NUCLIDE ACTIVITY

in a radioactive source - a value equal to the ratio of the total number of decays of radioactive nuclei of a nuclide in the source to the decay time. Unit A. n. (in SI) - becquerel(Bq). Non-systemic unit. - curie(Ki); 1 Ki = 3,700 * 10 10 Bq. Specific A. n is also applied: 1) mass A. n., Equal to the ratio A. n. to the mass of the source (Bq / kg); 2) volumetric A. n., Equal to the ratio A. n. to the volume of the source (Bq / m 3); 3) molar A.N., equal to the ratio A.N. to the number of source islands (Bq / mol).

Big Encyclopedic Polytechnic Dictionary. 2004 .

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The unit of isotope activity is becquerel (Bq), equal to the activity of a nuclide in a radioactive source, in which one decay occurs during 1 s.

1.2 The law of radioactive decay

The rate of radioactive decay is proportional to the number of available nuclei N:

where λ is the decay constant.

LnN = λt + const,

If t = 0, then N = N0 and, therefore, const = -lg N0. Finally

N = N0 e-λt (1)

where A - activity at time t; A0 - activity at t = 0.

Equations (1) and (2) characterize the law of radioactive decay. In kinetics, they are known as first-order reaction equations. As a characteristic of the rate of radioactive decay, the half-life T1 / 2 is usually indicated, which, like λ, is a fundamental characteristic of the process, independent of the amount of matter.

Half-life called the period of time during which a given amount of radioactive substance is reduced by half.

The half-lives of different isotopes differ significantly. It is about 1010 years old to a tiny fraction of a second. Of course, substances with a half-life of 10-15 minutes. and less difficult to use in the laboratory. Isotopes with a very long half-life are also undesirable in the laboratory, since in case of accidental contamination of surrounding objects with these substances, special work will be required to decontaminate the room and devices.

2. Methods of analysis based on the measurement of radioactivity

2.1. Using natural radioactivity in analysis

Elements with natural radioactivity can be quantified by this property. These are U, Th, Ra, Ac, etc., more than 20 elements in total. For example, potassium can be determined by its radioactivity in solution at a concentration of 0.05 M. The determination of various elements by their radioactivity is usually carried out using a calibration graph showing the dependence of activity on the content (%) of the determined element or by the method of additions.

Radiometric methods are of great importance in the prospecting work of geologists, for example, in the exploration of uranium deposits.

2.2. Activation analysis

When exposed to neutrons, protons, and other high-energy particles, many non-radioactive elements become radioactive. Activation analysis is based on the measurement of this radioactivity. Although, in principle, any particles can be used for irradiation, the process of neutron irradiation is of the greatest practical importance. The use of charged particles for this purpose is associated with overcoming more significant technical difficulties than in the case of neutrons. The main sources of neutrons for the activation analysis are the atomic reactor and the so-called portable sources (radium-beryllium, etc.). In the latter case, α-particles produced by the decay of any α-active element (Ra, Rn, etc.) interact with beryllium nuclei, releasing neutrons:

9Be + 4He → 12C + n

Neutrons enter into a nuclear reaction with the components of the analyzed sample,

for example

55Mn + n = 56Mn or Mn (n, γ) 56Mn

Radioactive 56Mn decays with a half-life of 2.6 hours:

55Mn → 56Fe + e-

To obtain information about the composition of the sample, its radioactivity is measured for some time and the resulting curve is analyzed. In carrying out such an analysis, it is necessary to have reliable data on the half-lives of the various isotopes in order to decipher the summary curve.

Another option for activation analysis is the γ-spectroscopy method, based on the measurement of the γ-radiation spectrum of the sample. The energy of γ-radiation is qualitative, and the counting rate is a quantitative characteristic of the isotope. Measurements are performed using multichannel γ-spectrometers with scintillation or semiconductor counters. This is a much faster and more specific, although somewhat less sensitive, method of analysis than radiochemical.

An important advantage of activation analysis is its low detection limit. With its help, up to 10-13 - 10-15 g of a substance can be detected under favorable conditions. In some special cases, it was possible to achieve even lower detection limits. For example, it is used to control the purity of silicon and germanium in the semiconductor industry, detecting impurities up to 10-8 - 10-9%. Such contents cannot be determined by any other method other than activation analysis. When receiving heavy elements of the periodic system, such as mendelevium and curchatovium, researchers were able to count almost every atom of the resulting element.

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In the SI system, the absolute radioactivity is estimated at becquerell (Bq). 1 Bq is understood as the amount of any radioactive isotope in which, on average, one decay occurs in 1 s

1 Bq = 1 decay / s.

Derived from becquerel: megabecquerel (10 6 Bq), gigabecquerel (10 9), etc.

In practice, a non-systemic unit of activity is sometimes used. curie - the amount of radioactive isotope in which 3.7 × 10 10 decays occur in 1 s (the same as in 1 g of Ra).

1 Ci = 37 × 10 9 Bq.

According to the radioactive equilibrium equation (1.10), the activity of elements of the radioactive series can be expressed through the activity of its ancestor

where n- the number of elements in a row.

In other words, to estimate the radioactivity of the uranium or thorium series, it is enough to know the amount of uranium or thorium. This circumstance greatly simplifies the study of the radioactivity of rocks, since in the case of radioactive equilibrium there is no need to determine the contents of those radioactive elements that are part of the series.

The concentration of a radioactive isotope in a certain substance is estimated specific mass Bq / kg and specific volume Bq / m 3 activity ... The concentrations of radon and other gaseous radioelements are expressed in Bq / l.

The activity of the isotope is proportional to the product of the decay constant l on the number of nuclei of a radioactive substance N... In this case, the number of nuclei isotope corresponding to an activity of 1 Bq:

where M- the relative atomic mass of the isotope;

- its half-life;

Is Avogadro's number.

It follows from the formula that the mass of radioactive elements corresponding to a given activity increases with an increase in the half-life.

For example, the mass of radium with an activity of 10 6 Bq = 1590 years is 27 × 10 -6 g. The mass of uranium with the same activity (= 4.49 × 10 9 years) is 80 g.

To characterize the g-activity of a substance, use the value of the radium g-element E g and the off-system unit milligram-equivalent of radium (mg-eq. Ra) - the amount of isotope, the g-radiation of which has the same ionizing ability (in air) as g- radiation of 1 mg 226 Ra (together with its decay products) after passing through a platinum filter with a thickness of 0.5 mm.

Basic radiological quantities and units

A substance is considered radioactive, or it contains radionuclides and the process of radioactive decay takes place in it. The amount of a radioactive substance is usually determined not by units of mass (gram, milligram, etc.), but by the activity of this substance.

The activity of a substance is determined by the intensity or rate of decay of its nuclei. The activity is proportional to the number of radioactive atoms contained in a given substance, i.e. increases with an increase in the amount of this substance. Activity is a measure of the amount of radioactive material, which is expressed by the number of radioactive transformations (nuclear decays) per unit time. Since the decay rate of radioactive isotopes is different, radionuclides of the same mass have different activities. The more nuclei decay per unit of time, the higher the activity. Activity is usually measured in decays per second. The unit of activity in the International System of Units (SI) is one decay per second. This unit is named after Henri Becquerel, who first discovered the phenomenon of natural radioactivity in 1896, Becquerel (Bq). 1 Bq is the amount of a radionuclide in which one decay occurs in one second. Since becquerel is a very small value, multiples are used: kBq - calobecquerel (103 Bq), MBq - megabecquerel (106 Bq), GBq - gigabecquerel (109 Bq).

The off-system unit of activity is the curie (Ki). Curie is such activity when the number of radioactive decays per second is equal to
3.7 x 1010 (37 billion dec / s). Curie corresponds to the activity of 1 g of radium. Since the curie is a very large value, the derived quantities are usually used: mCi - millicurie (thousandth part of curie) - 3.7 x 107 dec / s; μCi - microcurie (millionth curie) - 3.7 x 104 dec / s; nCi - nanocurie (billionth fraction of curie) - 3.7x10 dec / s.

Knowing the activity in becquerels, it is not difficult to go over to the activity in curie and vice versa:

1 Ci = 3.7 x 1010 Bq = 37 gigabecquerel;

1 mCi = 3.7 x 107 Bq = 37 megabecquerel;

1 mCiCi = 3.7 x 104 Bq = 37 kilobecquerel;

1 Bq = 1 dec / s = 2.7 x 10-11 Ci.

In practice, the number of decays per minute is often used.

1 Ki = 2.22 x 1012 rpm.

1 mCi = 2.22 x 109 rpm.

1 mCi = 2.22 x 106 rpm.

When measuring the activity of a radioactive sample, it is usually referred to as mass, volume, surface area, or length. There are the following types of radionuclide activity. Specific activity is the activity per unit mass of a substance (activity per unit mass) - Bq / kg, Ci / kg. Volumetric activity is the activity per unit volume - Bq / l, Ci / l, Bq / m3, Ci / m3. In the case of distribution of radionuclides on the surface, the activity is called superficial (the ratio of the activity of the radionuclide on which the radionuclide is located) - Bq / m2, Ci / m2. To characterize the pollution of the territory, the value of Ki / km2 is used. Natural potassium-40 in soil corresponds to 5mCi / km2 (200 Bq / m2). When the area is polluted in
40 Ci / km2 for cesium-137 per 1 m2 of surface there are 2,000,000 billion nuclei, or 0.455 micrograms of cesium-137. Linear activity radionuclide - the ratio of the activity of the radionuclide contained along the length of the segment to its length.

The mass in grams with a known activity (for example, 1Ki) of the radionuclide is determined by the formula m = k x A x T½ x a, where m is the mass in grams; A is the atomic mass; T½ is the half-life; a - activity in curies or becquerels; k is a constant depending on the units in which the half-life and activity are given. If the half-life is given in seconds, then with activity in becquerel the constant is 2.4 x 10-24, with activity in curie it is 8.86 x 10-14. If the half-life is given in other units, then it is converted to seconds.

Let's calculate the mass of 131J with a half-life of 8.05 days to create an activity of 1 curie.

M = 8.86 x 10-14 x 131 x 8.05 x 24 x 3600 x 1 = 0.000008 g. For strontium-90, the mass is 0.0073, plutonium-239 - 16.3 g, uranium-238 - 3 t. It is possible to calculate the activity in becquerels or curies of a radionuclide with its known mass: a0 = lxm / (A x T 1/2), where l is the parameter inverse to the constant "k". With T½ measured in seconds, and activities - in becquerels,
l = 4.17 x 1023, with activity in Cu l = 1.13 x 1013 Thus, the activity of 32.6 g of plutonium-239 is

a0 = 1.13 x 1013 x 32.6 (239 x 24300 x 365 x 24 x 3600) = 2 Ki,

a0 = 4.17 x 1013 x 32.6 (239 x 24300 x 365 x 24 x 3600) = 7.4 x 1010 Bq.

The biological effect of radiation is due to the ionization of the irradiated biological environment. The radiation spends its energy on the ionization process. Those. as a result of the interaction of radiation with the biological environment, a certain amount of energy is transferred to a living organism. Part of the radiation that penetrates the irradiated object (without absorption) has no effect on it. The radiation effect depends on many factors: the amount of radioactivity outside and inside the body, the route of its entry, the type and energy of radiation during the decay of nuclei, the biological role of the irradiated organs and tissues, etc. An objective indicator linking all these various factors is the number of absorbed energy radiation from ionization, which this energy produces in the mass of a substance.

In order to predict the magnitude of the radiation effect, you need to learn how to measure the intensity of exposure to ionizing radiation. And this can be done by measuring the energy absorbed in the object or the total charge of the ions formed during ionization. This amount of absorbed energy is called the dose.