Radioactivity is the ability of atoms of some isotopes to decompose spontaneously, emitting radiation. For the first time such radiation emitted by the uranium found Becquerel, therefore in the beginning of radioactive radiations were called Becquerel rays. The main type of radioactive decay is a projection from the nucleus of an atom alpha particle alpha-decay (see alpha-radiation) or beta-particles - beta decay (see Beta-radiation).
In the radioactive decay of the original atom turns into an atom of another element. As result of ejection from the nucleus of an atom alpha particles, which is a set of two protons and two neutrons, the mass number of the resulting atom (see) is reduced to four units, and it is shifted in table D. I. Mendeleev two squares to the left, as the index of the item in the table is equal to the number of protons in the nucleus of an atom. When you dispose of beta particles (electrons) is turning in one core of a neutron into a proton, and consequently the resulting atom is shifted in table D. I. Mendeleev one cell to the right. Mass with almost no changes. Throwing beta particles usually associated with gamma rays (see).
The collapse of any radioactive isotope takes place at the following law: the number of disintegrating in a unit of time atoms (n) is proportional to the number of atoms (N), available at this point in time, i.e., n=λN; factor wavelengths do is called a permanent radioactive decay and is associated with a half-life of the isotope (T) by the relation λ= 0,693/T. the law of decay leads to the fact that for every period of time equal to the half-life period T, the number of isotope halved. If resulting from the radioactive decay of atoms are also radioactive, it is slow accumulation, until the radioactive equilibrium between parent and subsidiary isotopes; the number of atoms of the daughter isotope produced per unit time is equal to the number of atoms decaying at the same time.
It is known more than 40 natural radioactive isotopes. Most of them are located in three radioactive rows (families): uranium, radium, thorium and sea anemone. All of these radioactive isotopes are widely distributed in nature. The presence of them in rocks, waters, atmosphere, flora and living organisms causes natural or natural radioactivity.
In addition to the natural radioactive isotopes, is now known about thousands artificially radioactive. Receive them by nuclear reactions, mainly in nuclear reactors (see nuclear Reactors). Many natural and artificial radioactive isotopes commonly used in medicine for treatment (see Radiation therapy), and especially for the diagnosis of diseases (see Radioisotope diagnostics). Cm. also ionizing Radiation.

Radioactivity (from lat. radius - ray and activus - effective) - the ability of unstable nuclei of atoms spontaneously developing in other, more stable or stable kernel. Such transformations of nuclei are called radioactive, and the kernel or the corresponding atoms of radioactive nuclei (atoms). When radioactive transformations of nuclei emit energy either in the form of charged particles, or gamma-quanta of electromagnetic radiation or gamma radiation.
Transformation, in which the nucleus of a chemical element becomes the nucleus of another element with a different value atomic number, called radioactive decay. Radioactive isotopes (see), formed and existing in nature, called the naturally radioactive; the same isotopes, artificially derived by nuclear reactions,artificially radioactive. Between natural and artificial radioactive isotopes there is no fundamental difference, since the properties of nuclei of atoms and atoms are determined by the composition and structure of the nucleus and independent of the method of their formation.
Radioactivity was discovered in 1896 by Becquerel (A. N. Becquerel), who discovered the radiation of uranium (see)that can cause blackening emulsion and ionize the air. Curie-Sklodowska (M. Curie-Sklodowska) first measured the intensity of the radiation of uranium and at the same time a German scientist Schmidt (G. S. Schmidt) discovered radioactivity from thorium (see). Property isotopes spontaneously emit invisible radiation wife Curie called radioactivity. In July 1898, they announced the opening them in the uranium resin grades of a new radioactive element polonium (see). In December 1898, together with Belonom (G. Bemont) they discovered radium (see).
After the discovery of radioactive elements a number of authors (Becquerel, spouses Curie, Rutherford and others) found that these elements can emit three types of rays, which behave differently in a magnetic field. On the proposal of the Rutherford (E. Rutherford, 1902) these rays were named alpha (see alpha particles), beta (see Beta-radiation) and gamma-rays (see Gamma rays). Alpha rays consist of positively charged alpha particles (double-ionized helium atoms He4); beta-rays of negatively charged particles of a small mass of electrons, gamma rays on the similar nature of x-rays and represent a quantum of electromagnetic radiation.
In 1902, Rutherford and Sodi (F. Soddy) has explained the phenomenon of radioactivity, spontaneous conversion of atoms of one element into atoms of another element that occurs under the laws of chance and accompanied by the release of energy in the form of alpha-, beta-and gamma-rays.
In 1910, M. Curie-Sklodowska with Gobiernos (A. Debierne) received net metallic radium and explored its radioactive properties, in particular, measured in constant decay of radium. Soon opened a number of other radioactive elements. Debern and Gizel (F. Giesel) opened anemone. Gan (Halm Og) opened radiothorium, mesochori, Boltwood (centuries Boltwood) opened ioni, Hahn and Meitner (L. Meitner) opened procaccini. All these isotopes of radioactive elements. In 1903 Pierre Curie and Laborde (C. A. Laborde) showed that the drug radium has always increased temperature and that 1 g of radium with the decay products in 1 hour allocates about 140 kcal. In the same year Ramsay (W. Ramsay) and Sodi found that in a sealed ampoule with radium contained gaseous helium. The work of Rutherford, Dorn (F. Dorn), Debierne and Gisela it was shown that among the decay products of uranium and thorium are rapidly decaying radioactive gases, called the emanation of radium, thorium, and sea anemone (radon, thoron, action). Thus, it was proved that at decay of radium atoms into helium atoms and radon. The laws of radioactive transformation of one element into other when alpha and beta decays (laws displacement) were first formulated Sodi, Faience (K. Fajans) and Russell (W. J. Russell).
These laws are as follows. When the alpha-decay always from the source element is another, which is located in the periodic system D. I. Mendeleev two squares to the left of the source element (serial or atomic number 2 less than the original); in beta decay, always from the source element is another element, which is located in the periodic system one cell to the right of the source element (atomic number one greater than the original item).
The study of transformations of radioactive elements led to the discovery of isotopes, i.e. atoms that have the same chemical properties and atomic numbers, but differ from each other with weight and physical properties, in particular on radioactive properties (type of radiation, the rate of decay). From a large number of open radioactive substances with new elements were only radium (Ra), radon (Rn), polonium (Po) and protactinium (RA), and the rest formerly known isotopes of uranium (U), thorium (Th), lead (Pb), thallium (Tl) and bismuth (Bi).
After opening the Rutherford nuclear structure of atoms and evidence that the kernel determines all the properties of the atom, in particular the structure of its electronic shells and chemical properties (see Atom, atomic Nucleus), it became clear that the radioactive transformations associated with transmutation of atomic nuclei. Further study of the structure of atomic nuclei helped to decipher the mechanism of radioactive transformations.
The first artificial transmutation of nuclei, nuclear reactions (see) - was carried out by Rutherford in 1919 by bombarding the nuclei of the atoms of nitrogen alpha particles of polonium. When the nuclei of nitrogen emitted protons (see) and turned into the kernel oxygen O. In 1934, F. Joliot-Curie and I. Joliot-Curie (F. Joliot-Curie, I. Joliot-Curie) first received by artificial radioactive isotope of phosphorus bombing of alpha-particles, atoms Al. Kernel P30 unlike engines naturally radioactive isotopes, when the collapse was not emit electrons and positrons (see Cosmic radiation and turned into a stable kernel silicon Si30. Thus, in 1934, were both opened artificial radioactivity and radioactive decay - positron decay, or β+-decay.
, Joliot-Curie expressed the idea that all of fast particles (protons, Daytona, neutrons) cause nuclear reactions and can be used to produce natural radioactive isotopes. Fermi (E. Fermi) with al., bombing by neutrons of different elements, received radioactive isotopes almost all chemical elements. Currently, with the help of accelerated charged particles (see Accelerators of charged particles and neutrons implemented large variety of nuclear reactions, in which it became possible to obtain any radioactive isotopes.
In 1937 Alvarez (L. Alvarez) discovered a new type of radioactive transformations - electronic capture. Under electron capture the nucleus of the atom captures an electron shell of the atom and turns into the core of another element. In 1939 Gan and Strassmann (F. Strassmann) opened the fission of a uranium into lighter nuclei fission fragments) bombarded him with neutrons. In the same year, Flerov and Petrzhak showed that the process of fission of uranium is carried out without external influence, spontaneously. Thus, they discovered a new type of radioactive transformations - spontaneous fission of heavy nuclei.
Currently, the following types of radioactive transformations occurring without an external influences, spontaneously, due to domestic reasons, due to the structure of atomic nuclei.
1. Alpha decay. The nucleus with atomic number Z and mass number And emits an alpha particle is a helium nucleus He4 and turns into another engine with Z reduced by 2 units and smaller and 4 units than the original kernel. In General, the alpha decay is the following:

where X is the original kernel, Y is the core product of disintegration.
2. Beta decay there are two types of electron and positron, or β-a - and b+decay (see Beta-radiation). Under electron decay of nuclei emitted electron and neutrino and formed a new kernel with the same mass number A, but with atomic number Z of one more, it is the source of the kernel:

When positron decay of the nucleus emits a positron, and a neutrino and formed a new kernel with the same mass number, but with Z to be one less than the original kernel:

In beta decay, on average, 2/3 of the energy kernel is entrained particles, neutrinos (neutral particles is very small mass, very weakly interacting with matter).
3. Electronic capture (formerly K-capture). The core electron captures to one of the shells of the atom, often with K-shell, emit neutrinos and turns into a new kernel with the same mass number A, but with atomic number Z is less than 1 than the original kernel.

The transformation of nuclei on electron capture and positron decay same, so these two kinds of decay are observed simultaneously for the same nuclei, i.e. they are competing. Because after the capture of an electron with the inner shell of the atom in its place goes electron from one of the more remote from the nucleus orbit, electronic capture always accompanied by the emission of characteristic x-ray radiation.
4. Isomeric transition. After emission of alpha or beta particles, some types of cores are in an excited state (condition with excessive energy and emit the excitation energy in the form of gamma rays (smame radiation). In this case when the radioactive decay of a nucleus, in addition to the alpha or beta particles, also emits gamma-rays. So, nuclei of the isotope Sr90 emit only b-particles, nuclei Na24 emit, in addition to b-particles, also gamma-quanta. Most kernels is in the excited state is very small intervals of time, not measurable (less than 10-9 seconds). However, only a relatively small number of cores can be in the excited state is relatively long periods - up to several months. Such nuclei are called isomers, and the transitions from an excited state to normal, accompanied by the emission of the gamma ray - isomer. When isomeric transitions of a and Z kernel does not change. Radioactive nuclei emitting alpha or beta particles, called pure alpha - or beta-emitters. Kernel, in which alpha or beta decay is accompanied by the emission of a gamma-ray, called a gamma-ray emitters. Pure gamma-emitters are only the kernel, which is a long time in the excited state, i.e. undergoing isomeric transition.
5. Spontaneous fission. In the result of the division of one core is formed by two lighter nuclei - the fission fragments. As the same kernel can share different way on two cores, then in the process of fission produces a lot of different pairs of lighter nuclei with different Z and A. In the fission neutrons are released, on average 2-3 neutrons of one act of fission of nuclei and gamma-quanta. All formed by the fission fragments are unstable and undergo β--decay. The probability of fission is very small for uranium, but increases with increasing Z. This explains the absence on Earth heavier than uranium nuclei. In stable nuclei there is a definite correlation between the number of protons and neutrons, under which the nucleus has the greatest resistance, i.e. the highest binding energy of particles in the nucleus. For light and medium nuclei most of their stability corresponds approximately equal amount of protons and neutrons. For heavier nuclei there is a relative increase in the number of neutrons in a stable nuclei. At surplus in the nucleus of protons or neutrons of the nucleus with a mean value And are unstable and undergo β-or β+-decays, in which there is a mutual transformation of the neutron and proton. At the excess of neutrons (heavy isotopes) is the transformation of one of neutron into proton with the emission of an electron and a neutrino:

At the excess of protons (light isotopes) is the transformation of one of the protons in the neutron emitting or positron, and a neutrino (?+decay)or only neutrinos (electronic capture):

All heavy nucleus with atomic number greater than what Pb82, are unstable due to the significant number of protons, repelling each other. Chain of consecutive alpha and beta decays in these nuclei occur until, until it forms stable nuclei of isotopes of lead. With the improvement of experimental techniques in an increasing number of cores, once regarded as stable, detect very slow radioactive decay. Currently, there are 20 of radioactive isotopes with Z less 82.
As a result of any radioactive transformations of the number of atoms of this isotope is continuously decreasing. The law of decreasing over time the number of active atoms (the law of radioactive decay) is common to all types of transformations and all isotopes. He is statistical in nature (only applies to a large number of radioactive atoms) and consists in the following. The number of active atoms of a given isotope decaying per unit time ΔN/Dt, in proportion to the number of atoms N, i.e. per unit of time disintegrates always the same proportion to the active atoms of a given isotope regardless of their number. The value of K is called the constant of radioactive decay, and a share of active atoms decaying per unit of time, or the relative decay rate. it is measured in units of inverse time measurement units, i.e., in sec.-1 (1/sec.), day-1 year-1, and so on, for each radioactive isotope has a certain value, which varies widely for different isotopes. The value characterizing the absolute rate of decay is called the activity of this isotope or drug. Activity 1 g of substance called specific activity material.
From the radioactive decay law, it follows that the decrease of the amount of active atoms N first occurs quickly, and then slower. The time during which the number of active atoms or activity of this isotope is reduced by half is called the half-life (T) of this isotope. The law descending N time t is exponential and has the following analytical expression: N=N0e-λt, where N0 is the number of active atoms in the beginning of the countdown (g=0), N is the number of active atoms later time t, e is the base of natural logarithms (number equal 2,718...). Between the constant decay and half-life λ there is the following relationship: λТ-0,693. Here

The half-life measured in seconds, minutes, etc., and for various isotopes vary widely from small fractions of seconds up to 10+21 years. Isotopes with large + (a) and small T, called short-lived isotopes with the small + (a) and large T are called long-lived. If the active substance is composed of several radioactive isotopes with different half-lives, genetically unrelated, then over time the activity of the substance will also be continuously reduced and the isotopic composition of the drug will all the time be changed: it will decrease the share of short-lived isotopes, and to increase the share of long-lived isotopes. After a sufficiently long period of time practically in the drug will remain only the long-lived isotopes. On the curve of decay of radioactive substances, consisting of one or a mixture of isotopes, you can determine the half-life of separate isotopes and their relative activity for any moment of time.
Laws of change of activity genetically related isotopes qualitatively different; they depend on the ratio of the periods of their half. For two genetically related isotopes period T1 for the source of isotope and T2 - product of the collapse of these laws have the most simple form. With T1>T2 activity source of isotope Q1 all the time decreases exponentially with half-lives T1. Due to the decay of nuclei source of isotope will form the nucleus of the end of the isotope and its activity Q2 will increase. After a certain time the rate of decay of nuclei second isotope (will be close to the speed of the formation of nuclei of the isotope from the source (the decay rate of the source of isotope Q1) and these rates will be determined and constant ratio further time comes radioactive equilibrium.
The activity of the source of isotope continuously decreases with periods T1, so after reaching radioactive equilibrium activity end of the isotope Q2 and the total activity of the two isotopes Q1+Q2 will also decrease with a half-life source isotope T1. With T1>T2 Q2=Q1. If the source of long-lived isotope produced several short-lived isotopes, as is the case in radioactive number of uranium and radium, after reaching equilibrium of activity of each short-lived isotope become almost equal to the activity of the parent isotope. The total activity equal to the sum of all activities short-lived decay products and decreases with period: half-life source of long-lived isotope, as the activity of all isotopes in equilibrium.
Radioactive equilibrium is almost over time, equal to 5-10 half-life of the isotope of decay products, which has the longest half-life. If T1<T2, then the equilibrium cannot be realized as never reach the constancy of the ratio between the activities of these isotopes. The ratio Q1 : Q2 will continuously decrease over time from OO to 0. Practical interest is the case of radioactive equilibrium, as it occurs in all chains of natural radioactive decay of elements in the ranks of uranium, radium, thorium, and sea anemone and in the reference radium preparations from products of its disintegration (see radium).
Among the natural radioactive isotopes is about 40 isotopes of the periodic system of elements with Z 82, which form three consecutive number of radioactive transformations: a number of uranium (Fig. 1), a number of thorium (Fig. 2) and a number of sea anemone (Fig. 3). By successive alpha - and beta-decay from the source of isotopes come up with the final number of stable isotopes of lead.

a number of uranium
Fig. 1. A number of uranium.
a number of thorium
Fig. 2. A number of thorium.
a number of sea anemone
Fig. 3. A number of sea anemone.

The arrows in figures specified serial radioactive transformations with the indication of the decay and percentage of atoms undergoing decay of this type. Horizontal arrows indicate the transformation that takes place almost in 100% of cases, and the inclined - in some parts of the cases. In the designation of isotopes specified periods of their half. In brackets are given earlier names of members of a number indicating a genetic link, without the parentheses - the currently adopted designation of isotopes that are relevant to their chemical and physical nature. In the framework of concluded long-lived isotopes, and in double-framework - final stable isotopes. Alpha decay is usually accompanied by very low-intensity of gamma-radiation, part of the beta-emitters emits intense gamma rays. Natural background due to natural radioactivity-radiation and the influence of natural radioactive isotopes contained on the surface of the Earth, biosphere and the air, and space radiation (see). In addition to these isotopes in various substances are also isotope 40 and about 20 other radioactive isotopes with very long half-lives (from 109 to 1021 years), so that their relative activity is very small in comparison with the activity of other isotopes.
Radioactive isotopes contained in the shell of the Earth, have played a crucial role in the development of our planet, in particular in the development and preservation of life, as they compensate heat loss occurring on Earth, and provided practical constancy of temperature on the planet for millions of years. Radioactive isotopes, like all other isotopes found in nature mainly in scattered condition and are present in all substances, plants and animals.
Due to differences in physico-chemical properties of isotopes relative content in soils and waters is varied. Gaseous decomposition products of uranium, thorium, and sea anemone - toron, radon and action - from soil water continuously entered into the air. In addition to these gaseous products in the air also contains alpha - and beta-active products of decay of radium, thorium, and sea anemone (in the form of aerosols). From the soil radioactive elements as stable, with soil water coming into the plants, therefore, stems and leaves of plants always contain uranium, radium, thorium with their degradation products, potassium and a number of other isotopes, albeit in relatively small concentrations. In plants and animals are also present isotopes C14, H3 Be7 and others, which are formed in the air under the influence of neutron cosmic radiation. Due to the fact that committed a continuous exchange between the human body and the environment, all of radioactive isotopes contained in food, water and air, are contained in the body. Isotopes are in the body in the following doses: in the soft tissues-31 mrem/year, in the bones, ber/year. The dose of cosmic radiation is 80-90 mrem/yr dose from external gamma-radiation - 60-80 mrem/year. The total dose equal 140-200 mrem/year. Dose falling on light, - 600-800 mrem/year.
Artificially radioactive isotopes are obtained by the bombing of stable isotopes neutrons or charged particles as a result of various nuclear reactions, as sources of charged particles using different types of accelerators.
About flow measurements and doses of different types of ionizing radiation - see Dosimetry, the Dose of ionizing radiation, Neutron.
Because large doses of radiation harmful for people's health, when working with radiation sources and radioactive isotopes are special measures of protection (see Antiradiation protection).
In medicine and biology isotopes used to study the metabolism, diagnostic and therapeutic purposes (see Radiation therapy). Radioactive isotopes in the body and the dynamics of their currency determined using a counter of external radiation from man.