A substance is considered radioactive, or it contains radionuclides in its composition and the process of radioactive decay takes place in it. Quantity radioactive substance usually determined not by mass units (gram, milligram, etc.), but by the activity of a given substance.
The activity of a substance is determined by the intensity or rate of decay of its nuclei. Activity is proportional to the number of radioactive atoms contained in a given substance, i.e. increases with the amount of the given substance. Activity is a measure of the amount of radioactive material, which is expressed as the number of radioactive transformations (nucleus decays) per unit of time. Since the rate of decay of radioactive isotopes is different, radionuclides of the same mass have various activities. The more nuclei decay per unit time, the higher the activity. Activity is usually measured in disintegrations per second. The unit of activity in the International System of Units (SI) is one disintegration per second. This unit is named after Henri Becquerel, who first discovered the phenomenon of natural radioactivity in 1896, the becquerel (Bq). 1 Bq is the amount of a radionuclide in which one decay occurs in one second. Since the 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 (Ci). Curie is such activity when the number of radioactive decays per second is equal to
3.7 x 1010 (37 GCR/s). The curie corresponds to the activity of 1 g of radium. Since the curie is a very large value, derivative quantities are usually used: mCi - millicurie (thousandth of a curie) - 3.7 x 107 dis / s; mkCi - microcurie (millionth of a curie) - 3.7 x 104 dis / s; nCi - nanocurie (billionth of a curie) - 3.7x10 dis / s.
Knowing the activity in becquerels, it is not difficult to pass to the activity in curies and vice versa:
1 Ci \u003d 3.7 x 1010 Bq \u003d 37 gigabecquerel;
1 mCi = 3.7 x 107 Bq = 37 megabecquerel;
1 mCiCi = 3.7 x 104 Bq = 37 kilobecquerel;
1 Bq \u003d 1 distribution / s \u003d 2.7 x 10-11 Ci.
In practice, disintegrations per minute are often used.
1 Ci \u003d 2.22 x 1012 dis / min.
1 mCi \u003d 2.22 x 109 dis / min.
1 mCi \u003d 2.22 x 106 dis / min.
When measuring the activity of a radioactive sample, it is usually referred to as mass, volume, surface area, or length. The following types of radionuclide activity are distinguished. Specific activity
- this is the activity per unit mass of a substance (activity per unit mass) - Bq / kg, Ci / kg. Volume activity
- this is the activity per unit volume - Bq / l, Ci / l, Bq / m3, Ci / m3. In the case of the 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 contaminated
40 Ci / km2 for cesium-137 per 1m2 of the surface accommodates 2,000,000 billion nuclei, or 0.455 micrograms of cesium-137. Line Activity radionuclide - the ratio of the activity of the radionuclide contained in the length of the segment to its length.
The mass in grams at 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; BUT - atomic mass; T½ - 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 becquerels the constant is 2.4 x 10-24, with activity in curie - 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 tons. It is possible to calculate the activity in becquerels or curies of a radionuclide with its known mass: a0 = l x m / (A x T 1/2), where l is the parameter inverse to the constant "k". With T½ measured in seconds and activity in becquerels,
l \u003d 4.17 x 1023, with activity in Ki l \u003d 1.13 x 1013 So, the activity of 32.6 g of plutonium-239 is equal to
a0 = 1.13 x 1013 x 32.6 (239 x 24300 x 365 x 24 x 3600) = 2 Ci,
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. Radiation wastes 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. The part of the radiation that penetrates the irradiated object (without absorption) does not affect it. The radiation effect depends on many factors: the amount of radioactivity outside and inside the body, the way it enters, 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 absorbed energy radiation from the ionization that this energy produces in the mass of matter.
In order to predict the magnitude of the radiation effect, one must 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.
Lecture 2. The basic law of radioactive decay and the activity of radionuclides
The rate of decay of radionuclides is different - some decay faster, others slower. The rate of radioactive decay is radioactive decay constant, λ [sec-1], which characterizes the probability of decay of one atom in one second. For each radionuclide, the decay constant has its own value, the larger it is, the faster the nuclei of matter decay.
The number of decays registered in a radioactive sample per unit of time is called activity (a ), or the radioactivity of the sample. The activity value is directly proportional to the number of atoms N radioactive material:
a =λ· N , (3.2.1)
where λ is the radioactive decay constant, [sec-1].
At present, according to the current International System of Units SI, the unit of measurement of radioactivity is becquerel [Bq]. This unit got its name in honor of the French scientist Henri Becquerel, who discovered in 1856 the phenomenon of natural uranium radioactivity. One becquerel is equal to one disintegration per second 1 Bq = 1 .
However, an off-system unit of activity is still quite often used. – curie [Key], introduced by the Curies as a measure of the decay rate of one gram of radium (in which ~3.7 1010 decays per second occurs), therefore
1 Key= 3.7 1010 Bq.
This unit is convenient for assessing the activity of large quantities of radionuclides.
The decrease in the radionuclide concentration over time as a result of decay obeys an exponential dependence:
, (3.2.2)
where N t- the number of atoms of a radioactive element remaining after a while t after the start of observation; N 0 is the number of atoms at the initial moment of time ( t =0 ); λ is the radioactive decay constant.
The relationship described is called basic law of radioactive decay .
The time it takes for half of the total number of radionuclides to decay is called half life, T½ . After one half-life, out of 100 atoms of the radionuclide, only 50 remain (Fig. 2.1). Over the next same period, of these 50 atoms, only 25 remain, and so on.
The relationship between half-life and decay constant is derived from the equation for the basic law of radioactive decay:
at t=T½ and
we get https://pandia.ru/text/80/150/images/image006_47.gif" width="67" height="41 src="> Þ ;
https://pandia.ru/text/80/150/images/image009_37.gif" width="76" height="21">;
i.e..gif" width="81" height="41 src=">.
Therefore, the law of radioactive decay can be written as follows:
https://pandia.ru/text/80/150/images/image013_21.gif" width="89" height="39 src=">, (3.2.4)
where at - the activity of the drug over time t ; a0 – the activity of the drug at the initial moment of observation.
It is often necessary to determine the activity of a given amount of any radioactive substance.
Remember that the unit of quantity of a substance is the mole. A mole is the amount of a substance containing as many atoms as there are in 0.012 kg = 12 g of the 12C carbon isotope.
One mole of any substance contains Avogadro's number NA atoms:
NA = 6.02 1023 atoms.
For simple substances(elements) the mass of one mole numerically corresponds to the atomic mass BUT element
1mol = BUT G.
For example: For magnesium: 1 mol 24Mg = 24 g.
For 226Ra: 1 mole of 226Ra = 226 g, etc.
In view of what has been said in m grams of the substance will N atoms:
https://pandia.ru/text/80/150/images/image015_20.gif" width="156" height="43 src="> (3.2.6)
Example: Let's calculate the activity of 1 gram of 226Ra, which has λ = 1.38 10-11 sec-1.
a\u003d 1.38 10-11 1 / 226 6.02 1023 \u003d 3.66 1010 Bq.
If a radioactive element is part of chemical compound, then when determining the activity of the drug, it is necessary to take into account its formula. Taking into account the composition of the substance is determined mass fraction χ radionuclide in a substance, which is determined by the ratio:
https://pandia.ru/text/80/150/images/image017_17.gif" width="118" height="41 src=">
Problem solution example
Condition:
Activity A0 radioactive element 32P on the day of observation is 1000 Bq. Determine the activity and number of atoms of this element in a week. Half life T½ 32P = 14.3 days.
Decision:
a) Find the activity of phosphorus-32 after 7 days:
https://pandia.ru/text/80/150/images/image019_16.gif" width="57" height="41 src=">
Answer: in a week, the activity of the 32P drug will be 712 Bq, and the number of atoms of the radioactive isotope 32P is 127.14 106 atoms.
test questions
1) What is the activity of a radionuclide?
2) Name the units of radioactivity and the relationship between them.
3) What is the radioactive decay constant?
4) Define the basic law of radioactive decay.
5) What is the half-life?
6) What is the relationship between activity and mass of a radionuclide? Write a formula.
Tasks
1. Calculate activity 1 G 226Ra. T½ = 1602 years.
2. Calculate activity 1 G 60Co. T½ = 5.3 years.
3. One M-47 tank shell contains 4.3 kg 238U. T½ = 2.5 109 years. Determine projectile activity.
4. Calculate the activity of 137Cs after 10 years, if at the initial moment of observation it is 1000 Bq. T½ = 30 years.
5. Calculate the 90Sr activity a year ago if it is 500 at the present time Bq. T½ = 29 years.
6. What activity will 1 create kg radioisotope 131I, T½ = 8.1 days?
7. Using the reference data, determine activity 1 G 238U. T½ = 2.5 109 years.
Using the reference data, determine activity 1 G 232Th, Т½ = 1.4 1010 years.
8. Calculate the activity of the compound: 239Pu316O8.
9. Calculate the mass of the radionuclide with activity in 1 Key:
9.1. 131I, T1/2=8.1 days;
9.2. 90Sr, Т1/2=29 years;
9.3. 137Cs, Т1/2=30 years;
9.4. 239Pu, Т1/2=2.4 104 years.
10. Determine the mass 1 mCi radioactive isotope of carbon 14C, T½ = 5560 years.
11. It is necessary to prepare a radioactive preparation of phosphorus 32P. How long will it take for 3% of the drug to remain? Т½ = 14.29 days.
12. The natural mixture of potassium contains 0.012% of the radioactive isotope 40K.
1) Determine the mass of natural potassium, which contains 1 Key 40K. T½ = 1.39 109 years = 4.4 1018 sec.
2) Calculate the radioactivity of the soil by 40K if it is known that the potassium content in the soil sample is 14 kg/t.
13. How many half-lives are required for the initial activity of a radioisotope to decrease to 0.001%?
14. To determine the effect of 238U on plants, the seeds were soaked in 100 ml solution UO2(NO3)2 6H2O, in which the mass of the radioactive salt was 6 G. Determine the activity and specific activity of 238U in solution. Т½ = 4.5 109 years.
15. Define Activity 1 grams 232Th, Т½ = 1.4 1010 years.
16. Determine the mass 1 Key 137Cs, Т1/2=30 years.
17. The ratio between the content of stable and radioactive isotopes of potassium in nature is a constant value. The content of 40K is 0.01%. Calculate the radioactivity of the soil by 40K if it is known that the potassium content in the soil sample is 14 kg/t.
18. Lithogenic radioactivity environment is formed mainly due to three main natural radionuclides: 40K, 238U, 232Th. The share of radioactive isotopes in the natural amount of isotopes is 0.01, 99.3, ~100, respectively. Calculate radioactivity 1 t soil, if it is known that the relative content of potassium in the soil sample is 13600 g/t, uranium - 1 10-4 g/t, thorium - 6 10-4 g/t.
19. In the shells of bivalve mollusks found 23200 Bq/kg 90Sr. Determine the activity of samples after 10, 30, 50, 100 years.
20. The main pollution of the closed reservoirs of the Chernobyl zone took place in the first year after the accident at the nuclear power plant. In the bottom sediments of the lake. Azbuchin in 1999 discovered 137Cs with a specific activity of 1.1 10 Bq/m2. Determine the concentration (activity) of 137Cs deposited per m2 of bottom sediments as of 1986-1987. (12 years ago).
21. 241Am (T½ = 4.32 102 years) is formed from 241Pu (T½ = 14.4 years) and is an active geochemical migrant. Taking advantage reference materials, calculate with an accuracy of 1% the decrease in the activity of plutonium-241 in time, in which year after Chernobyl disaster the formation of 241Am in the environment will be maximum.
22. Calculate the activity of 241Am in the products of emissions from the Chernobyl reactor as of April
2015, provided that in April 1986 the activity of 241Am was 3.82 1012 Bq,Т½ = 4.32 102 years.
23. 390 found in soil samples nCi/kg 137Cs. Calculate the activity of samples after 10, 30, 50, 100 years.
24. The average concentration of pollution in the bed of the lake. Deep, located in the Chernobyl exclusion zone, is 6.3 104 Bq 241Am and 7.4 104 238+239+240Pu per 1 m2. Calculate the year in which these data were obtained.
The word radiation, translated from English "radiation" means radiation and is used not only for radioactivity, but for a number of others. physical phenomena, For example: solar radiation, thermal radiation, etc. Therefore, in relation to radioactivity, the concept of "ionizing radiation" adopted by the ICRP (International Commission on Radiation Protection) and the Radiation Safety Standards should be applied.
ionizing radiation ( IONIZING RADIATION)?
Ionizing radiation - radiation (electromagnetic, corpuscular), which, when interacting with a substance, directly or indirectly causes ionization and excitation of its atoms and molecules. The energy of ionizing radiation is large enough to create a pair of ions of different signs when interacting with matter, i.e. ionize the medium into which these particles or gamma quanta have fallen.
Ionizing radiation consists of charged and uncharged particles, which also include photons.
What is radioactivity?
Radioactivity - spontaneous transformation atomic nuclei into the nuclei of other elements. Accompanied by ionizing radiation. Four types of radioactivity are known:
- alpha decay - radioactive transformation of an atomic nucleus in which an alpha particle is emitted;
- beta decay - radioactive transformation of the atomic nucleus in which beta particles are emitted, i.e. electrons or positrons;
- spontaneous fission of atomic nuclei - spontaneous fission of heavy atomic nuclei (thorium, uranium, neptunium, plutonium and other isotopes of transuranic elements). The half-lives of spontaneously fissile nuclei range from a few seconds to 1020 for Thorium-232;
- proton radioactivity - radioactive transformation of the atomic nucleus in which nucleons (protons and neutrons) are emitted.
What are isotopes?
Isotopes are varieties of atoms of the same chemical element that have different mass numbers, but have the same electric charge atomic nuclei and therefore occupying D.I. Mendeleev is the same place. For example: 55Cs131, 55Cs134m, 55Cs134, 55Cs135, 55Cs136, 55Cs137. There are stable (stable) and unstable isotopes - spontaneously decaying by radioactive decay, the so-called radioactive isotopes. About 250 stable and about 50 natural radioactive isotopes are known. An example of a stable isotope is Pb206, Pb208, which is the end product of the decay of radioactive elements U235, U238 and Th232.
INSTRUMENTS FOR measuring radiation and radioactivity.
To measure the levels of radiation and the content of radionuclides at various objects, special measuring instruments are used:
- to measure the exposure dose rate of gamma radiation, X-ray radiation, alpha and beta radiation flux density, neutrons, dosimeters for various purposes are used;
- to determine the type of radionuclide and its content in environmental objects, spectrometric paths are used, consisting of a radiation detector, an analyzer and a personal computer with an appropriate program for processing the radiation spectrum.
At present, there are various types of radiation meters different types, purposes, and with ample opportunities. For example, here are several models of devices that are most popular in professional and household activities:
A professional dosimeter-radiometer was developed for the radiation monitoring of banknotes by bank tellers, in order to comply with the "Instruction of the Bank of Russia dated 04.12.2007 N 131-I" On the procedure for detecting, temporary storage, cancellation and destruction of banknotes with radioactive contamination "".
The best household dosimeter from a leading manufacturer, this portable radiation meter has proven itself over time. Due to its easy use, small size and low price, users have called it folk, recommend it to friends and acquaintances without fear of a recommendation.
SRP-88N (scintillation search radiometer) - a professional radiometer designed to search and detect sources of photon radiation. It has digital and pointer indicators, the ability to set the threshold for the operation of an audible alarm, which greatly facilitates the work when examining territories, checking scrap metal, etc. The detection unit is remote. A NaI scintillation crystal is used as a detector. Autonomous power supply 4 elements F-343.
DBG-06T - designed to measure the exposure dose rate (EDR) of photon radiation. Power source galvanic element of the "Korund" type.
DRG-01T1 - designed to measure the exposure dose rate (EDR) of photon radiation.
DBG-01N - designed to detect radioactive contamination and assess the power level of the equivalent dose of photon radiation using a sound signaling device. Power source galvanic element of the "Korund" type. Measurement range from 0.1 mSv*h-1 to 999.9 mSv*h-1
RKS-20.03 "Pripyat" - designed to control the radiation situation in places of residence, stay and work.
Dosimeters allow you to measure:
- the magnitude of the external gamma background;
- levels of radioactive contamination of residential and public premises, territory, various surfaces
- the total content of radioactive substances (without determining the isotopic composition) in food and other environmental objects (liquid and bulk)
- levels of radioactive contamination of residential and public premises, territory, various surfaces;
- the total content of radioactive substances (without determining the isotopic composition) in food and other environmental objects (liquid and bulk).
How to choose a radiation meter and other devices for measuring radiation you can read in the article " Household dosimeter and indicator of radioactivity. how to choose?"
What types of ionizing radiation exist?
Types of ionizing radiation. The main types of ionizing radiation that we most often encounter are:
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Of course, there are other types of radiation (neutron), but we encounter them in Everyday life much less frequently. The difference between these types of radiation lies in their physical characteristics, origin, properties, radiotoxicity and damaging effect on biological tissues.
Sources of radioactivity can be natural or artificial. natural springs ionizing radiation is natural radioactive elements that are in earth's crust and creating natural radiation background, is ionizing radiation coming to us from outer space. The more active the source (ie, the more atoms decay in it per unit time), the more it emits particles or photons per unit time.
Artificial sources of radioactivity may contain radioactive substances obtained in nuclear reactors on purpose or being by-products of nuclear reactions. Various electrovacuum physical devices, charged particle accelerators, etc. can be used as artificial sources of ionizing radiation. For example: a TV kinescope, an X-ray tube, a kenotron, etc.
The main suppliers of radium-226 to the environment are enterprises engaged in the extraction and processing of various fossil materials:
- mining and processing uranium ores;
- Oil and gas; coal industry;
- building materials industry;
- energy industry enterprises, etc.
Radium-226 lends itself well to leaching from minerals containing uranium, this property explains the presence of significant amounts of radium in some types of groundwater (radon used in medical practice), in mine waters. The range of radium content in groundwater ranges from a few to tens of thousands of Bq/L. Radium content in surface natural waters much lower and can range from 0.001 to 1-2 Bq/l. Essential component natural radioactivity is a decay product of radium-226 - radium-222 (radon). Radon- inert radioactive gas, the longest-lived (half-life 3.82 days) emanation isotope*, alpha emitter. It is 7.5 times heavier than air, therefore it mainly accumulates in cellars, basements, basement floors of buildings, in mine workings, etc. * - emanation - the property of substances containing radium isotopes (Ra226, Ra224, Ra223) to emit emanation (radioactive inert gases) formed during radioactive decay.
It is believed that up to 70% harmful effects per population is associated with radon in residential buildings (see diagram). The main sources of radon in residential buildings are (in order of increasing importance):
- tap water and household gas;
- construction materials (crushed stone, clay, slag, ash and slag, etc.);
- soil under buildings.
Radon spreads in the bowels of the Earth extremely unevenly. Its accumulation in tectonic disturbances is characteristic, where it enters through systems of cracks from pores and microcracks in rocks. It enters the pores and cracks due to the process of emanation, being formed in the substance of rocks during the decay of radium-226.
Soil radon release is determined by the radioactivity of rocks, their emanation and collector properties. So, relatively weakly radioactive rocks, the foundations of buildings and structures can pose a greater danger than more radioactive ones, if they are characterized by high emanation, or are dissected by tectonic disturbances that accumulate radon. With a kind of "breathing" of the Earth, radon enters the atmosphere from rocks. And in largest quantities- from areas where there are radon collectors (shifts, cracks, faults, etc.), i.e. geological disturbances. Our own observations of the radiation situation in the coal mines of Donbass showed that in mines characterized by complex mining and geological conditions (the presence of multiple faults and cracks in the coal-bearing rocks, high water content, etc.), as a rule, the concentration of radon in the air of mine workings significantly exceeds the established standards.
The construction of residential and public-economic structures directly above the faults and cracks of rocks, without preliminary determination of radon release from the soil, leads to the fact that ground air enters them from the bowels of the Earth, containing high concentrations of radon, which accumulates in indoor air and creates a radiation hazard .
Technogenic radioactivity arises as a result of human activity during which the redistribution and concentration of radionuclides occurs. Man-made radioactivity includes the extraction and processing of minerals, the combustion of coal and hydrocarbons, the accumulation of industrial waste, and much more. The levels of human exposure to various man-made factors are illustrated by the presented diagram 2 (A.G. Zelenkov "Comparative effects on humans of various sources of radiation", 1990)
What are "black sands" and what danger do they pose?
Black sands are a mineral monazite - anhydrous phosphate of elements of the thorium group, mainly cerium and lanthanum (Ce, La)PO4, which are replaced by thorium. Monazite contains up to 50-60% oxides of rare earth elements: yttrium oxide Y2O3 up to 5%, thorium oxide ThO2 up to 5-10%, sometimes up to 28%. The specific gravity of monazite is 4.9-5.5. With an increase in the content of thorium sp. weight increases. It occurs in pegmatites, sometimes in granites and gneisses. During the destruction of rocks including monazite, it accumulates in placers, which are large deposits.
Such deposits are also observed in the south of the Donetsk region.
Placers of monazite sands located on land, as a rule, do not significantly change the existing radiation situation. But the deposits of monazite located near the coastal strip of the Sea of Azov (within the Donetsk region) create a number of problems, especially with the onset of the swimming season.
The fact is that as a result of the sea surf during the autumn-spring period on the coast, as a result of natural flotation, a significant amount of "black sand" accumulates, characterized by a high content of thorium-232 (up to 15-20 thousand Bq * kg-1 and more ), which creates gamma radiation levels of the order of 300 or more μR * h-1 in local areas. Naturally, it is risky to rest in such areas, therefore, this sand is collected annually, warning signs are put up, and certain sections of the coast are closed. But all this does not prevent a new accumulation of "black sand".
Let me express my personal point of view on this. The reason contributing to the removal of "black sand" to the coast may be the fact that dredgers are constantly working on the fairway of the Mariupol seaport to clear the shipping channel. The soil raised from the bottom of the canal dumps to the west of the shipping canal, 1-3 km from the coast (see the map of soil dump locations), and in case of strong sea waves, with a run-up on the coastal strip, the soil containing monazite sand is carried to the coast, where enriched and accumulated. However, all this requires careful examination and study. And if so, then it would be possible to reduce the accumulation of "black sand" on the coast simply by moving the site of the soil dump to another place.
Basic rules for performing dosimetric measurements.
When conducting dosimetric measurements, first of all, it is necessary to strictly adhere to the recommendations set forth in the technical documentation for the device.
When measuring the exposure dose rate of gamma radiation or the equivalent dose of gamma radiation, the following rules must be observed:
- when carrying out any dosimetric measurements, if they are supposed to be constantly carried out in order to monitor the radiation situation, it is necessary to strictly observe the measurement geometry;
- to improve the reliability of the results of dosimetric monitoring, several measurements are taken (but not less than 3), and the arithmetic mean is calculated;
- when performing measurements on the territory, sites are selected away from buildings and structures (2-3 heights); - measurements on the territory are carried out at two levels, at a height of 0.1 and 1.0 m from the ground surface;
- when measuring in residential and public premises, measurements are taken in the center of the room at a height of 1.0 m from the floor.
When measuring levels of contamination with radionuclides on various surfaces, it is necessary to place the remote sensor or the device as a whole, if there is no remote sensor, in a plastic bag (to prevent possible contamination), and measure at the closest possible distance from the measured surface.
Main physical quantities used in radiation protection and their units
|
Physical quantity |
Ratio between units |
|||
|
SI systems |
off-system |
SI system and off-system |
off-system and in the SI system |
|
|
Nuclide activity in a radioactive source. Expresses the number of decays per unit of time. |
Becquerel (Bq, Vq) |
Curie (Ki, Si) |
1 Bq = 1 spread. in s, 1 Bq = 2.7 10 -11 Ci |
1 Ci \u003d 3.7 10 10 Bq |
|
Specific activity. |
Becquerel per kilogram (Bq/kg). |
Curie per kilogram (Ci/kg). |
1 Bq/kg = 2.7 10 -11 Ci/kg |
1 Ci/kg = 3.7 10 10 Bq/kg |
|
Absorbed radiation dose. The amount of energy of ionizing radiation, |
Gray (Gy, Gy). |
Glad (rad, rad). |
1 Gy=1 J/kg; 1 Gy = 100 rad; 1 J \u003d 10 5 rad / g |
1 rad \u003d 100 erg / g \u003d 0.01 Gy \u003d 10 2 J / kg \u003d 10 -2 Gy; 1 rad/g |
Continuation of the table. 1.4.
|
Physical quantity |
Name and designation of the unit |
Ratio between units |
||
|
SI systems |
off-system |
SI system and off-system |
off-system and in the SI system |
|
|
absorbed by a unit of mass of a physical body, for example, by body tissues. | ||||
|
Dose equivalent. Absorbed dose multiplied by a factor that takes into account the unequal radiation hazard different types ionizing radiation (see Table 1.6). |
Sievert (3c, Sv). |
Rem (rem, rem). |
1Sv = 1Gy = 1 J/kg = 100 rem (for β- and γ radiation); 1 Sv = 2.58 10 -4 C/kg. |
1 rem = 0.01 Sv = 10 mSv. |
|
The dose is effective (effective equivalent). The sum of the average equivalent doses in various bodies or tissues, weighted with coefficients for taking into account the different sensitivity of organs and tissues to the occurrence |
Sievert (3c, Sv). |
Rem (rem, rem). |
1Sv = 1Gy = 1 J/kg = 100 rem (for β- and γ radiation). |
1 rem = 0.01 Sv = 10 mSv. |
Continuation of the table. 1.4.
|
Physical quantity |
Name and designation of the unit |
Ratio between units |
||||||
|
SI systems |
off-system |
SI system and off-system |
off-system and in the SI system |
|||||
|
stochastic effects of radioactive exposure (see Table 1.7). | ||||||||
|
Exposure dose radiation. The ratio of the total charge of all ions of the same sign, arising from the complete deceleration of electrons and positrons formed by photons in an elementary volume of air, to the mass of air in this volume. |
Coulomb per kilogram (C/kg) |
X-ray (R) |
1 C / kg \u003d 3876 R \u003d 3.88 10 3 R. |
1 P \u003d 2.58 10 -4 C / kg |
||||
|
Dose rate exposure- the dose received by the body per unit of time. |
Gray per second (Gy/s = J/kg s = W/kg); Sievert per second (Sv/s), Amp per kilogram (A/kg). |
Rad per second (rad/s), Rem per second (rem/s), Roentgen per second (R/s). |
1 Gy/s = 100 rad/s, 1 Gy/s=1 Sv/s = 100 R/s (for β- and γ-radiation); 1 Sv/s = 100 rem/s 1 A/kg = 3876 R/s. |
1 rad/s = 0.01 Gy/s, 100 R/s = 1 3v/s=1 µGy/s. |
||||
Continuation of the table. 1.4.
absorbs energy of 1 joule (J). 1 Gy \u003d 1 J / kg \u003d 2.388 10 -4 kcal / kg \u003d 6.242 10 15 eV / g \u003d 10 4 erg / g \u003d 100 rad.
Particle energy is measured in electron volts (eV). An electron volt is the energy that an electron acquires under the influence of an electric field with a potential difference (voltage) of 1 volt.
1 eV = 1.6 10 -12 erg = 1.6 10 -19 joules = 3.83 10 -20 calories
Based on the ratios: 1 J \u003d 0.239 cal \u003d 6.25 10 18 electron volts \u003d 10 7 erg,
1 glad = 10 -2 j/kg = 100 erg/g= 0.01 Gy = 2.388× 10 -6 cal/g
Multiple units of the absorbed dose are kilogray (1 kGy = 1 Gy 10 3), milligray (1 mGy = 1 Gy 10 -3). The principle of formation of multiple units of measurement of ionizing radiation is presented in Table. 1.5.
The absorbed energy is spent for heating a substance, as well as for its chemical and physical transformations. It increases with increasing irradiation time and depends on the composition of the substance, the type of radiation (X-rays, neutron flux, etc.), the energy of its particles, their flux density, and the composition of the irradiated substance. For example, for X-ray and γ-radiation, it depends on the atomic number (Z) of the elements that make up the substance.
The nature of this dependence is determined photon energy, depending on the frequency of electromagnetic oscillations - hv In this formula: h - constantPlank; introduced by M. Planck in 1900 with
establishment of the law of distribution of energy in the radiation spectrum of an absolutely black body. The most accurate value h = (6.626196 ± 0.000050) 10 -34 joules = (6.626196 ± 0.000050) 10 -27 erg s. However, h = h/2π is more often used = (1.0545919 ± 0.0000080) 10 -27 erg s , also called Planck's constant, and v is the frequency of electromagnetic oscillations.
As a result of such interactions in biological tissues, physiological processes are disturbed, and radiation sickness of varying severity develops in a number of cases. The absorbed dose of radiation is the main physical quantity that determines the degree of radiation exposure.
Absorbed dose rate– dose increment per unit time. It is characterized by the rate of radiation dose accumulation and can increase or decrease over time. Its SI unit is gray per second (Gy/s). This is such an absorbed dose rate of radiation at which a radiation dose of 1 Gy is absorbed in a substance in 1 s. In practice, to assess the absorbed dose rate, an off-system unit of absorbed dose rate is still widely used - rad per hour (rad/h) or rad per second (rad/s). This dose can be created both after external and after internal exposure.One of the first discovered natural radioactive elements was "radium" - emitting rays, radiating. Education for him and others natural radionuclides proceeds in the process of spontaneous transformations (decays) of nuclides of the family of uranium and thorium. As an example, we present in Fig. 1.6 a chain of numerous transformations of radionuclides of the 238 U family, accompanied by α- or β-radiation and culminating in the formation of a stable lead nuclide.
A person receives the highest dose of radiation (50%) from radon-222 (222 Rn) and its derivatives - representatives of the 238 U family (Fig. 1.6). 14% of the dose is created by g-rays from the ground and buildings, 12% - by food and drinks, 10% - by cosmic rays (internal exposure due to cosmogenic radionuclides: carbon-14 - 14 C (12 μSv / year), beryllium-7 - 7 Be (3 µSv/year), sodium-22 - 22 Na (0.2 µSv/year) and tritium - 3 H (0.01 µSv/year).
External absorbed dose is the dose received by a person from a source located outside the body. It accounts for almost 33% of the total radiation dose and is created by the flux of particles or quanta from the ground and buildings (mainly potassium-40), cosmic radiation and anthropogenic sources. Residents of Belarus also receive additional exposure due to Chernobyl radionuclides. 90% of it is created by caesium-137, 9% by strontium-90 and 1% by plutonium isotopes. After nuclear explosion penetrating radiation is created by a stream of γ-rays and neutrons emitted within about 10-25 seconds from the moment of a nuclear explosion.
Flux of γ-rays - photons (F) is the ratio of the number of ionizing particles (photons) dN passing through a given surface in a time interval dt to this interval: F= dN/dt. The unit of measure for the flow of ionizing particles is particle / s (one particle per second).
Fluence (transfer) of ionizing particles (photons)- the ratio of the number of ionizing particles (photons) dN penetrating into the volume of the elementary sphere to the area of the central cross section dS of this sphere: Ф = dN/dS. The particle fluence unit is particle / m 2 (one particle per square meter).
Flux density of ionizing particles (photons, φ)- the ratio of the flux of ionizing particles (photons) dF penetrating into the volume of the elementary sphere, to the area of the central cross section dS of this sphere: φ = dF/dS = dФ / dt = dN/dt dS. The flux density unit is particle/s -1 m -2 (one particle or quantum per second per square meter).
During the passage of these photons (gamma radiation), a narrow and a wide beam are distinguished. Geometry narrow beam characterized by the fact that the detector registers only non-scattered radiation from the source. The geometry in which the detector registers non-scattered and scattered radiation is called wide beam.
Specific absorbed dose (σ)- absorbed dose generated by radiation at fluence = one particle per square meter: σ = D / F.
internal absorbed dose- the dose received by any organ of the human body from a source of radiation located inside the body. This source of internal exposure can be a radioactive substance that penetrates into the body through the intestines with food (food and water), through the lungs (by breathing air) and, to a small extent, through the skin, or through wounds or cuts, as well as in medical radioisotope diagnostics. Sources of internal exposure can be conditionally divided into sources Chernobyl origin(at present, most of their cesium-137, strontium-90 and plutonium-239, 240 are found in food) and natural origin. The latter create almost 67% of the total radiation dose.
Source of internal exposure remains in the body for a certain time, during which it exerts its negative impact. The duration of exposure is determined by the half-life of the source that enters the body and the amount of time during which it is excreted from the body. The removal of radionuclides from the body is a very complex phenomenon. It can only be roughly described by the concept " biological half-life" - the time required for the elimination of half of the radioactive material from the body.
The state of the radiation situation on the ground or in the room characterizes exposure dose. Exposure dose (of photon radiation) is a quantitative characteristic of X-ray and γ-radiation with energies up to 3 MeV, based on their ionizing effect and expressed as the ratio of the total charge of all ions of the same sign dQ, arising from the complete deceleration of electrons and positrons that were formed by photons in elementary volume of air, to the mass dm of air in this volume: Х = dQ/dm. It represents the energy characteristic of radiation, estimated by the effect of ionization of dry atmospheric air, and the measure of the ionization effect of photon radiation, determined by the ionization of air under conditions of electronic equilibrium.
The unit of exposure dose in SI is coulomb per kilogram (C/kg). The non-systemic unit of exposure dose is also widespread - x-ray (R)(named after the German physicist Wilhelm Conrad Roentgen, who discovered X-rays in 1895): one X-ray (1 R) - this is such a dose of photon radiation, under the influence of which in 1 cm 3 dry air under normal conditions (0°С and 760mm rt. st.) ions are formed that carry one electrostatic unit of the amount of electricity of each sign.
A dose of 1 R corresponds to the formation of 2.083 10 9 pairs of ions per 1 cm 3 of air (at 0 ° C and 760 mm Hg), or 1.61 10 12 pairs of ions per 1 g of air. If we take into account that the electron charge is equal to 1.6 10 -19 coulombs, and the mass of 1 cm 3 of air = 1.29 10 -6 kg, then 1 P is 2.57976 10 -4 C / kg. In turn, 1 C / kg \u003d 3.876 10 3 R. To create such a number of ions, it is necessary to spend energy equal to 0.114 erg / cm 3 or 88 erg / g, i.e., 88 erg / g is the energy equivalent of X-rays.
The ratios between the units of measurement of exposure and absorbed doses are: for air 1 P = 0.88 rad, for biological tissue 1 P = 0.93 rad, 1 rad is equal to an average of 1.44 R.
Exposure dose rate is the increment of the exposure dose per unit time. Its SI unit is ampere per kilogram (A/kg).
1 R/s = 2.58 10 -4 A/kg.
In the Chernobyl nuclear power plant accident zone, there are areas where soil radioactivity reaches 1200 microroentgens per hour. The absorbed dose of X-ray and γ-radiation in any substance can also be calculated from the exposure dose. To do this, it is necessary to know the composition of matter and the energy of radiation photons.
It should be remembered that, according to the adopted GOST, after January 1, 1990 it is generally not recommended to use the concept of exposure dose and its power. Therefore, during the transitional period, these quantities should be indicated not in SI units, but in non-SI units - roentgens and roentgens per second (R / s).
Distinguish as lump sum, and permanent(chronic) radiative forcing. One-time impact occurs under extraordinary circumstances, in particular, accidents and is estimated by the absorbed dose. Permanent same impact, which can occur as a result of regular releases of radioactivity into air or water or the constant presence of radionuclides in the environment, as a rule, has a long-term damaging effect on humans. Radiation has such an impact on people living on lands contaminated with radionuclides after the Chernobyl accident. To evaluate these radiation doses use concepts such as equivalent and effective equivalent doses of radiation.
Equivalent radiation dose- the value used to assess the radiation hazard of chronic human exposure to various types of ionizing radiation and determined by the sum of the products of the absorbed doses of individual types of radiation and their quality factors. We can say that this is the average absorbed dose of radiation D in an organ or tissue T, multiplied by the weighting radiation coefficient W R (or, as it is also called, the radiation quality factor - K, see Table 1.6) for biological tissue of standard composition(10.1% - hydrogen; 11.1% - carbon; 2.6% - nitrogen; 76.2% - oxygen, by mass):
H T, R = D W R = Σ D T, R W R ,
where R is the index of the type and energy of radiation.
Quality factor radiation shows how many times the expected biological effect from the studied radiation is greater than for radiation with linear energy transfer (LET) ≤ 3.5 keV per 1 μm path in water. For various radiations, the weighting radiation coefficient (W R) is set in accordance with the "Radiation Safety Standards - NRB-2000" depending on the linear energy transfer (Table 1.5):
Table 1.5
|
LET, keV/µm water | |||||
Linear power transfer- LET (LET - Linear Energy Transfer) - the intensity of energy transfer (and, consequently, the level of damage) per unit of distance traveled. For example, an α-particle belongs to high LET radiation, while photons and electrons belong to low LET radiation.
Radiation weighting factor W R(quality factor K) shows how many times the radiation hazard for a certain type of radiation is higher than the radiation hazard for X-rays at the same absorbed dose in
Table 1.6
The unit of activity of an isotope is the becquerel (Bq), which is equal to the activity of a nuclide in a radioactive source in which one decay event occurs in a time of 1 s.
1.2 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 hence const = -lg N0 . Finally
N = N0 e-λt (1)
where A is activity at time t; А0 – 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 that does not depend on the amount of substance.
half-life called the period of time during which a given amount of radioactive material is reduced by half.
The half-life of different isotopes varies significantly. It is from about 1010 years to a tiny fraction of a second. Of course, substances with a half-life of 10 - 15 minutes. and smaller, 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 instruments.
2. Methods of analysis based on the measurement of radioactivity
2.1. Use of natural radioactivity in analysis
Elements that are naturally radioactive 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 element being determined or by the addition method.
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 irradiated with 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 particle can be used for irradiation, most practical value has a neutron irradiation process. The use of charged particles for this purpose involves overcoming more significant technical difficulties than in the case of neutrons. The main neutron sources for activation analysis are the nuclear reactor and the so-called portable sources (radium-beryllium, etc.). In the latter case, α-particles resulting from the decay of any α-active element (Ra, Rn, etc.) interact with beryllium nuclei, releasing neutrons:
9Be + 4He →12C + n
Neutrons come into 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. When conducting such an analysis, it is necessary to have reliable data on the half-lives of various isotopes in order to decipher the summary curve.
Another variant of activation analysis is the γ-spectroscopy method based on the measurement of the γ-radiation spectrum of a sample. The energy of γ-radiation is qualitative, and the counting rate is quantitative characteristic isotope. Measurements are made 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, under favorable conditions, up to 10-13 - 10-15 g of a substance can be detected. In some special cases even lower detection limits have been achieved. For example, it is used to control the purity of silicon and germanium in the semiconductor industry, detecting the content of impurities up to 10-8 - 10-9%. Such contents cannot be determined by any other method than activation analysis. Upon receipt heavy elements periodic system, such as mendelevium and kurchatovium, the researchers were able to count almost every atom of the resulting element.
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