weakening of the neutron flux mainly occurs due to collisions with the nuclei of other atoms and due to the capture of neutrons by the nuclei of atoms. So, when colliding with light nuclei, neutrons lose their energy to a greater extent, but light hydrogen-containing substances such as water, paraffin, human body tissue, wet concrete, soil are the best moderators and absorbers of neutrons.
In nature, many chemical elements emit radiation. These elements are called radioactive elements, and the process itself is called natural radioactivity. Neither enormous pressures and temperatures, nor magnetic and electric fields have any effect on the processes of radioactive radiation. Radioactive radiation is associated with the transformation of the nuclei of an element. There are two types of natural radioactive decay.
Alpha decay, in which a nucleus emits an alpha particle. With this type of decay, one nucleus always produces a nucleus of another element, which has a charge of two units less and a mass of four units less. So, for example, radium decays, turning into radon:
Ra 88 226 > He 2 4 + Rn 86 222
Beta decay, in which a beta particle is emitted from the nucleus. Since a beta particle can be differently charged, beta decay can be either electron or positron.
Electronic decay produces an element with the same mass, but with a charge greater than one. This is how thorium turns into protactinium:
Th 90 233 >Pa 91 233 + e -1 + g - quantum.
During positron decay, a radioactive element loses a positive particle and turns into an element with the same mass, but with a charge less by one. So the isotope of magnesium turns into sodium:
Mg 12 23 > Na 11 23 + e +1 + g- quantum.
By directing a beam of alpha particles onto an aluminum plate, for the first time they obtained an artificial radioactive isotope of phosphorus P 15 30:
Al 13 27 + He 2 4 > P 15 30 + n 0 1
The isotopes obtained in this way were called artificially radioactive, and their ability to decay was called artificial radioactivity. Currently, over 900 artificial radioactive isotopes have been obtained.
They are widely used in medicine and biology to study chemical transformations in the body. This method is called the tagged atom method.
yanLaw of conservation of mass - The mass of substances entering into a chemical reaction is equal to the mass of substances formed as a result of the reaction
Atomic-molecular theory was developed by M.V. Lomonosov in 1741. Main provisions of the law:
1) all substances consist of “corpuscles” (molecules);
2) molecules consist of “elements” (atoms);
3) particles - molecules and atoms - are in continuous motion. The thermal state of bodies is the result of the movement of their particles;
4) molecules of simple substances consist of identical atoms, and molecules complex substances– from different atoms. The atomic-molecular theory was finally established in 1860.
Pgrowth substances- substances consisting exclusively of atoms of one chemical element, as opposed to complex substances. Depending on the type of chemical bond between atoms, simple substances can be metals(Na, Mg, Al, Bi, etc.) and non-metals(H 2, N 2, Br 2, Si, etc.)
Chemical element- a collection of atoms with the same nuclear charge and the number of protons, coinciding with the serial (atomic) number in the periodic table. Each chemical element has its own name and symbol, which are given in Mendeleev's Periodic Table of Elements.
The law of constancy of composition - any specific chemically pure compound, regardless of the method of its preparation, consists of the same chemical elements
The law of multiple ratios is one of the stoichiometric laws of chemistry: if two elements form more than one compound with each other, then the masses of one of the elements per the same mass of the other element
are treated as integers, usually small.
The law of volumetric ratios: the volumes of reacting gases under the same conditions (temperature and pressure) relate to each other as whole numbers.
Atomic mass of the element- is the ratio of the mass of its atom to 1/12 of the mass of a 12C atom
Atoms in molecules are connected to each other in a certain sequence according to their valences. The sequence of interatomic bonds in a molecule is called its chemical structure and is reflected by one structural formula (structure formula). Molecular weight the mass of a molecule, expressed in atomic mass units. Numerically equal to molar mass.
A mole is a unit of quantity of a substance. This is the amount of a substance (or its portion) that contains 6.02 1023 particles (molecules, atoms or other particles)
Avagadro's Law equal volumes different gases taken at the same temperature and pressure contain the same number of molecules
A mole is a unit of quantity of a substance. This is the amount of a substance (or its portion) that contains 6.02 1023 particles (molecules, atoms or other particles)
Equivalent- is a real or fictitious particle that can attach, release, or otherwise be equivalent to a hydrogen cation in ion exchange reactions or an electron in redox reactions
law of equivalents: all substances react in equivalent ratios. Valency is a property of atoms of this element add or replace a certain number of atoms of another element in a compound
Avogadro's law allows us to determine the number of atoms that make up the molecules of simple gases. By studying the volumetric ratios in reactions involving hydrogen, oxygen, nitrogen and chlorine, it was found that the molecules of these gases are diatomic. Therefore, by determining the relative molecular mass of any of these gases and dividing it in half, one could immediately find the relative atomic mass of the corresponding element. For example, it was found that molecular weight chlorine is 70.90; hence the atomic mass of chlorine is equal to or 35.45.
Valence the ability of atoms of chemical elements to form a certain number chemical bonds with atoms of other elements.
Internal e is the sum of the energies of molecular interactions and thermal movements of the molecule. Internal energy is a unique function of the state of the system
A covalent bond is formed by two electrons with opposite spins, and this electron pair belongs to two atoms.
energy state of electrons in an atom.
Mainquantum number - an integer indicating the energy level number. Characterizes electron energy occupying a given energy level. Is the first in a series of quantum numbers, which includes the principal, orbital and magnetic quantum numbers, as well as spin
Orbital quantum number- in quantum physics, the quantum number ℓ, which determines the shape of the amplitude distribution of the electron wave function in an atom, that is, the shape of the electron cloud. Defines the sublevel of the energy level specified by the main (radial) quantum number n and can take values
Is the eigenvalue of the operator orbital moment electron, different from the angular momentum of the electron j only on the spin operator s:
Ionization energy- represents the lowest energy required to remove an electron from a free atom. The following factors have the most significant influence on the ionization energy of an atom:
effective nuclear charge, which is a function of the number of electrons in the atom that shield the nucleus and are located in deeper internal orbitals;
radial distance from the nucleus to the maximum charge density of the outer electron, most weakly bound to the atom and leaving it during ionization;
a measure of the penetrating power of that electron;
interelectron repulsion among outer (valence) electrons.
Electron affinity- the amount of energy released when an electron attaches to an atom, molecule or radical. Electron affinity is usually expressed in electron volts. The value of Electron Affinity is important for understanding the nature of chemical bonds and the processes of formation negative ions. The greater the Electron Affinity, the more easily the atom attaches an electron. The electron affinity of metal atoms is zero; for non-metal atoms, the electron affinity is greater, the closer the element (non-metal) is to inert gas in the periodic system of D.I. Mendeleev. Therefore, within the period they intensify non-metallic properties as we approach the end of the period.
An atom consists of a nucleus and an electron “cloud” surrounding it. Located in the electronic cloud electrons carry negative electric charge. Protons, included in the core, carry positive charge. In any atom, the number of protons in the nucleus is exactly equal to the number of electrons in the electron cloud, so the atom as a whole is a neutral particle that does not carry a charge. An atom can lose one or more electrons or, conversely, capture someone else’s electrons. In this case, the atom becomes positive or negative charge and is called ion.
Isotopes(from ancient Greek ισος - "equal", "same", and τόπος - "place") - varieties of atoms (and nuclei) of a chemical element that have the same atomic number, but at the same time different mass numbers. The name is due to the fact that all isotopes of one atom are placed in the same place (in one cell) of the periodic table: 16 8 O, 17 8 O, 18 8 O - three stable isotopes of oxygen.
Radioactive elements and their decay.
Radioactive decay- spontaneous change in the composition of unstable atomic nuclei through the emission of elementary particles or nuclear fragments. There are alpha, beta and gamma decays. Accordingly, they emit alpha, beta and gamma particles. The decay with the strongest penetrating ability is gamma decay (not rejected by a magnetic field). Alphas are positively charged particles. Beta are negatively charged particles.
Nuclei of radioactive elements or isotopes can decay in three main ways, and the corresponding nuclear decay reactions are named by the first three letters of the Greek alphabet. At alpha decay A helium atom consisting of two protons and two neutrons is released - it is usually called an alpha particle. Since alpha decay entails a decrease in the number of positively charged protons in an atom by two, the nucleus that emitted the alpha particle turns into the nucleus of an element two positions lower from it in the periodic table. At beta decay the nucleus emits an electron and the element moves one position forward according to the periodic table (in this case, essentially, a neutron turns into a proton with the radiation of this very electron). Finally, gamma decay - This decay of nuclei with the emission of high-energy photons, which are commonly called gamma rays. In this case, the nucleus loses energy, but the chemical element does not change. Radioactive element- a chemical element, all isotopes of which are radioactive.
Artificial radioactivity- spontaneous decay of nuclei of elements obtained artificially through appropriate nuclear reactions. All three types of radiation - a, b and g, characteristic of natural radioactivity - are also emitted by artificially radioactive substances. However, among artificially radioactive substances there is often another type of decay that is not characteristic of naturally radioactive elements. This is a decay with the emission of positrons - particles that have the mass of an electron, but carry a positive charge. By absolute value The charges of the positron and electron are equal. Artificially radioactive substances can be produced by a wide variety of nuclear reactions. An example is the reaction of neutron capture by silver. To carry out such a reaction, it is enough to place a silver plate near a neutron source surrounded by paraffin.
38. Nuclear reactions.
nuclear reaction- the process of formation of new nuclei or particles during their collisions. The nuclear reaction was first observed by Rutherford in 1919, bombarding the nuclei of nitrogen atoms with α particles; it was detected by the appearance of secondary ionizing particles that had a range in the gas greater than that of the α particles and were identified as protons. Subsequently, photographs of this process were obtained using a cloud chamber.
39. Theory of chemical structure.
This theory has four provisions: 1) The atoms in the molecule are connected in a certain sequence in accordance with their valence. This sequence is called chemical structure. 2) The properties of a substance depend not only on the qualitative and quantitative composition of the molecule, but also on its chemical structure. Substances that have the same composition but different structures are called isomers, and their very existence isomerism. 3) Atoms and groups of atoms in a molecule mutually influence each other directly or through other atoms. 4) The structure of matter is knowable; synthesis of substances with a given structure is possible. Butlerov.1861
40. Covalent bond.
Covalent bond- a chemical bond formed by the overlap of a pair of valence electron clouds. Electronic clouds that provide communication are called shared electron pair. It can be polar or non-polar. An important characteristic of a covalent bond is its polarity. If a molecule consists of 2 atoms that are connected by a polar bond, then such a molecule is a polar molecule. It is a dipole. A dipole is an electrically neutral system in which the centers of positive and negative charge are located at a certain distance from each other. The polarity of the molecule is quantified dipole moment, which is equal to the product of the length of the dipole and the value of the effective charge. Effective charge = 1.6 * 10 -19 C. The ability of molecules and individual bonds to become polyrized under the influence of external electric field called polyrizability. The ability of an atom to participate in the formation of a limited number of covalent bonds is called the saturation of a covalent bond. The direction of the covalent bond determines the spatial structure of the molecules, i.e. overlap of electron clouds. Occurs only at a certain mutual orientation of the orbitals, which provides the highest electron density in the overlap region.
The phenomenon of radioactivity consists of the spontaneous decay of nuclei with the emission of one or more particles. Nuclei that undergo such decay are called radioactive. It is obvious that a necessary, but not always sufficient condition for radioactive decay is its energetic advantage - the mass of the radioactive nucleus must exceed the sum of the masses of the fragment nucleus and particles emitted during decay (it is quite obvious that a similar inequality must be satisfied if it
replace the masses of nuclei with the masses of the corresponding atoms; it is precisely these inequalities that are usually used when considering radioactive decays).
exists in nature large number naturally radioactive nuclei, i.e. nuclei that have not had time to decay from the moment of their formation to the present time or are continuously formed under the influence of cosmic rays. At the same time, radioactive nuclei can be obtained artificially - by bombarding stable nuclei with particles. There is no physical boundary between natural and artificial radioactivity.
Radioactivity was first discovered by A. Becquerel in 1896. Shortly before this, x-rays, and Becquerel studied the relationship between fluorescence and x-rays. Uranium salts capable of fluorescing were placed on a photographic plate wrapped in black paper and placed on sunlight. It was believed that under the influence sun rays uranium fluoresces, and if the fluorescence spectrum includes x-rays, then, passing through black paper, they will cause the plate to blacken. There was no sun for several days, and prepared
the uranium plates lay in a black box. However, after development, a strong blackening of the plates was discovered. Thus, it turned out that uranium salts themselves emit some kind of rays.
Very soon other scientists joined the study of this phenomenon. 1898 P. Curie, together with M. Sklodowska-Curie, discovered new radioactive elements - polonium and radium. Using the enrichment method they developed, they were able in 1902, through painstaking work on processing large quantities of uranium tar, to obtain several decigrams of pure radium salt. In 1903, for research into the phenomenon of radioactive
The Curies, together with A. Becquerel, were awarded the Nobel Prize in Physics. The term “radioactivity” itself was introduced into science. Sklodowska-Curie.
Laws of radioactive decay. Radioactive decay is characterized by the time it takes to occur, the type of particles emitted, their energy, and when several particles are emitted, by angular correlation, i.e., the relative angle between the directions of their emission. The initial radioactive nucleus is called the mother nucleus, and the product of its decay is called the daughter nucleus.
Since the decay process occurs spontaneously (spontaneously), the change dN in the number of nuclei N due to decay over an arbitrary period of time dt is determined only by the number of radioactive nuclei at the moment t and is proportional to the time interval dt:
DN = λNdt, (10.34)
where λ is a constant characterizing the decay rate. Integrating (10.34)
and assuming that at t = 0 the number of cores is equal to the original N = N 0, we obtain
N = N 0 e - λt (10.35)
i.e., the number of cores decreases exponentially.
The quantity A, which determines in (10.35) the rate of decrease in the number of radioactive nuclei, is called the decay constant. It has a dimension [s -1 ] and, as will be shown a little further, characterizes the probability of the decay of one atom in one second. To characterize radioactive elements, the concept of half-life T 1/2 is also introduced. It refers to the time during which half the available number of atoms decays.
The law of radioactive decay (10.35) was first established in 1903 by P. Curie. He also introduced the concept of half-life and showed its independence from external conditions. Based on this, P. Curie proposed using the half-life as a time standard for determining absolute age terrestrial rocks.
Let us now calculate the average lifetime of a radioactive nucleus. Substituting
condition N(T 1/2) = N 0 /2 into equation (10.35), we obtain
N 0 /2 = N 0 e - λTl/2, (10.36)
from where, taking logarithms, we find that
λT 1/2 = 1n2 = 0.693,
and the half-life
T 1/2 = 0.693/λ. (10.37)
At exponential law radioactive decay at any time t there is a non-zero probability of finding undecayed nuclei. The lifetime of such nuclei exceeds t. At the same time, other nuclei that had decayed by this time lived different times, less than t. The average lifetime for a given radioactive isotope is usually determined as follows:
Consequently, the average lifetime g of a radioactive nucleus is equal to reciprocal from the decay constant A. Over time t, the initial number of nuclei decreases by e times.
Magnitude
A = - dN/dt = λN
called the activity of a given drug,
it determines the number of decays per second. Activity is a characteristic of a certain amount of decaying matter, and not of an individual nucleus. The unit of activity is the becquerel: 1 becquerel (Bq) is equal to 1 decay per second. Often in practice, an extra-systemic, previously used, unit of activity is used - the curie: 1 curie (Ci) is equal to the number of decays of nuclei contained in 1 g of radium in 1 s 3.7 10 10 decays per second).
Types of radioactive decay.To the number radioactive processes include α- and β-decays (including electron capture from atomic shell), γ-radiation, nuclear fission, as well as the emission of delayed neutrons and protons. Two latest process belong to the cascade two-stage type, since the emission of delayed neutrons (protons) occurs after the preliminary emission of an electron (positron) by the nucleus. Therefore, the emission is delayed by a time characterizing the preceding β-decay. Let's consider the processes we have listed.
Alpha decay. Only heavy nuclei with Z > 83 and a small group of rare earth nuclei in the region A = 140-160 are subject to spontaneous α decay. During α-decay, the original mother nucleus emits a helium nucleus (α-particle) and turns into a daughter nucleus, the numbers of protons and neutrons in which decrease by two units each. The half-life of α-active nuclei varies within extremely wide limits. So, for example, for the polonium isotope 214 84 Po it is equal to 3 10 ~7 s, and for the lead isotope 204 82 Pb - 1.4 * 10 17 years. The range of changes in the energy of emitted α particles is much smaller - from 4 to 9 MeV, and the lower their energy, the longer the half-life. The functional relationship between the energy of the alpha particle E and the half-life of the radioactive nucleus T 1/2 is good
described by the formula
logT 1/2 = a/√Ё + b, (10.39)
obtained on the basis of experimental data by G. Geiger and J. Nattall in 1911. Theoretical background The Geiger-Nattall law was received only after the creation of quantum mechanics in 1928 in the works of G. Gamow and, independently, R. Gurney and E. Condon, who showed that the probability of an alpha particle leaving the nucleus is determined by the probability of its penetration through the Coulomb barrier. The exponential nature of this process arises due to the exponential decay of the wave function in the region under
a barrier where the potential energy is greater than the energy of the particle.
Four elementary particles, of which the α-particle consists (two protons and two neutrons), participate in complex movement nucleons in a nucleus, and there is no way to distinguish them from other particles in that nucleus. At the same time, there is a noticeable (~ 10 ~6) probability of the formation of an alpha particle in the nucleus for some short time as a result of the random approach of four nucleons. However, only when the alpha particle leaves the nucleus and is sufficiently far from it can it and the nucleus be considered as two separate particles.
Energetically, α-decay is possible if the binding energy of the original mother nucleus E A, z is less than the sum of the binding energies of the daughter nucleus E A-4, z-2 and the α particle E α, i.e. the relation must be satisfied
ΔE = E A-4, z-2 + E α - E A, z > 0. (10.40)
The binding energy of an α particle is 28 MeV, which is 7 MeV/nucleon.
Therefore, α-decay of medium-sized nuclei, whose binding energy is
nucleon ~ 8 MeV.
Consider the view potential energy a-particles in the nucleus and its surroundings (Fig. 10.9). Outside the nucleus, short-range nuclear forces quickly vanish, and the a-particle is affected only by electrostatic Coulomb repulsion, the potential of which U coul is equal to
U cool = 2(Z-2)e 2 /r (10.41)
At the boundary of the nucleus, a strong attraction comes into play due to nuclear forces, and the potential curve goes down sharply. Inside the nucleus, the potential can be considered approximately constant.
Even if the total energy of the alpha particle in the nucleus is greater than zero, as shown in Fig. 10.9, and thus energetically a-decay is allowed; according to the concepts of classical physics, this process cannot occur without imparting additional energy to it, since the particle is in a potential well. However, quantum mechanics allows the passage or, more precisely, the leakage of a particle through a potential barrier. It is said that alpha particle tunneling through the barrier can occur. The point is that
the properties of a quantum particle are described using the wave function ψ, the square of whose modulus |ψ(r)| 2 is proportional to the probability of detecting a particle at point r. IN
in the case of a finite potential (potential with walls of finite height) the ψ-function
is everywhere different from zero. Therefore, there is, although small, a probability of detecting a particle outside the nucleus, and this means the possibility of α-decay.
Let us show qualitatively where the above-mentioned laws of α-decay follow. The permeability D of the barrier for an α-particle with energy E is determined by the following expression:
(10.42)
where integration is carried out within the range from the core radius R i to the turning point R n , determined from the condition
2(Z–2)e 2 /R n = E
(we took into account that the α-particle outside the nucleus is affected by the Coulomb potential of the residual nucleus with charge Z-2). We will assume that tunneling occurs deep under the barrier, i.e.
Due to the smallness of the Planck constant in the expression for the permeability of the barrier in the exponential, in fact the contribution of the region where U ~ E is small, and the condition we impose is physically justified. Under these assumptions, formula (10.42) takes the form
where A, B, C are constants. Since the half-life T 1/2 is inversely proportional to the permeability of the barrier, the experimentally observed Geiger-Nattall law follows from expression (10.43)
logT 1/2 = a/√E + b , (10.44)
relating the half-life to the energy of the emitted α-particle. In reality, coefficients a and b are not constants, but they very weakly depend on the atomic number of the parent nucleus Z:
a ~ 1.6 Z; b ~ -1.6 Z 2/3 - 21.4 (10.45)
(if T 1/2 is expressed in seconds, E is in megaelectronvolts, and Z is the charge of the daughter nucleus). As can be seen, T 1/2 does not depend on the atomic weight A, weakly depends on Z and in strong degree- on the energy of the emitted α-particles.
So far we have only talked about the permeability of the potential barrier.
To find the decay constant λ, it is necessary to multiply the permeability of the barrier by the number of attempts of an α-particle ν per unit time to overcome this barrier, i.e.
λ= 0.693/T 1/2 = νD. (10.46)
A rough estimate of the pre-exponential factor in (10.46) can be made if by v we mean the frequency of impacts of an alpha particle on the surface of the nucleus, determined by the formula
ν = v/(2R n), (10.47)
where v is the speed of the a-particle inside the nucleus. Of course, the pre-exponential factor also depends on energy (according to our rough estimate, it is proportional to √E), but, compared to the exponential dependence, it is a slowly varying function of energy, so it is the barrier permeability that determines
all the basic laws of α-decay.
The energy spectrum of alpha particles of many alpha-active nuclei consists of several lines, one of which is predominant. As an example in Fig. Figure 10.10 shows the α spectrum of ThC(212 83 Bi).
Rice. 10.10 α spectrum of ThC(212 83 Bi).
The discreteness of the lines and their relative intensity are easily explained. The fact is that α-particles can either be emitted by a nucleus in an excited state (the so-called long-range α-particles), or α-decay can occur from the ground state of the mother nucleus into the excited states of the daughter nucleus (short-range α-particles). In Fig. 10.11 shows two examples of such transitions - the decay of 238 Pu and 212 Po.
In the first case (238 Pu) α-particles of maximum energy correspond to transitions from the ground state to the ground state. In addition, α decay can occur in the excited states of the daughter nucleus 234 U with subsequent γ transitions to the ground state. The decay of 212 Po is an example of the emission of α particles from an excited state. This situation arises from the fact that 212 Po is formed as a result of the β-decay of 212 Bi. Being in an excited state, the 212 Po nucleus can either emit an α particle or go to the ground state by γ radiation.
Beta decay. Beta decay is the process of spontaneous transformation of an unstable nucleus into an isobaric nucleus (a nucleus with the same atomic number) with a charge different from the original one by ΔZ = ±1, due to the emission of an electron (positron) or the capture of an electron from the atomic shell. Main featureβ-decay is that it is caused not by nuclear or electromagnetic forces, but by weak interaction (see Chapter 12), the probability
which is approximately 10 14 times less than nuclear. Therefore the half-lives
β-active nuclei are on average quite large - on the order of several minutes and even hours. IN general case other than that equal conditions during β-decay, the same trend is observed as during α-decay: the greater the energy Q released during decay, the less period half-life
Half-lives shorter than 10 ~2 s do not occur, since with them the Q values would be greater than 10 MeV, i.e., greater than the average binding energy of nucleons in the nucleus; With such an excess of energy, the nucleus turns out to be unstable with respect to the emission of a nucleon, and this process (when it is possible) occurs much faster than beta decay, in a time of the order of 10~20 s. β-decay processes always occur when they are energetically possible. The Coulomb barrier for β-decay is insignificant due to the very low mass electron.
Characteristic featureβ-decay is the energy spectrum of the escaping particles (Fig. 10.12). Unlike alpha particles, in this case we have a continuous energy spectrum of β-decay electrons. The observed continuity is a consequence of the participation in the decay process of another particle - the neutrino, which has zero rest energy (according to the latest data upper limit neutrino rest energy is 3 eV). Therefore, during a single decay event, the energy ratio of the electron and neutrino can be any, i.e., the electron energy can take any value from zero to the maximum possible energy (total energy released).
Let us dwell in more detail on the energy processes during β-decay.
Consider an atom with charge Z+1 and full energy E z+1 . Let him zero energy corresponds to the system “singly ionized atom plus a free electron at rest.” The latter means that the energy of a neutral atom with charge Z + 1 is slightly negative and has the order of energy
ionization potential of the atom (Fig. 10.13). The following cases are possible.
A. The energy Ez of an atom with charge Z is higher than Ez+1. Energetically possible is β-decay, i.e. decay with the emission of an electron, and the Z atom transforms into an ionized Z + 1 atom. The process E z+1 -> E z is prohibited.
B. The transition E z+1 -> E z is possible only if the Z + 1 nucleus absorbs an electron from the atomic K-, L-, M-shells. Typically, a K-electron is captured by the nucleus, and therefore the process is often called K-capture. A new Z atom is formed in the excited state B*, respectively, with a vacancy (hole) in the K- or L-shell. Then a transition to the ground state occurs, accompanied by the emission characteristic radiation:
B* -> B + hv. (10.48)
C. The energy of atom Z is such that E z + 2m 2<= E z +1. Также возможен процесс К-захвата, но, кроме того, ядро может претерпевать β + -распад (позитронный распад). Приведенное энергетическое соотношение легко получить.
If m is the mass of the electron (positron), M z is the mass of the final nucleus, and
M z+1 is the mass of the original nucleus, then the inequality must be satisfied
M z+1 c 2 >= M z c 2 + mс 2 . (10.49)
But the masses of the atoms (A M Z and A M z+1) Z and Z+1, taking into account the mass of electrons, are equal
A M Z =M Z + Zm, A M Z +1 = M z +1 + (Z + l)m. (10.50)
Substituting these relations into condition A0.49), we obtain
A M Z +1 >= A M z +2m (10.51)
E z +1 >= E z +2mc 2 . (10.52)
It is important to emphasize that β-decay is not an intranuclear process, but an intranucleon one. A single nucleon - a neutron or proton - decays in the nucleus.
Electron decay is related to neutron decay
n° -> p + + e - + ν > . (10.53)
In positron decay, a single proton decays in a nucleus
р + -> n° + e + + ν. (10.54)
In formula (10.53), the “tilde” sign above the neutrino means that when a neutron decays, an antineutrino is formed. Why this happens will be discussed in detail further in Chapter. 12. Note that in a free state the neutron is unstable, its half-life is 10.5 minutes. A free proton does not decay, since its mass is less than the mass of a neutron, but for a proton bound in the nucleus, such a transformation is possible; the missing energy is replenished by the nucleus.
One of the amazing discoveries of the 20th century is associated with β-decay. - discovery of parity non-conservation. It seems quite obvious that the choice of coordinate system in which to write mathematically physical equations and, accordingly, the evolution of the system in time occurs, which is completely arbitrary. Consequently, there can be no difference between the descriptions of the same process in the left and right systems coordinates Mathematically, this means that all equations must be symmetrical with respect to the operation of spatial inversion, that is, replacing r with -r. Change
coordinate signs of any point corresponds to the position of the point obtained as a result of its mirror reflection in three coordinate planes, and therefore such a change in the coordinate system can be interpreted as a transition to a set of events that are mirror image given set of events. Conversion
spatial inversion has a physical meaning due to the fact that, as experience shows, natural processes are basically symmetrical with respect to
such a transformation. This means that for any process in nature, a “mirror symmetric” process is carried out and proceeds with the same probability.
Symmetry with respect to the transformation of spatial inversion leads, in a quantum mechanical description, to the existence of a certain spatial parity for the system. In other words, wave function the system is either even or odd under this transformation. Spatial parity is preserved in the processes of strong and electromagnetic interactions. As for weak interactions,
responsible for β-decay, the situation here is different. The hypothesis of parity nonconservation in weak interactions was put forward by T.D. Lee (b.1926) and C.N. Young (b. 1922), who proposed a corresponding experiment conducted by C.S. By (b. 1913).
Schematic diagram experience is extremely simple. The beta-active isotope 60 Co was placed in a magnetic field N solenoid, which polarized the cobalt nuclei, i.e. oriented them magnetic moments along the field (Fig. 10.14).
The entire system is mirror symmetric relative to the plane of the current loop, therefore, it would seem that the intensity of β-electron radiation should be the same on both sides of the plane of symmetry. In fact, the experiment observed a sharp asymmetry (by about 40%), i.e., an asymmetry of weak interactions relative to left and right.
Gamma radiation. In the case when the decay of a nucleus with the emission of a nucleon is energetically impossible, the excitation is removed due to the emission of γ-quanta - high-energy photons. The emission of γ-quanta by the nucleus with an energy exceeding the binding energy of a nucleon occurs only in the case of a ban on parity and angular momentum for the emission of nucleons (or other particles), which makes the process of emission of γ-quanta relatively more probable. If this kind of prohibition does not exist, then the emission of such “nuclear” particles as neutrons, protons,
α-particles are much more likely than γ-radiation. The latter is due to the fact that γ~ radiation is due to electromagnetic interaction, while the emission of nucleons or a-particles occurs due to a stronger nuclear interaction(this type of fundamental interaction is usually called strong interaction - see Chapter 12).
Unlike β-decay, γ~ radiation is not an intranucleon phenomenon, but an intranuclear one. An isolated free nucleon cannot emit (or absorb) a γ-quantum due to the combined action of the laws of conservation of energy and momentum. The latter is completely similar to the fact that the photoelectric effect on free electrons impossible. At the same time, inside the nucleus, a nucleon can emit a quantum, transferring part of the momentum to other nucleons.
In ch. 8 we showed that since the photon is a massless particle, there is no coordinate system for it in which it is at rest. In addition, it makes no sense for a photon to divide its total angular momentum into spin and orbital. The total moment can, in principle, have any integer (in units of K) value, starting from unity. This is why it is often said that the photon spin is equal to 1, although the more correct statement is “ minimum value The angular momentum of a photon is equal to 1.”
As mentioned in § 8.1, the state of a photon emitted by any system is characterized by multipoleity, i.e., by a certain total angular momentum and parity.
The 2 L multipole photon has angular momentum L, absolute value which, according to quantum mechanics, is equal to √(L(L + 1)), i.e. exactly the same as in the case of a particle of finite mass. In accordance with the law of conservation of angular momentum, the following relationship must be satisfied between the moments I n and I K of the initial and final nucleus and the moment L carried away by the γ-quantum:
|I n -I to |<=L<=I H + I K . (10.55)
It is a selection rule based on angular momentum. According to (10.55), dipole γ quanta (L = 1) can be emitted during transitions between states with
ΔI = 0, ±1, except for (0-0) transitions; quadrupole γ-quanta (L = 2) - during transitions between states with ΔI = ± 2, ±1, 0, except for (0-0)-, (0-1)- and (1-0)-transitions, etc. d.
Another selection rule is associated with the fulfillment of the law of conservation of parity of the wave function. Parity, as we said earlier, is determined by the influence on the sign of the wave function of the system of reflection of all three axes relative to the origin.
Such reflection in the case of a static dipole leads to a mutual rearrangement of the position of each charge (Fig. 10.15). Consequently, if viewed from the original coordinate system, there is an obvious change in the signs of all charges. However, the same reflection in the case of a magnetic dipole (circular current) does not change the direction (sign) of the current in the magnetic dipole (see also Fig. 8.1).
Therefore, the allowed change in the parity of a nucleus emitting electric γ-radiation of multipole L is described by the formula
P and /P k = (-1) L, (10.56)
and for a nucleus emitting magnetic L-multipole radiation, by the formula
R n / R k = (-1) L+1 , (10.57)
where R n and R k are the parity of the initial and final states of the nucleus, respectively.
Often the excitation in the nucleus is removed not by a direct transition to the ground state, but by the emission of a cascade of γ quanta with less multipolarity. It turns out that there is an angular correlation of sequentially emitted γ-quanta, i.e., a predominant direction of emission of the second quantum is observed.
The appearance of the correlation dependence is due to the fact that the projection m of the total moment of the γ-quantum onto its momentum can only take the values m = ±1 (the unit of measurement is Planck’s constant ћ).
The value m = 0 is excluded by the condition of transverse electromagnetic waves.
Therefore, if, for example, a nucleus at a level with a zero moment emitted a γ-quantum emitted in a certain direction, i.e., registered in this direction by a detector, then the projection of the spin of the nucleus in a new, lower energy state onto this direction can only be ± 1, but not zero. Thus, it turns out that the core is no longer completely chaotically oriented in space. Therefore, cascade γ-quanta fly out of it in different directions with different probabilities. Corner
the correlation depends significantly on the moments of successively decaying states.
The lifetimes of γ-active nuclei are short on average and are usually on the order of 10 ~7 -10 ~11 s. In rare cases, when a high degree of prohibition is combined with a low transition energy, γ-active nuclei can be observed with lifetimes of the macroscopic order - up to several hours, and sometimes even years. Such excited long-lived states of nuclei are called isomers. This phenomenon was discovered in 1935 by I.V. Kurchatov and his staff. An isomeric level must have a spin very different from the spins of the levels below it and a low excitation energy. As a rule, the isomeric state refers to the first excited level of the nucleus. So, for example, in the 115 49 In nucleus, the ground state has a characteristic of 9/2 +, and the first excited level with an energy of 335 keV has a characteristic of 1/2 ~. This transition is so strongly forbidden that the lifetime of the excited level turns out to be 14.4 hours.
It should be noted that all laboratory sources of γ quanta are actually long-lived β-active nuclei, and γ radiation arises due to
β-decay of the mother nucleus into excited levels of the daughter nucleus. For example, in the widespread source of γ-radiation 60 Co (T 1/2 = 5.3 g), the emission of electrons with an energy of 0.3 MeV and subsequent γ-transitions in the 60 Ni nucleus with energies of 1.17 and 1 occur. 33 MeV.
In addition to γ-radiation, there is another mechanism for the loss of energy by an excited nucleus - the emission of internal conversion electrons. In this process, the excitation energy of the nucleus is transferred directly to one of the orbital electrons, which receives all the energy of the quantum. With the greatest probability, the process of internal conversion occurs on K-electrons, whose wave function overlaps most with the nucleus. However
if the energy released during a nuclear transition is less than the binding energy of the X-electron, then conversion on L-electrons is observed, etc. In addition to conversion electrons, during internal conversion one can also observe X-ray quanta that arise when one of the outer electrons passes to the level K- or L-shell vacated by an emitted electron. The monoenergetic nature of the electrons emitted during internal conversion makes it possible to distinguish them from β-decay electrons, whose spectrum is continuous.
As an illustration of this process, Fig. Figure 10.16 shows the spectrum of electrons emitted from the β-active nucleus of mercury 203 Hg.
The process of internal conversion is in some sense analogous to oscillations in a coupled system with two degrees of freedom. The simplest example of such a system is two pendulums connected by a spring: the oscillations of one of the pendulums, thanks to the spring, excite oscillations of the other. In the case of internal conversion, the role of a “spring” is played by the electric field. Thus, internal conversion is a primary, and not a secondary, process of interaction of electromagnetic radiation with orbital electrons: the excitation energy of the nucleus is transferred to the orbital electrons, as they say, by virtual rather than real quanta.
Nuclear fission. The fission of atomic nuclei is a process characteristic only of the heaviest nuclei, starting from thorium and further towards high Z.
Now it is difficult to imagine with what bewilderment and distrust physicists greeted in 1938 the message of O. Hahn and F. Strassmann about the fission of the atomic nucleus by slow neutrons, since it was well known that to tear out one nucleon from the nucleus requires energy of millions of electron volts. According to the figurative expression of R. Leachman, this is equivalent to the fact that a hard stone is split by lightly tapping a pencil. The first explanation for the observed process was put forward by N. Bohr and J. Wheeler and independently, Ya.I. Frenkel within a few months based on
analogies between nuclear fission and the fission of a charged drop of liquid during deformation.
When hit by a neutron, the droplet nucleus begins to oscillate and at some point in time takes on an elongated shape. The nuclear forces acting between nucleons, like the cohesion forces of molecules in a liquid, lead to the appearance of surface tension. They strive to return the nucleus to its original almost spherical shape (heavy nuclei in the ground state are slightly deformed and have the shape of an elongated ellipsoid).
However, if the elongation of the nucleus at some point in time turns out to be large enough, the electrostatic forces of repulsion of like charges can exceed the forces of surface tension.
Then the core will begin to stretch even more until it breaks into two fragments. At the moment of fission, two or three neutrons, α-particles and even light nuclei are emitted as “small splashes”, although with a very low probability. Successive stages of the process
fission of the atomic nucleus is shown in Fig. 10.17.
In 1940 G.N. Flerov and K.A. Pietrzak discovered that uranium nuclei can also fission spontaneously. Half-life of spontaneous fission of 238 U
equals 8 10 15 years. As it turned out later, all nuclei heavier than thorium undergo spontaneous fission, and the heavier the nucleus and the greater its charge, the higher on average the probability of this process, i.e., the shorter the period of its spontaneous fission. The period of spontaneous fission decreases very quickly as we move to heavier nuclei. Thus, for the plutonium isotope 242 Pu it is 6.8 * 10 10 years, for californium 252 Cf it is already 85 years, and for fermium 256 Fm it is 2.7 hours.
Spontaneous nuclear fission is a purely quantum mechanical effect. As mentioned above, it is the result of the competition of two processes - surface tension, which tends to return the nucleus to its original state, and Coulomb repulsion of charged fragments. Thus, a potential barrier appears at the nucleus, preventing its division.
In Fig. Figure 10.18 shows the potential energy of a nucleus as a function of the deviation ΔR of the surface of the nucleus from a spherical shape. The ground state of the nucleus is slightly
deformed.
Therefore, spontaneous nuclear fission is a tunneling process, just as it occurs during the tunneling of α particles. This is where the strong dependence of the spontaneous fission period on the nuclear charge appears: as the nuclear charge increases, the barrier value decreases and the probability of fission sharply increases. For the 235 U isotope, the fission barrier is approximately 6 MeV, which is exactly the energy that a slow neutron contributes to the nucleus, and that is why 235 U fissions so easily when it absorbs a neutron.
The appearance and influence of the Coulomb barrier is easily explained using the semi-empirical Weizsäcker formula for the binding energy of nuclei. Let the core change its shape, for example, from spherical to ellipsoidal. The volume of the nucleus does not change (nuclear matter is practically incompressible), but the surface increases, and the Coulomb energy decreases (the average distance between protons increases). The ability of a nucleus to fission is naturally characterized by the ratio of the Coulomb energy to the surface energy, i.e.
(10.58)
Since the coefficients γ and β are constant for all nuclei, the probability of fission is determined by the value Z 2 /A, which, according to the proposal of Bohr and Wheeler, was chosen as the nuclear fissibility parameter. Calculations show that for nuclei with Z 2 /A >= 49, fission occurs almost instantly, in a time of the order of 10 ~ 23 s.
This means that spontaneous fission determines the limit of the existence of stable nuclei, i.e. nuclei with Z >= 120 do not have an energy barrier that prevents spontaneous division. The nature of the change in the fission barrier E f and the energy Q f released during nuclear fission as the nucleus deforms ε. At different meanings The divisibility parameter is shown in Fig. 10.19, and in Fig. Figure 10.20 shows the lifetimes for spontaneous fission of even-even nuclei. Nuclei with odd N or Z have several orders of magnitude longer half-lives for spontaneous fission than neighboring even-even nuclei.
Based on the above reasoning, it is easy to predict the following basic properties of the fission process.
1 . During the fission of a heavy nucleus, a large energy Q should be released, since the binding energy per nucleon in heavy nuclei ε strand is approximately 0.8 MeV less than the corresponding energy ε sr for medium nuclei; so, for example, for a 238 U core
Q f ~ A(ε heavy - ε sr) ~ 238 0.8 ~ 200 MeV. (10.59)
2. The overwhelming majority of fission energy is released in the form kinetic energy fission fragments Ek, since fragment nuclei must inevitably fly apart under the influence of Coulomb repulsion. The Coulomb energy of two fragments with charges Z 1 and Z 2 located at a distance δ is equal to
. (10.60)
where R 1, R 2 are the radii of the fragment nuclei that
can be calculated using the formula
R = 1.23 10 ~13 A 1/3 cm,
a Z 1 = Z 2 = Z 0 /2 ~ 46
(assuming that the uranium nucleus is divided in half), we get
i.e. the value is of the same order as Q f
3 . The fission fragments produced must be β-radioactive and can emit neutrons. The reason is that as the nuclear charge increases, the ratio of the number of neutrons in the nucleus to the number of protons increases due to the increase in the Coulomb energy of the protons. Therefore, fragment nuclei will have the same N/Z ratio during fission as, say, uranium, i.e. they will be overloaded with neutrons, and such nuclei undergo β-decay (due to the large overload of neutrons, the products of this decay are also β-active, so fission fragments give rise to fairly long chains of radioactive nuclei). In addition, part of the energy can be carried away by the direct emission of fission neutrons or secondary, i.e., neutrons emitted from fission fragments. Average energy fission neutrons is about 2 MeV.
The average number of neutrons ν emitted per fission event depends on the mass number of the fissile nucleus and increases with increasing Z. If for the 240 Pu nucleus ν ~ 2.2, then for 252 Cf ν ~ 3.8. Since 252 Cf also decays quite quickly (relative to spontaneous fission T 1/2 = 85 years; in reality its lifetime is determined by α-decay and is 2.64 years), it is an intense source of neutrons.
Currently, it is considered as one of the most promising radioactive neutron sources.
The large energy release and emission of secondary neutrons during nuclear fission have a huge practical significance. The operation of nuclear reactors, which will be discussed in the next chapter, is based on this process.
Natural and artificial radioactivity
The nuclei of some isotopes can spontaneously transform into the nuclei of other elements, releasing energy. This process is called radioactivity. Natural radioactivity first discovered on uranium salts in 1896. French physicist A. Becquerel and then studied by Pierre and Marie Curie. It was found that radioactive decay is accompanied by the emission of α-, β-, and γ-rays. Most naturally occurring radioactive elements form radioactive families, where each radioactive element arises from the previous one and, in turn, transforms into the next. The process of radioactive transformations continues until a stable isotope is formed. For some natural radioactive elements (40 K, 87 Rb, 152 Sm, etc.), the decay is limited to one stage of transformation.
Artificial radioactivity was discovered in 1934. French scientists Irene and Frederic Joliot-Curie. They found that when stable elements are irradiated with alpha particles, radioactive isotopes of phosphorus, nitrogen and silicon are formed - elements that do not have natural radioactive isotopes. Subsequently, when stable elements were irradiated with alpha particles, protons, deuterons and neutrons, radioactive isotopes of all chemical elements were obtained, from hydrogen to uranium, and for most elements several radioactive isotopes were obtained.
There are the following main types of decay of natural radioactive elements.
1. Emission of an α-particle, which is a positively charged helium nucleus with atomic number Z=2 and mass number M=4. The nucleus formed as a result of α decay has a mass number of four units, and an atomic number two units less than that of the original nucleus, for example:
2. Emission of negative or positive α-particles - electron (denoted e - or β -) or positron (e + or β +), which are charged particles with approximately the same mass ( m e=0.9035-10 -27 g), constituting only 1/1835 of the mass of a proton. In this case, the mass number of the decay product is the same as that of the original nucleus, and the atomic number increases or decreases by one unit, for example:
.
The above reaction records highlight an important feature of β-decay: it is always accompanied by the emission of a neutral particle with zero mass - a neutrino v with β+ decay and antineutrinos v during β - -decay. Very often, the main (obligatory) decay products, α- and β-particles, as well as neutrinos (antineutrinos), do not carry away all the energy of the decay reaction. Excess energy is emitted in the form of one or more -quanta
.
3. Capture of an electron by one of the atomic shells by the nucleus. As a result of this process, called electron capture (EC), the atomic number (as in β + decay) is reduced by one unit, and the reaction energy is carried away by neutrinos and in some cases also by -γ radiation, for example:
.
When occupying a vacant position on electron shell another electron also produces a characteristic x-ray radiation element - the product of the reaction.
Electronic gripper with K-, L-shells are usually called accordingly ( TO-grab, L-grab, etc.
4. Spontaneous fission of some heavy nuclei (238 U, 232 Th) into two parts, usually with unequal mass. During spontaneous fission, in addition to the fission fragments, two or three neutrons and sometimes other particles are emitted. Newly formed nuclei are usually unstable and decay by emitting several neutrons and β - particles. In nuclear geophysics, the emission of so-called delayed neutrons accompanying β-decay by some fission products is of interest, for example:
The registration of such neutrons is used to determine the uranium content.
5. The emission of one or two protons, in which the mass and charge are reduced by one or two units, is observed only in some artificial radioactive isotopes with exclusively big deficit neutrons (respectively with an excess of protons), for example:
.
This type of decay was recently discovered by Soviet scientists, and its significance for nuclear geophysics has not yet been studied.
Sometimes radioactive decay also includes the transition of some nuclei from a metastable (relatively stable excited) state to the ground state with the emission of one or more γ quanta. In this case, no nuclear transformation (in the sense of a change in its mass or charge) occurs. However, the law of decrease in the number of active (metastable) nuclei coincides with the law of radioactive decay, which justifies the attribution of this process, called isomeric transition (IT), to special type radioactivity.
An excited isomer nucleus of some element M X is usually denoted M m X. Isomers are usually obtained by excitation of nuclei during bombardment with nuclear particles or sometimes as an intermediate product during the decay of certain nuclei. For example, during the decay of UX 1, in addition to the isotope 234 Pa(UZ), its isomer 234 m Pa(UX 2) is formed, which has a different half-life.
Typically, a radioactive element decays in one of the ways listed above. However, many of them can break down in different ways. For example, 226 Ra in 99% of cases turns into 222 Rn, emitting an alpha particle with an energy of 4.9 MeV. However, the transition of radium into radon is observed with the emission of two particles: an α-particle with an energy of 4.7 MeV and a γ-quantum with an energy of 0.2 MeV. Some radioactive elements decay to form two or more new elements. Thus, about 12% of atoms experience 40 K TO-capture and turn into argon atoms 40 Ar with subsequent emission of γ -quanta with an energy of 1.46 MeV. The remaining 88% of 40 K is converted into calcium 40 Ca atoms with β-particle emission. The decay of artificial radioactive elements is usually accompanied by the emission of electrons (or positrons) and γ rays.
More than 50 naturally occurring radioactive elements have been discovered in nature. Most common heavy elements, which are part of the radioactive families of uranium, actinouranium AcU and thorium (Fig. 5.1). Elements of the neptunium family are found in nature in negligible quantities, the decay of which is limited to one link of transformations. From the analysis of Fig. 5.1 it follows that the nature of the decay of these families has much in common.
