Transformation of atomic nuclei laws of radioactive decay. Radioactive transformations of atomic nuclei Radioactive transformations of atomic nuclei briefly physics

What happens to matter during radioactive radiation?
Already at the very beginning of radioactivity research, many strange and unusual things were discovered.

Firstly What was surprising was the consistency with which the radioactive elements uranium, thorium and radium emitted radiation.
Over the course of days, months and even years, the radiation intensity did not change noticeably.
It was unaffected by such usual influences as heat and increased pressure.
The chemical reactions into which radioactive substances entered also did not affect the intensity of the radiation.

Secondly, very soon after the discovery of radioactivity, it became clear that radioactivity is accompanied by the release of energy.
Pierre Curie placed an ampoule of radium chloride in a calorimeter.
α-, β- and γ-rays were absorbed in it, and due to their energy the calorimeter was heated.
Curie determined that radium weighing 1 g emits energy approximately equal to 582 J in 1 hour.
And such energy is released continuously for many years!

Where does the energy come from, the release of which is not affected by all known influences?
Apparently, during radioactivity, a substance experiences some profound changes, completely different from ordinary chemical transformations.
It was assumed that the atoms themselves undergo transformations.
Now this thought may not cause much surprise, since a child can hear about it even before he learns to read.
But at the beginning of the 20th century. it seemed fantastic, and it took great courage to dare to express it.
At that time, indisputable evidence for the existence of atoms had just been obtained.
Democritus's idea of ​​the atomic structure of matter finally triumphed.
And almost immediately after this, the immutability of atoms is called into question.

So, during radioactive decay, a chain of successive transformations of atoms occurs.
Let us dwell on the very first experiments begun by Rutherford and continued by him together with the English chemist F. Soddy.

Rutherford discovered that activity thorium, defined as the number of alpha particles emitted per unit time, remains unchanged in a closed ampoule.
If the preparation is blown with even very weak air currents, then the activity of thorium is greatly reduced.
The scientist suggested that, simultaneously with α-particles, thorium emits some kind of radioactive gas.

By sucking air from an ampoule containing thorium, Rutherford isolated the radioactive gas and examined its ionizing ability.
It turned out that the activity of this gas (unlike the activity of thorium, uranium and radium) decreases very quickly with time.
Every minute the activity decreases by half, and after ten minutes it becomes almost equal to zero.
Soddy studied the chemical properties of this gas and found that it does not enter into any reactions, i.e., it is an inert gas.
Subsequently, this gas was called radon and placed in the periodic table of D. I. Mendeleev under serial number 86.

Other radioactive elements also experienced transformations: uranium, actinium, radium.
The general conclusion that scientists made was precisely formulated by Rutherford: “The atoms of a radioactive substance are subject to spontaneous modifications.
At each moment, a small portion of the total number of atoms becomes unstable and disintegrates explosively.
In the overwhelming majority of cases, a fragment of an atom - an α-particle - is ejected at enormous speed.
In some other cases, the explosion is accompanied by the ejection of a fast electron and the appearance of rays, which, like X-rays, have great penetrating power and are called γ-radiation.

It was discovered that as a result of an atomic transformation, a completely new type of substance is formed, completely different in its physical and chemical properties from the original substance.
This new substance, however, is itself also unstable and undergoes a transformation with the emission of characteristic radioactive radiation.

Thus, it is precisely established that the atoms of certain elements are subject to spontaneous disintegration, accompanied by the emission of energy in quantities enormous in comparison with the energy released during ordinary molecular modifications.”

After the atomic nucleus was discovered, it immediately became clear that it was this nucleus that underwent changes during radioactive transformations.
After all, there are no alpha particles in the electron shell at all, and a decrease in the number of shell electrons by one turns the atom into an ion, and not into a new chemical element.
The ejection of an electron from the nucleus changes the charge of the nucleus (increases it) by one.

So, radioactivity is the spontaneous transformation of some nuclei into others, accompanied by the emission of various particles.

Offset rule

Nuclear transformations are subject to the so-called displacement rule, first formulated by Soddy.

During α decay, the nucleus loses its positive charge 2e and its mass M decreases by approximately four atomic mass units.
As a result, the element is shifted two cells to the beginning of the periodic table.

Here, the element is designated, as in chemistry, by generally accepted symbols: the nuclear charge is written as an index at the bottom left of the symbol, and the atomic mass is written as an index at the top left of the symbol.
For example, hydrogen is represented by the symbol
For an α particle, which is the nucleus of a helium atom, the notation, etc., is used.
During beta decay, an electron is emitted from the nucleus
As a result, the nuclear charge increases by one, but the mass remains almost unchanged:

Here it denotes an electron: the index 0 at the top means that its mass is very small compared to the atomic unit of mass; an electron antineutrino is a neutral particle with a very small (possibly zero) mass that carries away part of the energy during β-decay.
The formation of an antineutrino is accompanied by the β-decay of any nucleus, and this particle is often not indicated in the equations of the corresponding reactions.

After β decay, the element moves one cell closer to the end of the periodic table..

Gamma radiation is not accompanied by a change in charge; the mass of the nucleus changes negligibly.

According to the displacement rule, during radioactive decay the total electric charge is conserved and the relative atomic mass of nuclei is approximately conserved.
New nuclei formed during radioactive decay can also be radioactive and undergo further transformations.

So,
During radioactive decay, atomic nuclei transform.

What happens to matter during radioactive radiation? To answer this question at the beginning of the 20th century. it wasn't very easy. Already at the very beginning of radioactivity research, many strange and unusual things were discovered.

First, the amazing consistency with which the radioactive elements uranium, thorium and radium emit radiation. Over the course of days, months and years, the radiation intensity did not change noticeably. It was unaffected by ordinary influences such as heat or increased pressure.

The chemical reactions into which radioactive substances entered also did not affect the intensity of the radiation.

Secondly, very soon after the discovery of radioactivity it became clear that radioactivity is accompanied by the release of energy. Pierre Curie placed an ampoule of radium chloride in a calorimeter. α-, β- and γ-rays were absorbed in it, and due to their energy the calorimeter was heated. Curie determined that 1 g of radium releases 582 J of energy in 1 hour. And this energy is released continuously over a number of years.

Where does the energy come from, the release of which is not affected by all known influences? Apparently, during radioactivity, a substance experiences some profound changes, completely different from ordinary chemical transformations. It was assumed that the atoms themselves undergo transformations!

Now this thought may not cause much surprise, since a child can hear about it even before he learns to read. But at the beginning of the 20th century. it seemed fantastic and it took great courage to decide to express it. At that time, indisputable evidence for the existence of atoms had just been obtained. The centuries-old idea of ​​Democritus about the atomic structure of matter finally triumphed. And almost immediately after this, the immutability of atoms is called into question.

We will not talk in detail about those experiments that ultimately led to complete confidence that during radioactive decay a chain of successive transformations of atoms occurs. Let us dwell only on the very first experiments begun by Rutherford and continued by him together with the English chemist F. Soddy (1877-1956).

Rutherford discovered that thorium activity, defined as the number of decays per unit time, remains unchanged in a closed ampoule. If the preparation is blown with even very weak air currents, then the activity of thorium is greatly reduced. Rutherford suggested that, simultaneously with the alpha particles, thorium emits some kind of gas, which is also radioactive. He called this gas emanation. By sucking air from an ampoule containing thorium, Rutherford isolated the radioactive gas and examined its ionizing ability. It turned out that the activity of this gas decreases rapidly with time. Every minute the activity decreases by half, and after ten minutes it is practically equal to zero. Soddy studied the chemical properties of this gas and found that it does not enter into any reactions, i.e., it is an inert gas. Subsequently, the gas was named radon and placed in the periodic table under serial number 86. Other radioactive elements also experienced transformations: uranium, actinium, radium. The general conclusion that scientists came to was accurately formulated by Rutherford: “The atoms of a radioactive substance are subject to spontaneous modifications. At each moment, a small portion of the total number of atoms becomes unstable and disintegrates explosively. In the overwhelming majority of cases, a fragment of an atom - an α-particle - is ejected at enormous speed. In some other cases, the explosion is accompanied by the ejection of a fast electron and the appearance of rays, which, like X-rays, have high penetrating power and are called γ-radiation. It was discovered that as a result of an atomic transformation, a completely new type of substance is formed, completely different in its physical and chemical properties from the original substance. This new substance, however, is itself also unstable and undergoes a transformation with the emission of characteristic radioactive radiation.

Thus, it is precisely established that the atoms of certain elements are subject to spontaneous disintegration, accompanied by the emission of energy in quantities enormous in comparison with the energy released during ordinary molecular modifications.”

After the atomic nucleus was discovered, it immediately became clear that it was this nucleus that underwent changes during radioactive transformations. After all, there are no os-particles in the electron shell at all, and a decrease in the number of shell electrons by one turns the atom into an ion, and not into a new chemical element. The ejection of an electron from the nucleus changes the charge of the nucleus (increases it) by one. The charge of the nucleus determines the atomic number of the element in the periodic table and all its chemical properties.

Note

Literature

Myakishev G.Ya. Physics: Optics. The quantum physics. 11th grade: Educational. for in-depth study of physics. - M.: Bustard, 2002. - P. 351-353.

Radioactive transformations

In 1903, Pierre Curie discovered that uranium salts continuously and without visible decrease over time release thermal energy, which, per unit mass, seemed enormous compared with the energy of the most energetic chemical reactions. Radium releases even more heat - about 107 J per hour per 1 g of pure substance. It turned out that the radioactive elements available in the depths of the globe were sufficient (under conditions of limited heat removal) to melt the magma

Where is the source of this seemingly inexhaustible energy? Marie Curie put forward at the very end of the 19th century. two hypotheses. One of them (shared by Lord Kelvin ) was that radioactive substances capture some kind of cosmic radiation, storing the necessary energy. In accordance with the second hypothesis, radiation is accompanied by some changes in the atoms themselves, which at the same time lose energy, which is emitted. Both hypotheses seemed equally incredible, but gradually more and more evidence accumulated in favor of the second.

Ernest Rutherford made a great contribution to understanding what happens to radioactive substances. Back in 1895, the English chemist William Ramsay, who became famous for the discovery of argon in the air, discovered another noble gas in the mineral kleveite - helium. Subsequently, significant amounts of helium were discovered in other minerals - but only those that contained uranium and thorium. It seemed surprising and strange - where could a rare gas come from in minerals? When Rutherford began to investigate the nature of the alpha particles that are emitted by radioactive minerals, it became clear that helium is a product of radioactive decay ( cm. RADIOACTIVITY). This means that some chemical elements are capable of “generating” others - this contradicted all the experience accumulated by several generations of chemists.

However, the “transformation” of uranium and thorium into helium was not limited to. In 1899, another strange phenomenon was observed in Rutherford’s laboratory (at that time he was working in Montreal): preparations of the thorium element in a closed ampoule maintained constant activity, but in the open air their activity depended on. Drafts. Rutherford quickly realized that thorium emits radioactive gas (it was called thorium emanation - from the Latin emanatio - outflow, or thoron), the activity of this gas decreased very quickly: by half in about one minute (according to modern data - in 55.6 s). A similar gaseous “emanation” was also discovered in radium (its activity decreased much more slowly) - it was called radium emanation, or radon. Actinium was also found to have its own “emanation,” which disappears in just a few seconds; it was called actinium emanation, or actinon. Subsequently, it turned out that all these “emanations” are isotopes of the same chemical element - radon ( cm. CHEMICAL ELEMENTS).

After assigning each member of the series to one of the isotopes of known chemical elements, it became clear that the uranium series begins with uranium-238 ( T 1/2 = 4.47 billion years) and ends with stable lead-206; since one of the members of this series is the very important element radium), this series is also called the uranium-radium series. The actinium series (its other name is the actinouranium series) also originates from natural uranium, but from its other isotope - 235 U ( T 1/2 = 794 million years). The thorium series begins with the nuclide 232 Th ( T 1/2 = 14 billion years). Finally, the neptunium series, which is not present in nature, begins with the artificially obtained longest-lived isotope of neptunium: 237 Np 233 Pa 233 U 229 Th 225 Ra 225 Ac 221 Fr 217 At 213 Bi 213 Po 2 09 Pb  209 Bi. There is also a “fork” in this series: 213 Bi with a 2% probability can turn into 209 Tl, which already turns into 209 Pb. A more interesting feature of the neptunium series is the absence of gaseous “emanations”, and the end member of the series is bismuth instead of lead. The half-life of the ancestor of this artificial series is “only” 2.14 million years, so neptunium, even if it had been present during the formation of the Solar system, could not “survive” to this day, because The age of the Earth is estimated at 4.6 billion years, and during this time (more than 2000 half-lives) not a single atom would remain of neptunium.

As an example, Rutherford unraveled the complex tangle of events in the radium transformation chain (radium-226 is the sixth member of the radioactive series of uranium-238). The diagram shows both the symbols of Rutherford's time and modern symbols for nuclides, as well as the type of decay and modern data on half-lives; in the above series there is also a small “fork”: RaC with a probability of 0.04% can transform into RaC""(210 Tl), which then turns into the same RaD ( T 1/2 = 1.3 min). This radioactive lead has a fairly long half-life, so during the experiment one can often ignore its further transformations.

The last member of this series, lead-206 (RaG), is stable; in natural lead it is 24.1%. The thorium series leads to stable lead-208 (its content in “ordinary” lead is 52.4%), the actinium series leads to lead-207 (its content in lead is 22.1%). The ratio of these lead isotopes in the modern earth's crust is, of course, related both to the half-life of the parent nuclides and to their initial ratio in the material from which the Earth was formed. And “ordinary”, non-radiogenic, lead in the earth’s crust is only 1.4%. So, if initially there were no uranium and thorium on Earth, the lead in it would not be 1.6 × 10 –3% (about the same as cobalt), but 70 times less (like, for example, such rare metals as indium and thulium!) . On the other hand, an imaginary chemist who flew to our planet several billion years ago would have found much less lead and much more uranium and thorium in it...

When F. Soddy in 1915 isolated lead formed from the decay of thorium from the Ceylon mineral thorite (ThSiO 4), its atomic mass turned out to be equal to 207.77, that is, more than that of “ordinary” lead (207.2). This is a difference from the “theoretical "(208) is explained by the fact that the thorite contained some uranium, which produces lead-206. When the American chemist Theodore William Richards, an authority in the field of measuring atomic masses, isolated lead from some uranium minerals that did not contain thorium, its atomic mass turned out to be almost exactly 206. The density of this lead was slightly less, and it corresponded to the calculated one: ( Pb)  206/207.2 = 0.994(Pb), where (Pb) = 11.34 g/cm 3 . These results clearly show why for lead, as for a number of other elements, there is no point in measuring atomic mass with very high accuracy: samples taken in different places will give slightly different results ( cm. CARBON UNIT).

In nature, the chains of transformations shown in the diagrams continuously occur. As a result, some chemical elements (radioactive) are transformed into others, and such transformations occurred throughout the entire period of the Earth’s existence. The initial members (they are called mother) of radioactive series are the longest-lived: the half-life of uranium-238 is 4.47 billion years, thorium-232 is 14.05 billion years, uranium-235 (also known as “actinouranium” is the ancestor of the actinium series ) – 703.8 million years. All subsequent (“daughter”) members of this long chain live significantly shorter lives. In this case, a state occurs that radiochemists call “radioactive equilibrium”: the rate of formation of an intermediate radionuclide from the parent uranium, thorium or actinium (this rate is very low) is equal to the rate of decay of this nuclide. As a result of the equality of these rates, the content of a given radionuclide is constant and depends only on its half-life: the concentration of short-lived members of the radioactive series is small, and the concentration of long-lived members is greater. This constancy of the content of intermediate decay products persists for a very long time (this time is determined by the half-life of the parent nuclide, which is very long). Simple mathematical transformations lead to the following conclusion: the ratio of the number of maternal ( N 0) and children ( N 1, N 2, N 3...) atoms are directly proportional to their half-lives: N 0:N 1:N 2:N 3... = T 0:T 1:T 2:T 3... Thus, the half-life of uranium-238 is 4.47 10 9 years, radium 226 is 1600 years, therefore the ratio of the number of atoms of uranium-238 and radium-226 in uranium ores is 4.47 10 9:1600 , from which it is easy to calculate (taking into account the atomic masses of these elements) that for 1 ton of uranium, when radioactive equilibrium is reached, there is only 0.34 g of radium.

And vice versa, knowing the ratio of uranium and radium in ores, as well as the half-life of radium, it is possible to determine the half-life of uranium, and to determine the half-life of radium you do not need to wait more than a thousand years - it is enough to measure (by its radioactivity) the decay rate (i.e. .d value N/d t) a small known quantity of that element (with a known number of atoms N) and then according to the formula d N/d t = –N determine the value  = ln2/ T 1/2.

Law of displacement. If the members of any radioactive series are plotted sequentially on the periodic table of elements, it turns out that the radionuclides in this series do not shift smoothly from the parent element (uranium, thorium or neptunium) to lead or bismuth, but “jump” to the right and then to the left. Thus, in the uranium series, two unstable isotopes of lead (element No. 82) are converted into isotopes of bismuth (element No. 83), then into isotopes of polonium (element No. 84), and then again into isotopes of lead. As a result, the radioactive element often returns back to the same cell of the table of elements, but an isotope with a different mass is formed. It turned out that there is a certain pattern in these “jumps”, which F. Soddy noticed in 1911.

It is now known that during  decay, an  particle (the nucleus of a helium atom) is emitted from the nucleus, therefore, the charge of the nucleus decreases by 2 (a shift in the periodic table by two cells to the left), and the mass number decreases by 4, which allows us to predict what isotope of the new element is formed. An illustration can be the -decay of radon:  + . During  decay, on the contrary, the number of protons in the nucleus increases by one, but the mass of the nucleus does not change ( cm. RADIOACTIVITY), i.e. there is a shift in the table of elements by one cell to the right. An example is two successive transformations of polonium formed from radon:   . Thus, it is possible to calculate how many alpha and beta particles are emitted, for example, as a result of the decay of radium-226 (see uranium series), if we do not take into account the “forks”. Initial nuclide, final nuclide - . The decrease in mass (or rather, mass number, that is, the total number of protons and neutrons in the nucleus) is equal to 226 – 206 = 20, therefore, 20/4 = 5 alpha particles were emitted. These particles carried away 10 protons, and if there were no  decays, the nuclear charge of the final decay product would be equal to 88 – 10 = 78. In fact, there are 82 protons in the final product, therefore, during the transformations, 4 neutrons turned into protons and 4  particles were emitted.

Very often, an -decay is followed by two -decays, and thus the resulting element returns to the original cell of the table of elements - in the form of a lighter isotope of the original element. Thanks to these facts, it became obvious that D.I. Mendeleev’s periodic law reflects the relationship between the properties of elements and the charge of their nucleus, and not their mass (as it was originally formulated when the structure of the atom was not known).

The law of radioactive displacement was finally formulated in 1913 as a result of painstaking research by many scientists. Notable among them were Soddy's assistant Alexander Fleck, Soddy's trainee A.S. Russell, the Hungarian physical chemist and radiochemist György Hevesy, who worked with Rutherford at the University of Manchester in 1911–1913, and the German (and later American) physical chemist Casimir Fajans (1887–1975). ). This law is often called the Soddy–Faience law.

Artificial transformation of elements and artificial radioactivity. Since the time of Becquerel, it has been noticed that the most ordinary substances that have been near radioactive compounds themselves become more or less radioactive. Rutherford called it “excited activity,” the Curies called it “induced activity,” but for a long time no one could explain the essence of the phenomenon.

In 1919, Rutherford studied the passage of alpha particles through various substances. It turned out that when fast-flying -particles hit the nuclei of light elements, for example, nitrogen, fast-flying protons (hydrogen nuclei) can occasionally be knocked out of them, while the -particle itself becomes part of the nucleus, which increases its charge by one. Thus, as a result of the reaction +  +, another chemical element is formed from nitrogen - oxygen (its heavy isotope). This was the first artificially carried out reaction of converting one element into another. In this, as well as all other nuclear processes, both the total charge (subscripts) and the mass number are conserved, i.e. total number of protons and neutrons (superscripts).

The age-old dream of alchemists came true: man learned to transform some elements into others, although no one expected a practical outcome from this skill in Rutherford’s time. Indeed, to obtain α-particles, it was necessary to have their source, for example, a radium preparation. Worse, for every million α-particles released on nitrogen, on average only 20 oxygen atoms were obtained.

Over time, other nuclear reactions were realized, and many of them found practical use. In April 1932, at a meeting of the English Academy of Sciences (Royal Society), Rutherford announced that his laboratory had successfully carried out reactions of splitting light elements (for example, lithium) with protons. To do this, protons obtained from hydrogen were accelerated using high voltages equal to tens or even hundreds of thousands of volts. Protons, having a smaller charge and mass than alpha particles, penetrate the nucleus more easily. Introducing itself into the lithium-7 nucleus, the proton transforms it into a beryllium-8 nucleus, which almost instantly “dumps” excess energy, falling apart into two -particles: +  ()  2. If we take a light isotope of lithium (in natural lithium is 7.5%), then nuclei of two isotopes of helium are formed: +  ()  + . When bombarded with oxygen protons, fluorine was obtained: +  + ; when shelling aluminum – magnesium: + + .

Many different transformations were carried out with deuterons, the nuclei of the heavy hydrogen isotope deuterium, accelerated to high speeds. Thus, during the reaction +  +, superheavy hydrogen – tritium – was produced for the first time. The collision of two deuterons can proceed differently: +  + , these processes are important for studying the possibility of a controlled thermonuclear reaction. The reaction +  ()  2 turned out to be important, since it occurs already at a relatively low energy of deuterons (0.16 MeV) and is accompanied by the release of colossal energy - 22.7 MeV (recall that 1 MeV = 10 6 eV, and 1 eV = 96.5 kJ/mol).

The reaction that occurs when beryllium is bombarded with -particles has gained great practical importance: +  ()  + , it led in 1932 to the discovery of the neutral neutron particle, and radium-beryllium neutron sources turned out to be very convenient for scientific research. Neutrons with different energies can also be obtained as a result of reactions +  + ; +  + ; +  + . Neutrons that have no charge penetrate particularly easily into atomic nuclei and cause a variety of processes that depend both on the nuclide being fired and on the speed (energy) of the neutrons. Thus, a slow neutron can simply be captured by the nucleus, and the nucleus is released from some excess energy by emitting a gamma quantum, for example: +  + . This reaction is widely used in nuclear reactors to control the fission reaction of uranium: cadmium rods or plates are pushed into the nuclear boiler to slow the reaction.

In 1934, husbands Irene and Frederic Joliot-Curie made an important discovery. Having bombarded some light elements with alpha particles (polonium emitted them), they expected a reaction similar to that already known for beryllium, i.e. knocking out neutrons, for example:

If the matter was limited to these transformations, then after stopping the -irradiation, the neutron flux should have dried up immediately, so, having removed the polonium source, they expected the cessation of all activity, but found that the particle counter continued to register pulses that gradually faded - in exact accordance with exponential law. This could be interpreted in only one way: as a result of alpha irradiation, previously unknown radioactive elements appeared with a characteristic half-life of 10 minutes for nitrogen-13 and 2.5 minutes for phosphorus-30. It turned out that these elements undergo positron decay:  + e + ,  + e + . Interesting results were obtained with magnesium, represented by three stable natural isotopes, and it turned out that upon -irradiation they all give radioactive nuclides of silicon or aluminum, which undergo 227- or positron decay:

The production of artificial radioactive elements is of great practical importance, since it allows the synthesis of radionuclides with a half-life convenient for a specific purpose and the desired type of radiation with a certain power. It is especially convenient to use neutrons as “projectiles”. The capture of a neutron by a nucleus often makes it so unstable that the new nucleus becomes radioactive. It can become stable due to the transformation of the “extra” neutron into a proton, that is, due to 227 radiation; There are a lot of such reactions known, for example: +   + e. The reaction of radiocarbon formation occurring in the upper layers of the atmosphere is very important: +  + ( cm. RADIOCARBON ANALYSIS METHOD). Tritium is synthesized by the absorption of slow neutrons by lithium-6 nuclei. Many nuclear transformations can be achieved under the influence of fast neutrons, for example: +  + ; +  + ; +  + . Thus, by irradiating ordinary cobalt with neutrons, radioactive cobalt-60 is obtained, which is a powerful source of gamma radiation (it is released by the decay product of 60 Co - excited nuclei). Some transuranium elements are produced by irradiation with neutrons. For example, from natural uranium-238, unstable uranium-239 is first formed, which during  decay ( T 1/2 = 23.5 min) turns into the first transuranium element neptunium-239, and it, in turn, also through -decay ( T 1/2 = 2.3 days) turns into the very important so-called weapons-grade plutonium-239.

Is it possible to artificially obtain gold by carrying out the necessary nuclear reaction and thus accomplish what the alchemists failed to do? Theoretically, there are no obstacles to this. Moreover, such a synthesis has already been carried out, but it did not bring wealth. The easiest way to artificially produce gold would be to irradiate mercury, the element next in the periodic table after gold, with a stream of neutrons. Then, as a result of the reaction +  +, a neutron would knock out a proton from the mercury atom and turn it into a gold atom. This reaction does not indicate specific mass numbers ( A) nuclides of mercury and gold. Gold in nature is the only stable nuclide, and natural mercury is a complex mixture of isotopes with A= 196 (0.15%), 198 (9.97%), 199 (1.87%), 200 (23.10%), 201 (13.18%), 202 (29.86%) and 204 (6.87%). Consequently, according to the above scheme, only unstable radioactive gold can be obtained. It was obtained by a group of American chemists from Harvard University back in early 1941, irradiating mercury with a stream of fast neutrons. After a few days, all the resulting radioactive isotopes of gold, through beta decay, again turned into the original isotopes of mercury...

But there is another way: if mercury-196 atoms are irradiated with slow neutrons, they will turn into mercury-197 atoms: +  + . These atoms, with a half-life of 2.7 days, undergo electron capture and finally transform into stable gold atoms: + e  . This transformation was carried out in 1947 by employees of the National Laboratory in Chicago. By irradiating 100 mg of mercury with slow neutrons, they obtained 0.035 mg of 197Au. In relation to all mercury, the yield is very small - only 0.035%, but relative to 196Hg it reaches 24%! However, the isotope 196 Hg in natural mercury is just the least, in addition, the irradiation process itself and its duration (irradiation will require several years), and the isolation of stable “synthetic gold” from a complex mixture will cost immeasurably more than the isolation of gold from the poorest ore ( see also GOLD). So the artificial production of gold is of only purely theoretical interest.

Quantitative patterns of radioactive transformations. If it were possible to track a specific unstable nucleus, it would be impossible to predict when it would decay. This is a random process and only in certain cases can the probability of decay be assessed over a certain period of time. However, even the smallest speck of dust, almost invisible under a microscope, contains a huge number of atoms, and if these atoms are radioactive, then their decay obeys strict mathematical laws: statistical laws characteristic of a very large number of objects come into force. And then each radionuclide can be characterized by a very specific value - half-life ( T 1/2) is the time during which half of the available number of nuclei decays. If at the initial moment there was N 0 cores, then after a while t = T 1/2 of them will remain N 0/2, at t = 2T 1/2 will remain N 0/4 = N 0/2 2 , at t = 3T 1/2 – N 0/8 = N 0/2 3 etc. In general, when t = nT 1/2 will remain N 0/2 n nuclei, where n = t/T 1/2 is the number of half-lives (it does not have to be an integer). It is easy to show that the formula N = N 0/2 t / T 1/2 is equivalent to the formula N = N 0e –   t, where  is the so-called decay constant. Formally, it is defined as the proportionality coefficient between the decay rate d N/d t and available number of cores: d N/d t = –N(the minus sign indicates that N decreases over time). Integrating this differential equation gives the exponential dependence of the number of cores on time. Substituting into this formula N = N 0/2 at t = T 1/2, we get that the decay constant is inversely proportional to the half-life:  = ln2/ T 1/2 = 0,693/T 1/2. The value  = 1/ is called the average lifetime of the nucleus. For example, for 226 Ra T 1/2 = 1600 years,  = 1109 years.

According to the given formulas, knowing the value T 1/2 (or ), it is easy to calculate the amount of radionuclide after any period of time, and from them you can calculate the half-life if the amount of radionuclide is known at different points in time. Instead of the number of nuclei, you can substitute radiation activity into the formula, which is directly proportional to the available number of nuclei N. Activity is usually characterized not by the total number of decays in the sample, but by the number of pulses proportional to it, which are recorded by the device measuring activity. If there is, for example, 1 g of a radioactive substance, then the shorter its half-life, the more active the substance will be.

Other mathematical laws describe the behavior of a small number of radionuclides. Here we can only talk about the probability of a particular event. Let, for example, there be one atom (more precisely, one nucleus) of a radionuclide with T 1/2 = 1 min. The probability that this atom will live 1 minute is 1/2 (50%), 2 minutes - 1/4 (25%), 3 minutes - 1/8 (12.5%), 10 minutes - (1/2 ) 10 = 1/10 24 (0.1%), 20 min – (1/2) 20 = 1/1048576 (0.00001%). For a single atom the chance is negligible, but when there are a lot of atoms, for example, several billion, then many of them, no doubt, will live 20 half-lives or much more. The probability that an atom will decay over a certain period of time is obtained by subtracting the obtained values ​​from 100. So, if the probability of an atom surviving 2 minutes is 25%, then the probability of the same atom decaying during this time is 100 - 25 = 75%, probability disintegration within 3 minutes - 87.5%, within 10 minutes - 99.9%, etc.

The formula becomes more complicated if there are several unstable atoms. In this case, the statistical probability of an event is described by a formula with binomial coefficients. If there N atoms, and the probability of the decay of one of them over time t equal to p, then the probability that during the time t from N atoms will decay n(and will remain accordingly N – n), is equal to P = N!p n (1–p) N – n /(N–n)!n! Similar formulas have to be used in the synthesis of new unstable elements, the atoms of which are obtained literally individually (for example, when a group of American scientists discovered the new element Mendelevium in 1955, they obtained it in the amount of only 17 atoms).

It was one of the most important stages in the development of modern physical knowledge. Scientists did not immediately come to the correct conclusions regarding the structure of the smallest particles. And much later, other laws were discovered - for example, the laws of motion of microparticles, as well as features of the transformation of atomic nuclei that occur during radioactive decay.

Rutherford's experiments

The radioactive transformations of atomic nuclei were first studied by the English researcher Rutherford. Even then it was clear that the bulk of the mass of an atom lies in its nucleus, since electrons are many hundreds of times lighter than nucleons. In order to study the positive charge inside the nucleus, in 1906 Rutherford proposed probing the atom with alpha particles. Such particles arose during the decay of radium, as well as some other substances. During his experiments, Rutherford gained an understanding of the structure of the atom, which was given the name “planetary model”.

First observations of radioactivity

Back in 1985, the English researcher W. Ramsay, who is known for his discovery of argon gas, made an interesting discovery. He discovered helium gas in a mineral called kleveite. Subsequently, large amounts of helium were also found in other minerals, but only in those containing thorium and uranium.

This seemed very strange to the researcher: where could gas come from in minerals? But when Rutherford began to study the nature of radioactivity, it turned out that helium was a product of radioactive decay. Some chemical elements “give birth” to others, with completely new properties. And this fact contradicted all the previous experience of chemists of that time.

Frederick Soddy's observation

Together with Rutherford, scientist Frederick Soddy was directly involved in the research. He was a chemist, and therefore all his work was carried out in relation to the identification of chemical elements according to their properties. In fact, the radioactive transformations of atomic nuclei were first noticed by Soddy. He managed to find out what the alpha particles that Rutherford used in his experiments are. After making measurements, scientists found that the mass of one alpha particle is 4 atomic mass units. Having accumulated a certain number of such alpha particles, the researchers discovered that they turned into a new substance - helium. The properties of this gas were well known to Soddy. Therefore, he argued that alpha particles were able to capture electrons from outside and turn into neutral helium atoms.

Changes inside the nucleus of an atom

Subsequent studies were aimed at identifying the features of the atomic nucleus. Scientists realized that all transformations occur not with electrons or the electron shell, but directly with the nuclei themselves. It was the radioactive transformations of atomic nuclei that contributed to the transformation of some substances into others. At that time, the features of these transformations were still unknown to scientists. But one thing was clear: as a result, new chemical elements somehow appeared.

For the first time, scientists were able to trace such a chain of metamorphoses in the process of converting radium into radon. The reactions that resulted in such transformations, accompanied by special radiation, were called nuclear by researchers. Having made sure that all these processes take place precisely inside the nucleus of an atom, scientists began to study other substances, not just radium.

Open types of radiation

The main discipline that may require answers to such questions is physics (grade 9). Radioactive transformations of atomic nuclei are included in her course. While conducting experiments on the penetrating power of uranium radiation, Rutherford discovered two types of radiation, or radioactive transformations. The less penetrating type was called alpha radiation. Later, beta radiation was also studied. Gamma radiation was first studied by Paul Villard in 1900. Scientists have shown that the phenomenon of radioactivity is associated with the decay of atomic nuclei. Thus, a crushing blow was dealt to the previously prevailing ideas about the atom as an indivisible particle.

Radioactive transformations of atomic nuclei: main types

It is now believed that during radioactive decay three types of transformations occur: alpha decay, beta decay, and electron capture, otherwise called K-capture. During alpha decay, an alpha particle is emitted from the nucleus, which is the nucleus of a helium atom. The radioactive nucleus itself is transformed into one that has a lower electrical charge. Alpha decay is characteristic of substances that occupy the last places in the periodic table. Beta decay is also included in the radioactive transformations of atomic nuclei. The composition of the atomic nucleus with this type also changes: it loses neutrinos or antineutrinos, as well as electrons and positrons.

This type of decay is accompanied by short-wave electromagnetic radiation. In electron capture, the nucleus of an atom absorbs one of the nearby electrons. In this case, the beryllium nucleus can turn into a lithium nucleus. This type was discovered in 1938 by an American physicist named Alvarez, who also studied the radioactive transformations of atomic nuclei. The photographs in which the researchers tried to capture such processes contain images similar to a blurry cloud due to the small size of the particles being studied.

In 1900, Rutherford told the English radiochemist Frederick Soddy about the mysterious thoron. Soddy proved that thoron was an inert gas similar to argon, discovered several years earlier in the air; it was one of the isotopes of radon, 220 Rn. The emanation of radium, as it turned out later, turned out to be another isotope of radon - 222 Rn (half-life T 1/2 = 3.825 days), and the emanation of actinium is a short-lived isotope of the same element: 219 Rn ( T 1/2 = 4 s). Moreover, Rutherford and Soddy isolated a new non-volatile element from the transformation products of thorium, different in properties from thorium. It was called thorium X (later it was established that it was an isotope of radium 224 Ra c T 1/2 = 3.66 days). As it turned out, the “thorium emanation” is released precisely from thorium X, and not from the original thorium. Similar examples multiplied: in initially chemically thoroughly purified uranium or thorium, over time there appeared an admixture of radioactive elements, from which, in turn, new radioactive elements were obtained, including gaseous ones. Thus, a-particles released from many radioactive drugs turned into a gas identical to helium, which was discovered in the late 1860s on the Sun (spectral method), and in 1882 discovered in some rocks.

The results of their joint work were published by Rutherford and Soddy in 1902–1903 in a number of articles in the Philosophical Magazine. In these articles, after analyzing the results obtained, the authors came to the conclusion that it is possible to transform some chemical elements into others. They wrote: “Radioactivity is an atomic phenomenon, accompanied by chemical changes in which new types of matter are born... Radioactivity must be considered as a manifestation of an intra-atomic chemical process... Radiation accompanies the transformation of atoms... As a result of an atomic transformation, a completely new type of substance is formed , completely different in its physical and chemical properties from the original substance."

At that time, these conclusions were very bold; other prominent scientists, including the Curies, although they observed similar phenomena, explained them by the presence of “new” elements in the original substance from the very beginning (for example, Curie isolated the polonium and radium contained in it from uranium ore). Nevertheless, Rutherford and Soddy turned out to be right: radioactivity is accompanied by the transformation of some elements into others

It seemed that the unshakable was collapsing: the immutability and indivisibility of atoms, because since the times of Boyle and Lavoisier, chemists had come to the conclusion about the indecomposability of chemical elements (as they said then, “simple bodies,” the building blocks of the universe), about the impossibility of their transformation into each other. What was going on in the minds of scientists of that time is clearly evidenced by the statements of D.I. Mendeleev, who probably thought that the possibility of “transmutation” of elements, which alchemists had been talking about for centuries, would destroy the harmonious system of chemicals that he had created and was recognized throughout the world. elements. In a textbook published in 1906 Basics of Chemistry he wrote: “... I am not at all inclined (on the basis of the harsh but fruitful discipline of inductive knowledge) to recognize even the hypothetical convertibility of some elements into each other and I do not see any possibility of the origin of argon or radioactive substances from uranium or vice versa.”

Time has shown the fallacy of Mendeleev’s views regarding the impossibility of converting some chemical elements into others; at the same time, it confirmed the inviolability of his main discovery - the periodic law. Subsequent work by physicists and chemists showed in which cases some elements can transform into others and what laws of nature govern these transformations.

Transformations of elements. Radioactive series.

During the first two decades of the 20th century. Through the work of many physicists and radiochemists, many radioactive elements were discovered. It gradually became clear that the products of their transformation are often themselves radioactive and undergo further transformations, sometimes quite intricate. Knowing the sequence in which one radionuclide transforms into another has made it possible to construct the so-called natural radioactive series (or radioactive families). There were three of them, and they were called the uranium row, the actinium row and the thorium row. These three series originated from heavy natural elements - uranium, known since the 18th century, and thorium, discovered in 1828 (unstable actinium is not the ancestor, but an intermediate member of the actinium series). Later, the neptunium series was added to them, starting with the first transuranium element No. 93, artificially obtained in 1940, neptunium. Many products of their transformation were also named after the original elements, writing the following schemes:

Uranium series: UI ® UХ1 ® UХ2 ® UII ® Io (ion) ® Ra ® ... ® RaG.

Sea anemone series: AcU ® UY ® Pa ® Ac ® AcK ® AcX ® An ® AcA ® AcB ® AcC ® AcC"" ® AcD.

Thorium series: Th ® MsTh1 ® MsTh2 ® RdTh ® ThХ ® ThEm ® ThA ® ThB ® ThC ® ThC" ® ThD.

As it turned out, these rows are not always “straight” chains: from time to time they branch. So, UX2 with a probability of 0.15% can turn into UZ, it then goes into UII. Similarly, ThC can decay in two ways: the transformation of ThC ® ThC" occurs at 66.3%, and at the same time, with a probability of 33.7%, the process ThC ® ThC"" ® ThD occurs. These are the so-called “forks”, the parallel transformation of one radionuclide into different products The difficulty in establishing the correct sequence of radioactive transformations in this series was also associated with the very short lifetime of many of its members, especially beta-active ones.

Once upon a time, each new member of the radioactive series was considered as a new radioactive element, and physicists and radiochemists introduced their own designations for it: ionium Io, mesothorium-1 MsTh1, actinouranium AcU, thorium emanation ThEm, etc. and so on. These designations are cumbersome and inconvenient; they do not have a clear system. However, some of them are still sometimes traditionally used in specialized literature. Over time, it became clear that all these symbols refer to unstable varieties of atoms (more precisely, nuclei) of ordinary chemical elements - radionuclides. To distinguish between chemically inseparable elements, but differing in half-life (and often in type of decay) elements, F. Soddy in 1913 proposed calling them isotopes

After assigning each member of the series to one of the isotopes of known chemical elements, it became clear that the uranium series begins with uranium-238 ( T 1/2 = 4.47 billion years) and ends with stable lead-206; since one of the members of this series is the very important element radium), this series is also called the uranium-radium series. The actinium series (its other name is the actinouranium series) also originates from natural uranium, but from its other isotope - 235 U ( T 1/2 = 794 million years). The thorium series begins with the nuclide 232 Th ( T 1/2 = 14 billion years). Finally, the neptunium series, which is not present in nature, begins with the artificially obtained longest-lived isotope of neptunium: 237 Np ® 233 Pa ® 233 U ® 229 Th ® 225 Ra ® 225 Ac ® 221 Fr ® 217 At ® 213 Bi ® 213 Po ® 209 Pb ® 209 Bi. There is also a “fork” in this series: 213 Bi with a 2% probability can turn into 209 Tl, which already turns into 209 Pb. A more interesting feature of the neptunium series is the absence of gaseous "emanations", as well as the end member of the series - bismuth instead of lead. The half-life of the ancestor of this artificial series is “only” 2.14 million years, so neptunium, even if it had been present during the formation of the Solar system, could not “survive” to this day, because The age of the Earth is estimated at 4.6 billion years, and during this time (more than 2000 half-lives) not a single atom would remain of neptunium.

As an example, Rutherford unraveled the complex tangle of events in the radium transformation chain (radium-226 is the sixth member of the radioactive series of uranium-238). The diagram shows both the symbols of Rutherford's time and modern symbols for nuclides, as well as the type of decay and modern data on half-lives; in the above series there is also a small “fork”: RaC with a probability of 0.04% can transform into RaC""(210 Tl), which then turns into the same RaD ( T 1/2 = 1.3 min). This radioactive lead has a fairly long half-life, so during the experiment one can often ignore its further transformations.

The last member of this series, lead-206 (RaG), is stable; in natural lead it is 24.1%. The thorium series leads to stable lead-208 (its content in “ordinary” lead is 52.4%), the actinium series leads to lead-207 (its content in lead is 22.1%). The ratio of these lead isotopes in the modern earth's crust is, of course, related both to the half-life of the parent nuclides and to their initial ratio in the material from which the Earth was formed. And “ordinary”, non-radiogenic, lead in the earth’s crust is only 1.4%. So, if initially there were no uranium and thorium on Earth, the lead in it would not be 1.6 × 10 –3% (about the same as cobalt), but 70 times less (like, for example, such rare metals as indium and thulium!) . On the other hand, an imaginary chemist who flew to our planet several billion years ago would have found much less lead and much more uranium and thorium in it...

When F. Soddy in 1915 isolated lead formed from the decay of thorium from the Ceylon mineral thorite (ThSiO 4), its atomic mass turned out to be equal to 207.77, that is, more than that of “ordinary” lead (207.2). This is a difference from the “theoretical "(208) is explained by the fact that the thorite contained some uranium, which produces lead-206. When the American chemist Theodore William Richards, an authority in the field of measuring atomic masses, isolated lead from some uranium minerals that did not contain thorium, its atomic mass turned out to be almost exactly 206. The density of this lead was also slightly less, and it corresponded to the calculated one: r ( Pb) ґ 206/207.2 = 0.994r (Pb), where r (Pb) = 11.34 g/cm3. These results clearly show why for lead, as for a number of other elements, there is no point in measuring atomic mass with very high accuracy: samples taken in different places will give slightly different results ( cm. CARBON UNIT).

In nature, the chains of transformations shown in the diagrams continuously occur. As a result, some chemical elements (radioactive) are transformed into others, and such transformations occurred throughout the entire period of the Earth’s existence. The initial members (they are called mother) of radioactive series are the longest-lived: the half-life of uranium-238 is 4.47 billion years, thorium-232 is 14.05 billion years, uranium-235 (also known as “actinouranium” is the ancestor of the actinium series ) – 703.8 million years. All subsequent (“daughter”) members of this long chain live significantly shorter lives. In this case, a state occurs that radiochemists call “radioactive equilibrium”: the rate of formation of an intermediate radionuclide from the parent uranium, thorium or actinium (this rate is very low) is equal to the rate of decay of this nuclide. As a result of the equality of these rates, the content of a given radionuclide is constant and depends only on its half-life: the concentration of short-lived members of the radioactive series is small, and the concentration of long-lived members is greater. This constancy of the content of intermediate decay products persists for a very long time (this time is determined by the half-life of the parent nuclide, which is very long). Simple mathematical transformations lead to the following conclusion: the ratio of the number of maternal ( N 0) and children ( N 1, N 2, N 3...) atoms are directly proportional to their half-lives: N 0:N 1:N 2:N 3... = T 0:T 1:T 2:T 3... Thus, the half-life of uranium-238 is 4.47 10 9 years, radium 226 is 1600 years, therefore the ratio of the number of atoms of uranium-238 and radium-226 in uranium ores is 4.47 10 9:1600 , from which it is easy to calculate (taking into account the atomic masses of these elements) that for 1 ton of uranium, when radioactive equilibrium is reached, there is only 0.34 g of radium.

And vice versa, knowing the ratio of uranium and radium in ores, as well as the half-life of radium, it is possible to determine the half-life of uranium, and to determine the half-life of radium you do not need to wait more than a thousand years - it is enough to measure (by its radioactivity) the decay rate (i.e. .d value N/d t) a small known quantity of that element (with a known number of atoms N) and then according to the formula d N/d t= –l N determine the value l = ln2/ T 1/2.

Law of displacement.

If the members of any radioactive series are plotted sequentially on the periodic table of elements, it turns out that the radionuclides in this series do not shift smoothly from the parent element (uranium, thorium or neptunium) to lead or bismuth, but “jump” to the right and then to the left. Thus, in the uranium series, two unstable isotopes of lead (element No. 82) are converted into isotopes of bismuth (element No. 83), then into isotopes of polonium (element No. 84), and then again into isotopes of lead. As a result, the radioactive element often returns back to the same cell of the table of elements, but an isotope with a different mass is formed. It turned out that there is a certain pattern in these “jumps”, which F. Soddy noticed in 1911.

It is now known that during a -decay, an a -particle (the nucleus of a helium atom) is emitted from the nucleus, therefore, the charge of the nucleus decreases by 2 (a shift in the periodic table by two cells to the left), and the mass number decreases by 4, which allows us to predict what isotope of the new element is formed. An illustration is the a -decay of radon: ® + . With b-decay, on the contrary, the number of protons in the nucleus increases by one, but the mass of the nucleus does not change ( cm. RADIOACTIVITY), i.e. there is a shift in the table of elements by one cell to the right. An example is two successive transformations of polonium formed from radon: ® ® . Thus, it is possible to calculate how many alpha and beta particles are emitted, for example, as a result of the decay of radium-226 (see uranium series), if we do not take into account the “forks”. Initial nuclide, final nuclide - . The decrease in mass (or rather, mass number, that is, the total number of protons and neutrons in the nucleus) is equal to 226 – 206 = 20, therefore, 20/4 = 5 alpha particles were emitted. These particles carried away 10 protons, and if there were no b-decays, the nuclear charge of the final decay product would be equal to 88 - 10 = 78. In fact, there are 82 protons in the final product, therefore, during the transformations, 4 neutrons turned into protons and 4 b particles were emitted.

Very often, an a-decay is followed by two b-decays, and thus the resulting element returns to the original cell of the table of elements - in the form of a lighter isotope of the original element. Thanks to these facts, it became obvious that D.I. Mendeleev’s periodic law reflects the relationship between the properties of elements and the charge of their nucleus, and not their mass (as it was originally formulated when the structure of the atom was not known).

The law of radioactive displacement was finally formulated in 1913 as a result of painstaking research by many scientists. Notable among them were Soddy's assistant Alexander Fleck, Soddy's trainee A.S. Russell, the Hungarian physical chemist and radiochemist György Hevesy, who worked with Rutherford at the University of Manchester in 1911–1913, and the German (and later American) physical chemist Casimir Fajans (1887–1975 ). This law is often called the Soddy–Faience law.

Artificial transformation of elements and artificial radioactivity.

Many different transformations were carried out with deuterons, the nuclei of the heavy hydrogen isotope deuterium, accelerated to high speeds. Thus, during the reaction + ® +, superheavy hydrogen was produced for the first time - tritium. The collision of two deuterons can proceed differently: + ® + , these processes are important for studying the possibility of a controlled thermonuclear reaction. The reaction + ® () ® 2 turned out to be important, since it occurs already at a relatively low energy of deuterons (0.16 MeV) and is accompanied by the release of colossal energy - 22.7 MeV (recall that 1 MeV = 10 6 eV, and 1 eV = 96.5 kJ/mol).

The reaction that occurs when beryllium is bombarded with a-particles has gained great practical importance: + ® () ® + , it led in 1932 to the discovery of the neutral neutron particle, and radium-beryllium neutron sources turned out to be very convenient for scientific research. Neutrons with different energies can also be obtained as a result of reactions + ® + ; + ® + ; + ® + . Neutrons that have no charge penetrate particularly easily into atomic nuclei and cause a variety of processes that depend both on the nuclide being fired and on the speed (energy) of the neutrons. Thus, a slow neutron can simply be captured by the nucleus, and the nucleus is released from some excess energy by emitting a gamma quantum, for example: + ® + g. This reaction is widely used in nuclear reactors to control the fission reaction of uranium: cadmium rods or plates are pushed into the nuclear boiler to slow the reaction.

If the matter was limited to these transformations, then after the cessation of a-irradiation the neutron flux should have dried up immediately, so, having removed the polonium source, they expected the cessation of all activity, but found that the particle counter continued to register pulses that gradually died out - in exact accordance with exponential law. This could be interpreted in only one way: as a result of alpha irradiation, previously unknown radioactive elements appeared with a characteristic half-life of 10 minutes for nitrogen-13 and 2.5 minutes for phosphorus-30. It turned out that these elements undergo positron decay: ® + e + , ® + e + . Interesting results were obtained with magnesium, represented by three stable natural isotopes, and it turned out that upon a-irradiation they all produce radioactive nuclides of silicon or aluminum, which undergo 227- or positron decay:

The production of artificial radioactive elements is of great practical importance, since it allows the synthesis of radionuclides with a half-life convenient for a specific purpose and the desired type of radiation with a certain power. It is especially convenient to use neutrons as “projectiles”. The capture of a neutron by a nucleus often makes it so unstable that the new nucleus becomes radioactive. It can become stable due to the transformation of the “extra” neutron into a proton, that is, due to 227 radiation; There are a lot of such reactions known, for example: + ® ® + e. The reaction of radiocarbon formation occurring in the upper layers of the atmosphere is very important: + ® + ( cm. RADIOCARBON ANALYSIS METHOD). Tritium is synthesized by the absorption of slow neutrons by lithium-6 nuclei. Many nuclear transformations can be achieved under the influence of fast neutrons, for example: + ® + ; + ® + ; + ® + . Thus, by irradiating ordinary cobalt with neutrons, radioactive cobalt-60 is obtained, which is a powerful source of gamma radiation (it is released by the decay product of 60 Co - excited nuclei). Some transuranium elements are produced by irradiation with neutrons. For example, from natural uranium-238, unstable uranium-239 is first formed, which, during b-decay ( T 1/2 = 23.5 min) turns into the first transuranium element neptunium-239, and it, in turn, also through b-decay ( T 1/2 = 2.3 days) turns into the very important so-called weapons-grade plutonium-239.

Is it possible to artificially obtain gold by carrying out the necessary nuclear reaction and thus accomplish what the alchemists failed to do? Theoretically, there are no obstacles to this. Moreover, such a synthesis has already been carried out, but it did not bring wealth. The easiest way to artificially produce gold would be to irradiate the element next to gold in the periodic table with a stream of neutrons. Then, as a result of the + ® + reaction, a neutron would knock out a proton from the mercury atom and turn it into a gold atom. This reaction does not indicate specific mass numbers ( A) nuclides of mercury and gold. Gold in nature is the only stable nuclide, and natural mercury is a complex mixture of isotopes with A= 196 (0.15%), 198 (9.97%), 199 (1.87%), 200 (23.10%), 201 (13.18%), 202 (29.86%) and 204 (6.87%). Consequently, according to the above scheme, only unstable radioactive gold can be obtained. It was obtained by a group of American chemists from Harvard University back in early 1941, irradiating mercury with a stream of fast neutrons. After a few days, all the resulting radioactive isotopes of gold, through beta decay, again turned into the original isotopes of mercury...

But there is another way: if mercury-196 atoms are irradiated with slow neutrons, they will turn into mercury-197 atoms: + ® + g. These atoms, with a half-life of 2.7 days, undergo electron capture and finally transform into stable gold atoms: + e ® . This transformation was carried out in 1947 by employees of the National Laboratory in Chicago. By irradiating 100 mg of mercury with slow neutrons, they obtained 0.035 mg of 197Au. In relation to all mercury, the yield is very small - only 0.035%, but relative to 196Hg it reaches 24%! However, the isotope 196 Hg in natural mercury is just the least, in addition, the irradiation process itself and its duration (irradiation will require several years), and the isolation of stable “synthetic gold” from a complex mixture will cost immeasurably more than the isolation of gold from the poorest ore(). So the artificial production of gold is of only purely theoretical interest.

Quantitative patterns of radioactive transformations.

If it were possible to track a specific unstable nucleus, it would be impossible to predict when it would decay. This is a random process and only in certain cases can the probability of decay be assessed over a certain period of time. However, even the smallest speck of dust, almost invisible under a microscope, contains a huge number of atoms, and if these atoms are radioactive, then their decay obeys strict mathematical laws: statistical laws characteristic of a very large number of objects come into force. And then each radionuclide can be characterized by a very specific value - half-life ( T 1/2) is the time during which half of the available number of nuclei decays. If at the initial moment there was N 0 cores, then after a while t = T 1/2 of them will remain N 0/2, at t = 2T 1/2 will remain N 0/4 = N 0/2 2 , at t = 3T 1/2 – N 0/8 = N 0/2 3 etc. In general, when t = nT 1/2 will remain N 0/2 n nuclei, where n = t/T 1/2 is the number of half-lives (it does not have to be an integer). It is easy to show that the formula N = N 0/2 t/T 1/2 is equivalent to the formula N = N 0e – l t, where l is the so-called decay constant. Formally, it is defined as the proportionality coefficient between the decay rate d N/d t and available number of cores: d N/d t= – l N(the minus sign indicates that N decreases over time). Integrating this differential equation gives the exponential dependence of the number of cores on time. Substituting into this formula N = N 0/2 at t = T 1/2, we get that the decay constant is inversely proportional to the half-life: l = ln2/ T 1/2 = 0,693/T 1/2. The value t = 1/ l is called the average lifetime of the nucleus. For example, for 226 Ra T 1/2 = 1600 years, t = 1109 years.

According to the given formulas, knowing the value T 1/2 (or l), it is easy to calculate the amount of radionuclide after any period of time, and you can also use them to calculate the half-life if the amount of radionuclide is known at different times. Instead of the number of nuclei, you can substitute radiation activity into the formula, which is directly proportional to the available number of nuclei N. Activity is usually characterized not by the total number of decays in the sample, but by the number of pulses proportional to it, which are recorded by the device measuring activity. If there is, for example, 1 g of a radioactive substance, then the shorter its half-life, the more active the substance will be.

Other mathematical laws describe the behavior of a small number of radionuclides. Here we can only talk about the probability of a particular event. Let, for example, there be one atom (more precisely, one nucleus) of a radionuclide with T 1/2 = 1 min. The probability that this atom will live 1 minute is 1/2 (50%), 2 minutes - 1/4 (25%), 3 minutes - 1/8 (12.5%), 10 minutes - (1/2 ) 10 = 1/10 24 (0.1%), 20 min – (1/2) 20 = 1/1048576 (0.00001%). For a single atom the chance is negligible, but when there are a lot of atoms, for example, several billion, then many of them, no doubt, will live 20 half-lives or much more. The probability that an atom will decay over a certain period of time is obtained by subtracting the obtained values ​​from 100. So, if the probability of an atom surviving 2 minutes is 25%, then the probability of the same atom decaying during this time is 100 - 25 = 75%, probability disintegration within 3 minutes - 87.5%, within 10 minutes - 99.9%, etc.

The formula becomes more complicated if there are several unstable atoms. In this case, the statistical probability of an event is described by a formula with binomial coefficients. If there N atoms, and the probability of the decay of one of them over time t equal to p, then the probability that during the time t from N atoms will decay n(and will remain accordingly N – n), is equal to P = N!p n(1–p) N–n /(N–n)!n! Similar formulas have to be used in the synthesis of new unstable elements, the atoms of which are obtained literally individually (for example, when a group of American scientists discovered the new element Mendelevium in 1955, they obtained it in the amount of only 17 atoms).

The application of this formula can be illustrated in a specific case. Let, for example, there be N= 16 atoms with a half-life of 1 hour. You can calculate the probability of the decay of a certain number of atoms, for example in time t= 4 hours. The probability that one atom will survive these 4 hours is 1/2 4 = 1/16, respectively, the probability of its decay during this time R= 1 – 1/16 = 15/16. Substituting these initial data into the formula gives: R = 16!(15/16) n (1/16) 16–n /(16–n)!n! = 16!15 n /2 64 (16–n)!n! The results of some calculations are shown in the table:

Thus, out of 16 atoms after 4 hours (4 half-lives), not one will remain at all, as one might assume: the probability of this event is only 38.4%, although it is greater than the probability of any other outcome. As can be seen from the table, the probability that all 16 atoms (35.2%) or only 14 of them will decay is also very high. But the probability that after 4 half-lives all atoms will remain “alive” (not one has decayed) is negligible. It is clear that if there are not 16 atoms, but, let’s say, 10 20, then we can say with almost 100% confidence that after 1 hour half of their number will remain, after 2 hours – a quarter, etc. That is, the more atoms there are, the more accurately their decay corresponds to the exponential law.

Numerous experiments conducted since the time of Becquerel have shown that the rate of radioactive decay is practically not affected by temperature, pressure, or the chemical state of the atom. Exceptions are very rare; Thus, in the case of electron capture, the value T 1/2 changes slightly as the oxidation state of the element changes. For example, the decay of 7 BeF 2 occurs approximately 0.1% slower than 7 BeO or metallic 7 Be.

The total number of known unstable nuclei - radionuclides - is approaching two thousand, their lifetime varies within very wide limits. There are known both long-lived radionuclides, for which half-lives amount to millions and even billions of years, and short-lived ones, which decay completely in tiny fractions of a second. The half-lives of some radionuclides are given in the table.

Properties of some radionuclides (for Tc, Pm, Po and all subsequent elements that do not have stable isotopes, data are given for their longest-lived isotopes).

The shortest-lived nuclide known is 5 Li: its lifetime is 4.4·10 –22 s). During this time, even light will travel only 10–11 cm, i.e. a distance only several tens of times greater than the diameter of the nucleus and significantly smaller than the size of any atom. The longest-lived is 128 Te (contained in natural tellurium in an amount of 31.7%) with a half-life of eight septillion (8·10 24) years - it can hardly even be called radioactive; for comparison, our Universe is estimated to be “only” 10 10 years old.

The unit of radioactivity of a nuclide is the becquerel: 1 Bq (Bq) corresponds to one decay per second. The off-system unit curie is often used: 1 Ci (Ci) is equal to 37 billion disintegrations per second or 3.7 . 10 10 Bq (1 g of 226 Ra has approximately this activity). At one time, an off-system unit of the rutherford was proposed: 1 Рд (Rd) = 10 6 Bq, but it was not widespread.

Literature:

Soddy F. History of atomic energy. M., Atomizdat, 1979
Choppin G. et al. Nuclear chemistry. M., Energoatomizdat, 1984
Hoffman K. Is it possible to make gold? L., Chemistry, 1984
Kadmensky S.G. Radioactivity of atomic nuclei: history, results, latest achievements. "Soros Educational Journal", 1999, No. 11

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