RADIOACTIVITY

From Big Medical Encyclopedia

RADIOACTIVITY — the spontaneous transformation of an unstable isotope of one chemical element from the basic or the excited (metastable) state in isotope of other element which is followed by emission of elementary particles or kernels. Such transformations of kernels call radioactive transformations, and such kernels or the corresponding atoms — radioactive.

Opening of the phenomenon of R. played a huge role in development of scientific and technical progress, production of radioisotopes and creation of nuclear power. In biology and medicine it matters, in particular, for studying of processes of permeability (see), a metabolism, in diagnosis and at treatment of a number of diseases (see. Radiation therapy , Radio isotope diagnosis ).

R.'s opening belongs to 1896 when A. Bekkerel found emission by uranium of an unknown type of the getting radiation. Radioactivity of thorium was soon found, and in 1898 M. Sklodovskaya-Curie and P. Curie opened two new radioelements — polonium and radium. They entered the term «radioactivity». Works of E. Rutherford and the mentioned scientists established existence of three types of radiation of radioelements — alpha, beta and gamma radiation and their nature is revealed (see. Alpha radiation , Beta radiation , Gamma radiation ). In 1903 E. Rutherford and Soddi found out that alpha radiation is followed by transformation of chemical elements, napr, radium in radon. In 1934 F. Zholio-Curie and I. Zholio-Curie opened the artificial River. From total number (apprx. 2 thousand) radioisotopes known nowadays (see) only apprx. 300 are natural, and the others are received in the artificial way as a result of nuclear reactions. Thanks to artificial R.'s studying new options of a beta decay — emission of positrons and electron capture were open (see). In 1940 K. A. Petrzhak and G. N. Flerov opened spontaneous (spontaneous) division of kernels. In 1971 I. Goldansky predicted two-proton R.'s existence, and in 1971 I. Goldansky and L. K. Peker — two-neutron radioactive division of kernels.

Now 6 types of elementary radioactive transformations are experimentally established or theoretically predicted: alpha decay (see), all options of a beta decay (see), spontaneous division of kernels, proton R., the two-proton, two-neutron River. Proton R. is experimentally found, however the probability of disintegration on this type is low. Two-proton and two-neutron R. experimentally are not found yet. Radioactive decay often is followed by the gamma radiation as as a result of any radioactive process the affiliated kernel can appear in wild spirits. At the same time there is no change of structure of a kernel, i.e. does not come transformations of one isotope into another. Therefore in this case gamma radiation is not considered as independent type of disintegration. However if gamma quanta are radiated at formation of ground states of atomic kernels from long-living — metastabilyiy or isomeric states, then such gamma transitions sometimes consider as special type P. (see. Isomerism ).

Spontaneous transformations of radioactive kernels lead to continuous reduction of number of atoms (kernels) of initial radioisotope and to formation of affiliated products.

The law of radioactive decay for any transformations of kernels establishes that for a unit of time the same share of not broken up kernels of this radionuclide breaks up always. This share is called a constant of disintegration a lambda. Follows from the law of radioactive decay that quantity of kernels of dN which are breaking up for a time term of dt, in direct ratio to quantity of not broken up: dN/dt = λN.

Rate of decay (dN/dt) of atoms of radioactive material is called activity or absolute activity (Q) of drug: Q = λN (i.e. activity of drug is proportional to quantity of kernels). In process of disintegration of kernels activity of radionuclide decreases under the law Q = Q 0 e - λt , where Q 0 — activity in initial timepoint; Q = activity in timepoint of t; е = 2,718.... Time, during to-rogo breaks up a half of initial quantity of radioactive kernels and activity decreases twice, is called half-life (see). Between a constant of disintegration and half-life (T) there is a ratio: T = Ln2/λ = 0,693/λ.

Half-life for various radionuclides matters from fractions of a second to billions of years. Respectively and radioisotopes divide on short-lived (hours, days) and long-living (many years).

1 disintegration in 1 sec. — becquerel (Bq) is taken for an activity unit in the International System of Units (SI). Specific activity is expressed in becquerels on kilogram (Bq/kg). Historically the activity unit of curie was widely adopted (To and) — activity of such radioactive drug, in Krom for 1 sec. occurs 3,7 • 10 10 acts of disintegration. For designation of specific activity often use unit of curie on kilogram (Ki/kg) or curie on liter (Ki/l) or fractional units, e.g. mkyuri/ml, mkkyuri/ml, mkkyuri/g, etc. In practical work with radioactive materials absolute activity of drugs, as a rule, is not defined directly. Measuring devices usually give the size proportional to the absolute radioactivity of drug; this size call the registered activity 1. During the work with counters of nuclear particles as the registered activity the speed of the account expressed in impulses in a minute is (imp / mines), and the coefficient connecting the size of the absolute and registered activity is called coefficient of the account (φ): J = φa.

Heavy-nuclei because of a large number of protons in them are unstable and as a result of radioactive decay form the affiliated kernels which are also radioactive and then formation of a stable isotope comes on a chain — from several acts of radioactive decay. Groups of genetically connected radioisotopes in which each subsequent isotope results from disintegration previous are called decay series. Heavy isotopes with mass number more than 209 form ranks of thorium ( 232 Th), actinouranium ( 235 U) and uranium (uranium — radium, 238 U). Half-lives of thorium-232 (1,41 • 10 10 years), uranium-235 (7,13 • 10 8 years) and uranium-238 (4,51 • 10 9 years) are commensurable Earth with age. As a result of consecutive disintegrations isotopes of radium are formed of initial isotopes of decay series ( 226 Ra, 224 Ra, 223 Ra) which by alpha decay turn into isotopes of radon ( 222 Rn, 220 Rn, 219 Rn), being radioactive gases (see). Ranks come to an end with stable isotopes of lead: 232 Th -> 208 Pb, 235 U -> 207 Pb, 238 U -> 206 Pb.

Some isotopes, members of a decay series, break up in two ways with various probability. The competition of different ways of disintegration is resulted by branchings of radioactive transformations. Each decay series contains long and short-lived isotopes. However if isotope belongs to a natural decay series, then it surely is in the nature even if the rate of decay of its kernels is very big. It is connected with the fact that in decay series so-called century balance is established eventually. Presence at the nature of short-lived radionuclides, such as actinium, radium, radon, polonium is explained by it.

Radioactivity of the environment is caused by the content of natural radionuclides in various natural educations: in crust, the soil, reservoirs, the atmosphere, the biosphere. The important factor determining the radioactivity of the environment is activity of the person. The distributional pattern of radioelements in the environment causes their external and internal action on a human body. Sources of external radiation are space radiation (see), the radiation of Earth and land-based sources and to a lesser extent the atmosphere. In a year this dose averages 70 — 92 mrad in gonads, 77 — 110 mrad in endosteal cells and 67 — 120 mrad in red marrow.

Impact of radioactivity on the person in the doses significantly exceeding a natural background is undesirable. Level of a background is natural border for assessment of impact of ionizing radiation on the person (see. background radiation ). In this regard radiation control behind the content of radioactive materials (see) in the environment and in a human body is established (see. Radiation control , Radiometry ). During the work with sources of ionizing radiation and radioactive materials special measures of protection are applied (see. Antiactinic protection , Radiation safety ).

See also Ionizing radiation .


Bibliography: Belousova I. M. and Shtukkenberg Yu. M. Natural radioactivity, M., 1961; Dubinin N. P. and Pashin Yu. V. Mutagenez and environment, M., 1978; Ivanov I. I., etc. Radioisotopes in medicine and biology, page 5, M., 1955; Isaev B. M. and B r e-@ and d z e Yu. I. Neutrons in a radio biological experiment, M., 1967; Sources and actions of ionizing radiation, the Report for 1977 of scientific UN committee on action of atomic radiation, the lane with English, t. 1, appendix B, page 71, New York, 1978; M. Radioaktivnost's Curie, the lane with fr., M., 1960; Standards of radiation safety of NRB-76, M., 1978; Pertsov of L *. A. Ionizing radiation of the biosphere, M., 1973; Radiation medicine, under the editorship of A. I. Burnazyan and A. V. Lebedinsky, page 5, M., 1963.

Yu. M. Shtukkenberg.

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