Since the middle of the last century, a new word has come into science - radiation. Its discovery made a revolution in the minds of physicists around the world and allowed to discard some of the Newtonian theories and make bold assumptions about the structure of the universe, its formation and our place in it. But that's all for the experts. The townsfolk only sigh and try to put together such disparate knowledge about this subject. Complicating the process is the fact that there are quite a few units of radiation measurement, and all of them are eligible.
Terminology
The first term worth getting to know is, in fact, radiation. This is the name given to the process of radiation by some substance of the smallest particles, such as electrons, protons, neutrons, helium atoms and others. Depending on the type of particle, the properties of radiation differ from each other. Radiation is observed either during the decay of substances into simpler ones, or during their synthesis.
Radiation units- these are conditional concepts that indicate how many elementary particles are released from matter. At the moment, physics operates with seven different units and their combinations. This makes it possible to describe various processes occurring with matter.
radioactive decay- arbitrary change in the structure of unstable nuclei of atoms by means of the release of microparticles.
decay constant- This is a statistical concept that predicts the probability of destruction of an atom for a certain period of time.
Half life is the time interval during which half of the total amount of matter decays. For some elements, it is calculated in minutes, while for others it is years, and even decades.
How is radiation measured?
Units of measurement of radiation are not the only ones that are used to evaluate properties. In addition, they use such quantities as:
- activity of the radiation source;
- flux density (number of ionizing particles per unit area).
In addition, there is a difference in the description of the effects of radiation on living and non-living objects. So, if the substance is inanimate, then the following concepts apply to it:
Absorbed dose;
- exposure dose.
If the radiation affected living tissue, then the following terms are used:
equivalent dose;
- effective equivalent dose;
- dose rate.
The units of radiation measurement are, as mentioned above, conditional numerical values adopted by scientists to facilitate calculations and build hypotheses and theories. Perhaps that is why there is no single generally accepted unit of measurement.
Curie
Curie is one of the units for measuring radiation. It does not belong to the system (does not belong to the SI system). In Russia, it is used in nuclear physics and medicine. The activity of a substance will be equal to one curie if 3.7 billion radioactive decays occur in it in one second. That is, we can say that one curie is equal to three billion seven hundred million becquerels.
This number was obtained due to the fact that Marie Curie (who introduced this term into science) conducted her experiments on radium and took its decay rate as a basis. But over time, physicists decided that the numerical value of this unit is better tied to another - the becquerel. This made it possible to avoid some errors in mathematical calculations.
In addition to curies, multiples or submultiples are often found, such as:
- megacurie (equal to 3.7 times 10 to the 16th power of becquerels);
- kilocurie (3.7 thousand billion becquerels);
- millicurie (37 million becquerels);
- microcurie (37 thousand becquerels).
Using this unit, you can express the volume, surface or specific activity of a substance.
becquerel
The becquerel unit of radiation dose is systemic and is included in the International System of Units (SI). It is the simplest, because a radiation activity of one becquerel means that there is only one radioactive decay per second in matter.
It got its name in honor of Antoine the French physicist. The name was approved at the end of the last century and is still in use today. Since this is a fairly small unit, decimal prefixes are used to denote activity: kilo-, milli-, micro- and others.
Recently, non-systemic units such as curie and rutherford have been used along with becquerels. One rutherford is equal to one million becquerels. In the description of volumetric or surface activity, one can find the designations becquerel per kilogram, becquerel per meter (square or cubic) and their various derivatives.
x-ray
The unit of measurement of radiation, the roentgen, is also not a systemic one, although it is used everywhere to indicate the exposure dose of the received gamma radiation. One roentgen is equal to such a radiation dose at which one cubic centimeter of air at standard atmospheric pressure and zero temperature carries a charge equal to 3.3 * (10 * -10). This is equal to two million pairs of ions.
Despite the fact that, according to the legislation of the Russian Federation, the use of most off-system units is prohibited, X-rays are used in the marking of dosimeters. But they will soon cease to be used, since it turned out to be more practical to write down and calculate everything in grays and sieverts.
Glad
The rad unit of radiation is outside the SI system and is equal to the amount of radiation at which one millionth of a joule of energy is transferred to one gram of a substance. That is, one rad is 0.01 joule per kilogram of matter.
The material that absorbs energy can be both living tissue and other organic and inorganic substances and substances: soil, water, air. As an independent unit, the rad was introduced in 1953 and in Russia has the right to be used in physics and medicine.
Gray
This is another unit of measure for the level of radiation, which is recognized by the International System of Units. It reflects the absorbed dose of radiation. A substance is considered to have received a dose of one gray if the energy that was transferred with the radiation is equal to one joule per kilogram.
This unit got its name in honor of the English scientist Lewis Gray and was officially introduced into science in 1975. According to the rules, the full name of the unit is written with a small letter, but its abbreviated designation is capitalized. One gray is equal to one hundred rads. In addition to simple units, multiple and submultiple equivalents are also used in science, such as kilogray, megagray, decigray, centigray, microgray and others.
Sievert
The unit of measure of radiation, the sievert, is used to denote effective and equivalent doses of radiation and is also part of the SI system, like gray and becquerel. Used in science since 1978. One sievert is equal to the energy absorbed by a kilogram of tissue after exposure to one heating of gamma rays. The unit got its name in honor of Rolf Sievert, a scientist from Sweden.
By definition, sieverts and grays are equal, that is, the equivalent and absorbed doses have the same size. But there is still a difference between them. When determining the equivalent dose, it is necessary to take into account not only the amount, but also other properties of the radiation, such as wavelength, amplitude, and which particles represent it. Therefore, the numerical value of the absorbed dose is multiplied by the radiation quality factor.
So, for example, with all other things being equal, the absorbed effect of alpha particles will be twenty times stronger than the same dose of gamma radiation. In addition, it is necessary to take into account the tissue coefficient, which shows how the organs respond to radiation. Therefore, the equivalent dose is used in radiobiology, and the effective dose is used in occupational health (to normalize exposure to radiation).
solar constant
There is a theory that life on our planet appeared due to solar radiation. The units of measurement of radiation from a star are calories and watts divided by a unit of time. This was decided because the amount of radiation from the Sun is determined by the amount of heat that objects receive, and the intensity with which it comes. Only half a millionth of the total amount of energy emitted reaches the Earth.
Radiation from stars travels through space at the speed of light and enters our atmosphere in the form of rays. The spectrum of this radiation is quite wide - from "white noise", that is, radio waves, to X-rays. The particles that also get along with the radiation are protons, but sometimes there may be electrons (if the energy release was large).
The radiation received from the Sun is the driving force behind all living processes on the planet. The amount of energy we receive depends on the season, the position of the star above the horizon, and the transparency of the atmosphere.
Effects of radiation on living beings
If living tissues identical in their characteristics are irradiated with different types of radiation (at the same dose and intensity), then the results will vary. Therefore, to determine the consequences, only the absorbed or exposure dose is not enough, as is the case with inanimate objects. Units of penetrating radiation appear on the scene, such as sieverts rems and grays, which indicate the equivalent dose of radiation.
An equivalent dose is a dose absorbed by living tissue and multiplied by a conditional (table) coefficient, which takes into account how dangerous this or that type of radiation is. The most commonly used measure is the sievert. One sievert is equal to one hundred rems. The higher the coefficient, the more dangerous the radiation, respectively. So, for photons this is one, and for neutrons and alpha particles it is twenty.
Since the accident at the Chernobyl nuclear power plant in Russia and other CIS countries, special attention has been paid to the level of radiation exposure to humans. The equivalent dose from natural sources of radiation should not exceed five millisieverts per year.
The action of radionuclides on non-living objects
Radioactive particles carry a charge of energy that they transfer to matter when they collide with it. And the more particles come into contact on their way with a certain amount of matter, the more energy it will receive. Its quantity is estimated in doses.
- Absorbed dose- this is what was received by a unit of substance. It is measured in grays. This value does not take into account the fact that the effect of different types of radiation on matter is different.
- Exposure dose- represents the absorbed dose, but taking into account the degree of ionization of the substance from the impact of various radioactive particles. It is measured in coulombs per kilogram or roentgens.
We examined the nature of radiation - what is radiation (ionizing radiation) and radioactivity, the concept radionuclides and half-life, the effect of radiation on human organism, and talked a little about the radioactive objects around us. The article gave information about methods for measuring radioactivity and background radiation, about dosimeters. We also gave several examples of dosimeters-radiometers, and explained that you should not panic if the device goes off scale. In the third part of the article on radiation we will talk about radiation doses ...
Exposure dose
The main characteristic of the interaction of ionizing radiation and the medium is the ionization effect. In the initial period of the development of radiation dosimetry, it was most often necessary to deal with X-rays propagating in the air. Therefore, the degree of air ionization of X-ray tubes or apparatuses was used as a quantitative measure of the radiation field. A quantitative measure based on the amount of ionization of dry air at normal atmospheric pressure, which is fairly easy to measure, is called exposure dose.
The exposure dose determines the ionizing capacity of X-rays and gamma rays and expresses the radiation energy converted into the kinetic energy of charged particles per unit mass of atmospheric air. The exposure dose is the ratio of the total charge of all ions of the same sign in an elementary volume of air to the mass of air in this volume.
In the SI system, the unit of exposure dose is the coulomb divided by the kilogram (C/kg). Off-system unit - x-ray (R). 1 C/kg = 3880 R
Absorbed dose
With the expansion of the range of known types of ionizing radiation and the scope of its application, it turned out that the measure of the effect of ionizing radiation on a substance cannot be simply determined due to the complexity and diversity of the processes occurring in this case. An important of them, giving rise to physicochemical changes in the irradiated substance and leading to a certain radiation effect, is the absorption of the energy of ionizing radiation by the substance. As a result, the concept absorbed dose. The absorbed dose shows how much radiation energy is absorbed per unit mass of any irradiated substance and is determined by the ratio of the absorbed ionizing radiation energy to the mass of the substance.
In SI units, absorbed dose is measured in joules per kilogram (J/kg) and has a special name - Gray (Gr). 1 Gr is the dose at which the mass 1 kg ionizing radiation energy is transferred 1 J. The off-system unit of absorbed dose is glad.1 Gy=100 rad.
The absorbed dose is a fundamental dosimetric value, it does not reflect the biological effect of irradiation.
Dose equivalent
Dose equivalent (E,HT,R) reflects the biological effect of irradiation. The study of individual effects of irradiation of living tissues has shown that at the same absorbed doses, different types of radiation produce unequal biological effects on the body. This is due to the fact that a heavier particle (for example, a proton) produces more ions per unit path in the tissue than a light one (for example, an electron). With the same absorbed dose, the radiobiological destructive effect is the higher, the denser the ionization created by the radiation. To take this effect into account, the notion equivalent dose. The equivalent dose is calculated by multiplying the value of the absorbed dose by a special coefficient - the coefficient of relative biological effectiveness ( OBE) or the quality factor of a given type of radiation ( WR), reflecting its ability to damage body tissues.
When exposed to different types of radiation with different quality factors, the equivalent dose is defined as the sum of the equivalent doses for these types of radiation.
The SI unit of equivalent dose is sievert (Sv) and is measured in joules per kilogram ( j/kg). Value 1 Sv equal to the equivalent dose of any type of radiation absorbed in 1 kg biological tissue and creating the same biological effect as the absorbed dose in 1 Gr photon radiation. The off-system unit of equivalent dose is Baer(until 1963 - biological equivalent x-ray, after 1963 - biological equivalent glad). 1 Sv = 100 rem.
Quality factor - in radiobiology, the average coefficient of relative biological effectiveness (RBE). Characterizes the danger of this type of radiation (compared to γ-radiation). The higher the coefficient, the more dangerous this radiation. (The term should be understood as "harm quality factor").
The values of the quality factor of ionizing radiation are determined taking into account the impact of the microdistribution of absorbed energy on the adverse biological consequences of chronic human exposure to low doses of ionizing radiation. For the quality factor, there is GOST 8.496-83. GOST as a standard is used to control the degree of radiation hazard for persons exposed to ionizing radiation during work. The standard is not applicable for acute exposures and during radiotherapy.
The RBE of a particular type of radiation is the ratio of the absorbed dose of X-ray (or gamma) radiation to the absorbed dose of radiation at the same equivalent dose.
| Quality factors for types of radiation: | |
| Photons (γ-radiation and X-rays), by definition | 1 |
| β radiation (electrons, positrons) | 1 |
| Muons | 1 |
| α-radiation with energy less than 10 MeV | 20 |
| Neutrons (thermal, slow, resonance), up to 10 keV | 5 |
| Neutrons from 10 keV to 100 keV | 10 |
| Neutrons from 100 keV to 2 MeV | 20 |
| Neutrons from 2 MeV to 20 MeV | 10 |
| Neutrons over 2 MeV | 5 |
| Protons, 2…5 MeV | 5 |
| Protons, 5…10 MeV | 10 |
| Heavy recoil nuclei | 20 |
Effective dose
Effective dose, (E, effective equivalent dose) is a value used in radiation protection as a measure of the risk of long-term effects of exposure ( stochastic effects) of the whole human body and its individual organs and tissues, taking into account their radiosensitivity.
Different parts of the body (organs, tissues) have different sensitivity to radiation exposure: for example, with the same dose of radiation, the occurrence of cancer in the lungs is more likely than in the thyroid gland. The effective equivalent dose is calculated as the sum of equivalent doses to all organs and tissues, multiplied by the weighting factors for these organs, and reflects the total effect of exposure to the body.
Weighted coefficients are established empirically and calculated in such a way that their sum for the whole organism is one. Units effective dose match the units of measurement equivalent dose. It is also measured in Sievertach or Baerach.
Fixed effective equivalent dose (CEDE - the committed effective dose equivalent) is an estimate of the doses of radiation per person, as a result of inhalation or consumption of a certain amount of a radioactive substance. CEDE is expressed in rems or sieverts (Sv) and takes into account the radiosensitivity of various organs and the time during which the substance remains in the body (up to a lifetime). Depending on the situation, CEDE may also refer to radiation dose to a specific organ rather than to the whole body.
Effective and equivalent doses- These are normalized values, i.e. values that are a measure of damage (harm) from the effects of ionizing radiation on a person and his descendants. Unfortunately, they cannot be directly measured. Therefore, operational dosimetric veins are introduced into practice, which are uniquely determined through the physical characteristics of the radiation field at a point, as close as possible to the normalized ones. The main operating value is ambient dose equivalent(synonyms - ambient dose equivalent, ambient dose).
Ambient dose equivalent H*(d) is the dose equivalent that was created in the spherical phantom ICRU(International Commission on Radiation Units) at a depth d (mm) from the surface along a diameter parallel to the direction of radiation, in a radiation field identical to that considered in composition, fluence and energy distribution, but unidirectional and homogeneous, i.e. The ambient dose equivalent H*(d) is the dose that a person would receive if they were at the location where the measurement is being taken. Unit of ambient dose equivalent — Sievert (Sv).
Group doses
By calculating the individual effective doses received by individuals, one can arrive at a collective dose - the sum of individual effective doses in a given group of people over a given period of time. The collective dose can be calculated for the population of a particular village, city, administrative-territorial unit, state, etc. It is obtained by multiplying the average effective dose by the total number of people who were exposed to radiation. The unit of measure for the collective dose is man-sievert (man-sound), off-system unit - man-rem (man-rem).
In addition, the following doses are distinguished:
- commitment- the expected dose, half a century dose. It is used in radiation protection and hygiene when calculating absorbed, equivalent and effective doses from incorporated radionuclides; has the dimension of the corresponding dose.
- collective- a calculated value introduced to characterize the effects or damage to health from irradiation of a group of people; unit - Sievert (Sv). The collective dose is defined as the sum of the products of average doses and the number of people in dose intervals. The collective dose can accumulate for a long time, not even one generation, but covering subsequent generations.
- threshold- the dose below which no manifestations of this irradiation effect are noted.
- maximum allowable doses (SDA)- the highest values of the individual equivalent dose per calendar year, at which uniform exposure for 50 years cannot cause adverse changes in the state of health detected by modern methods (NRB-99)
- preventable is the predicted dose due to a radiation accident that can be prevented by protective measures.
- doubling- a dose that doubles (or 100%) the rate of spontaneous mutations. The doubling dose is inversely proportional to the relative mutational risk. According to currently available data, the doubling dose for acute exposure is on average 2 Sv, and for chronic exposure is about 4 Sv.
- biological dose of gamma-neutron radiation- the dose of gamma irradiation equally effective in terms of damage to the body, taken as standard. Equal to the physical dose of the given radiation, multiplied by the quality factor.
- minimally lethal- the minimum dose of radiation that causes the death of all irradiated objects.
Dose rate
Dose rate (radiation intensity) is the increment of the corresponding dose under the influence of this radiation per unit of time. It has the dimension of the corresponding dose (absorbed, exposure, etc.) divided by a unit of time. Various special units are allowed (for example, microroentgen/hour, Sv/h, rem/min, cSv/year and etc.).
After beta radiation and alpha radiation were discovered, the question of evaluating these radiations when interacting with the environment became a question. The exposure dose for evaluating these radiations turned out to be unsuitable, since the degree of ionization from them turned out to be different in the air, in various irradiated substances, and in biological tissue. Therefore, a universal characteristic was proposed - the absorbed dose.
Absorbed dose - the amount of energy E transferred to a substance by ionizing radiation of any kind, calculated per unit mass m of any substance.
In other words, the absorbed dose (D) is the ratio of the energy dE, which is transferred to the substance by ionizing radiation in an elementary volume, to the mass dm of the substance in this volume:
1 J/kg = 1 Gray. The off-system unit is rad (radiation adsorption dose). 1 Gray = 100 rad.
You can also use fractional units, for example: mGy, µGy, mrad, µrad, etc.
Note. According to RD50-454-84, the use of the unit "rad" is not recommended. However, in practice there are devices with this calibration, and it is still used.
The definition of absorbed dose includes the concept of the average energy transferred to a substance in a certain volume. The fact is that due to the statistical nature of radiation and the probabilistic nature of the interaction of radiation with matter, the value of the energy transferred to matter is subject to fluctuations. It is impossible to predict its value during measurement in advance. However, after a series of measurements, you can get the average value of this value.
Dose in an organ or biological tissue (D,r) is the average absorbed dose in a specific organ or tissue of the human body:
D T = E T /m T ,(4)
where E T is the total energy transferred by ionizing radiation to a tissue or organ; m T is the mass of an organ or tissue.
When a substance is irradiated, the absorbed dose increases. The dose slew rate is characterized by the absorbed dose rate.
The absorbed dose rate of ionizing radiation is the ratio of the increment of the absorbed radiation dose dD over the time interval dt to this interval:
Dose rate units: rad/s, Gy/s, rad/h, Gy/h, etc.
The absorbed dose rate in some cases can be considered as a constant value over some short time interval or exponentially changing over a significant time interval, then we can assume that:
Kerma - an abbreviation of English words in translation means "kinetic energy of weakening in the material." The characteristic is used to assess the impact of indirect ionizing radiation on the environment. Kerma is the ratio of the sum of the initial kinetic energies dE k of all charged particles formed indirectly by AI in an elementary volume to the mass dm of matter in this volume:
K = dEk /dm. (7)
Units of measurement in SI and off-system: Gray and rad, respectively.
Kerma was introduced to take into account the radiation field more fully, in particular, the energy flux density, and is used to assess the impact of indirect ionizing radiation on the medium.
Dose equivalent
It has been established that when irradiating human biological tissue with the same energy (that is, when receiving the same dose), but with different types of rays, the health consequences will be different. For example, when exposed to alpha particles, the human body is much more likely to develop cancer than when exposed to beta particles or gamma rays. Therefore, for a biological tissue, a characteristic was introduced - an equivalent dose.
Equivalent dose (HTR) is the absorbed dose in an organ or tissue multiplied by the corresponding radiation quality factor WR of a given type of radiation R.
Introduced to assess the consequences of irradiation of biological tissue with low doses (doses not exceeding 5 maximum permissible doses for irradiation of the whole human body), that is, 250 mSv / year. It cannot be used to assess the effects of exposure to high doses.
The equivalent dose is:
H T . R = D T . R W R ,(8)
where D T . R is the absorbed dose by biological tissue by radiation R; W R - weight factor (quality factor) of radiation R (alpha particles, beta particles, gamma quanta, etc.), which takes into account the relative effectiveness of various types of radiation in inducing biological effects (Table 1). This factor depends on many factors, in particular, on the magnitude of the linear energy transfer, on the ionization density along the track of the ionizing particle, and so on.
Formula (8) is valid for assessing the doses of both external and internal irradiation of only individual organs and tissues or uniform exposure of the entire human body.
When exposed to different types of radiation simultaneously with different weighting factors, the equivalent dose is determined as the sum of equivalent doses for all these types of radiation R:
H T = Σ H T . R(9)
It has been established that at the same absorbed dose the biological effect depends on the type of ionizing radiation and the radiation flux density.
Note. When using formula (8), the average quality factor is taken in a given volume of biological tissue of a standard composition: 10.1% hydrogen, 11.1% carbon, 2.6% nitrogen, 76.2% oxygen.
The SI unit of equivalent dose is Sievert (Sv).
Sievert is a unit of equivalent dose of radiation of any nature in biological tissue, which creates the same biological effect as the absorbed dose of 1 Gy of exemplary X-ray radiation with a photon energy of 200 keV. Fractional units are also used - μSv, mSv. There is also an off-system unit - rem (the biological equivalent of a rad), which is gradually being withdrawn from use.
1 Sv = 100 rem.
Fractional units are also used - mrem, mkrem.
Table 1. Radiation quality factors
|
Type of radiation and energy range |
Quality factors WE |
|
Photons of all energies | |
|
Electrons of all energies | |
|
Neutrons with energy: | |
|
from 10 keV to 100 keV | |
|
> 100 keV up to 2 Msv | |
|
> 2 MeV to 20 MeV | |
|
Protons with energy over 2 MeV, except for recoil protons | |
|
Alpha particles, fission fragments, heavy nuclei | |
|
Note. All values refer to radiation incident on the body and, in the case of internal exposure, emitted during nuclear transformation. |
|
Note. The coefficient W R takes into account the dependence of the adverse biological effects of low-dose exposure on the total linear energy transfer (LET) of radiation. Table 2 shows the dependence of the quality weighting factor W R on the LET.
Table 2. Dependence of quality factor WR on LET
The equivalent dose rate is the ratio of the increment of the equivalent dose dH during the time dt to this time interval:
Units of equivalent dose rate mSv/s, µSv/s, rem/s, mrem/s, etc.
The impact of radiation on living organisms is characterized by dose of radiation.
Exposure dose X of ionizing radiation - the total charge formed due to radiation in 1 cm 3 of air for some time t.
measured in pendants on the kilogram (C/kg), off-system unit - x-ray (R).
At a dose of 1 R in 1 cm 3 under normal conditions, 2.08 is formed. 10 9 pairs of ions, which corresponds to 2.58. 10-4 C/kg. At the same time, in 1 cm 3 air due to ionization absorbs energy equal to 1.1. 10-8 J, i.e. 8.5 mJ/kg.
The absorbed dose of radiation D p. is a physical quantity equal to the ratio of the absorbed energy W p to the mass M p of the irradiated substance. The values of the absorbed dose are determined using the expression
D p \u003d W p / M p.
In the SI system, the unit of absorbed dose is Gray. This unit is named after the English physicist A. Gray. This dose is received by a body weighing 1 kg, if it absorbed energy in 1 J.
Until 1980, rad and roentgen were used as the unit of absorbed dose. These are non-systemic units.
Glad - from English. absorbed radiation dose.
1 glad= 10 -2 j/kg = 10 -2 Gr.
1 Gray (Gy) \u003d 100 rad » 110 R (for gamma radiation).
The unit "X-ray" is quite often used now; maybe it's just a tribute to tradition. By definition, the dose in 1 R corresponds to such radiation at which in 1 cm 3 air at n.o. ( P 0=760 mm. rt. st, T = 273 To) a certain number of pairs of ions is formed (N » 2.1 10 9), so that their total charge is 3.3 10 -10 Cl. The meaning of this definition is clear: knowing the discharge current and time, one can experimentally determine the total ionization charge and the number of pairs of ions that have arisen as a result of irradiation
N ion \u003d Q total /e.
For the same conditions (n.c.), we find the value of the absorbed dose:
D p \u003d W p / M p= 112.5 10 -10 / 0.128 10 -5 = 8.7 10 -3 j/kg.
Thus, a dose of 1 roentgen corresponds to an absorbed dose of 8.7 10 -3 j/kg or 8.7 10 mGy.
1 P \u003d 8.7 10 -3 J / kg \u003d 8.7 mGy.
A dose of 1 R is created by rays emitted by 1 gram of radium at a distance of 1 m from the source for 1 hour.
The absorbed dose rate D I P. is a physical quantity that characterizes the amount of energy absorbed by a unit of mass of any physical body per unit of time:
D 1 p \u003d D P / t \u003d W P / M Pp t.
The value of the background radiation is usually reported to us in microroentgen/hour, for example 15 microroentgen/hour. This value has the dimension of the absorbed dose rate, but it is not expressed in SI units.
Equivalent dose H equiv. - a value that characterizes the absorbed dose of a living organism. It is equal to the absorbed dose multiplied by a coefficient that reflects the ability of this type of radiation to damage body tissues:
H equiv. = KK × D P,
where KK is the average quality factor of ionizing radiation in a given volume element of biological tissue (Table 22.1).
Table 22.1.e.
It should be noted that the equivalent dose H eq characterizes the average value of the absorbed dose by a living organism, although the same tissues (bones, muscles, brain, etc.) for different people and under different conditions will absorb different energy.
In the SI system, the unit of dose equivalent is Sievert (1 Sv), named after the Swedish scientist - radiologist R. Sievert. In practice, a non-systemic unit of equivalent dose is often used - rem (the biological equivalent of a roentgen).
1 rem= 0,01 j/kg.
In practice, submultiple units are used: millirem (1 mbre = 10 -3 rem); microrem (1 microrem= 10 -6 rem); nanorem (1 nber = 10 -9 rem).
There is another definition of the concept rem.
Rem is the amount of energy absorbed by a living organism when exposed to any type of ionizing radiation and causing the same biological effect as an absorbed dose of 1 rad of X-ray or g-radiation with an energy of 200 keV.
The ratio between named units (1 Sv, 1 rem, 1 R) is:
1 Sv = 100 rem» 110 R(for gamma radiation).
As you move away from a point source, the dose decreases inversely with the square of the distance (~ 1 / r 2).
Absorbed dose
D p \u003d D 1 floor t region / r 2. [D 1 e t] = 1 R· 1m 2 / hour,
where D1 et - power of a point source; t region - exposure time, h; r - distance from the source, m.
The activity of a point emitter and the dose rate are related by the relation:
R = K g ,
where K g- ionization constant, r- distance from the radiation source, d- thickness of the protective screen, - radiation absorption coefficient in the screen material.
Ionization constant K g and the absorption coefficient of the screen depend in a complex way on the type and energy of the radiation. For gamma rays with an energy of about 1 MeV the ratio of the absorption coefficient to the density of the material for many materials (water, aluminum, iron, copper, lead, concrete, brick) is close to 7 . 10-3 m 2 /kg.
The natural radiation background (cosmic rays; radioactivity of the environment and the human body) is about a yearly radiation dose of about Gy per person. The International Commission on Radiation Protection has set a maximum allowable annual dose of 0.05 Gy for persons working with radiation. A radiation dose of 3-10 Gy received in a short time is lethal.
When working with any source of radiation (radioactive isotopes, reactors, etc.), it is necessary to take measures for the radiation protection of all people who can get into the radiation zone.
The simplest method of protection is the removal of personnel from the source of radiation at a sufficiently large distance. Even without taking into account absorption in air, the intensity of radiation decreases in proportion to the square of the distance from the source. Therefore, ampoules with radioactive preparations should not be taken by hand. It is necessary to use special tongs with a long handle.
In cases where it is impossible to move away from the radiation source at a sufficiently large distance, barriers made of absorbing materials are used to protect against radiation.
The most difficult protection against g-rays and neutrons because of their high penetrating power. The best absorber of g-rays is lead. Slow neutrons are well absorbed by boron and cadmium. Fast neutrons are pre-moderated with graphite.
Fon at 15 microroentgen/hour corresponds to dose rate 36.2 10 –12 Gy/s(or 4.16 10 -9 R/s). With such a dose rate, a person in one year, provided that tissue ionization occurs in the same way as air ionization, will receive a radiation dose equal to 1.1 mGy(or 0.13 R). This dose of radiation is very small and harmless to humans. But we must also keep in mind that radiation can accumulate in building materials used in the construction of residential and industrial buildings. The influence of radiation from structural materials can be more significant than from the background of outside air.
Knowing the total equivalent dose, one can find the equivalent absorbed dose of individual organs ( H org, i \u003d K pp × D equiv) and assess the probability of their radiation injury. At the same time, when using radiation therapy in medicine, it is very important to know and set the values of the power of the radiation source and the exposure time so that the equivalent absorbed dose for a given organ (for example, for the lungs) does not go beyond the allowable dose.
This article is devoted to the topic of absorbed radiation dose (i-tion), ionizing radiation and their types. It contains information about diversity, nature, sources, calculation methods, units of absorbed radiation dose and much more.
The concept of absorbed radiation dose
Radiation dose is a value used by such sciences as physics and radiobiology in order to assess the degree of impact of ionizing radiation on the tissues of living organisms, their life processes, and also on substances. What is called the absorbed dose of radiation, what is its value, the form of exposure and the variety of forms? It is mainly presented in the form of interaction between the medium and ionizing radiation, and is called the ionization effect.
The absorbed dose has its own methods and units of measurement, and the complexity and variety of the processes occurring under the influence of radiation give rise to some species diversity in the forms of the absorbed dose.
Ionizing form of radiation
Ionizing radiation is a stream of various types of elementary particles, photons or fragments formed as a result of atomic fission and capable of causing ionization in a substance. Ultraviolet radiation, like the visible form of light, does not belong to this type of radiation, nor do they include infrared radiation and emitted by radio bands, which is due to their small amount of energy, which is not enough to create atomic and molecular ionization in the ground state.
Ionizing type of radiation, its nature and sources
The absorbed dose of ionizing radiation can be measured in various SI units and depends on the nature of the radiation. The most significant types of radiation are: gamma radiation, beta particles of positrons and electrons, neutrons, ions (including alpha particles), x-rays, short-wave electromagnetic (high-energy photons) and muons.
The nature of sources of ionizing radiation can be very diverse, for example: spontaneously occurring radionuclide decay, thermonuclear reactions, rays from space, artificially created radionuclides, nuclear-type reactors, an elementary particle accelerator and even an X-ray apparatus.
How does ionizing radiation work?
Depending on the mechanism by which the substance and ionizing radiation interact, it is possible to single out a direct flow of particles of a charged type and radiation that acts indirectly, in other words, a photon or proton flow, a flow of neutral particles. The formation device allows you to select the primary and secondary forms of ionizing radiation. The absorbed radiation dose rate is determined in accordance with the type of radiation to which the substance is exposed, for example, the effect of the effective dose of rays from space on the earth's surface, outside the shelter, is 0.036 μSv / h. It should also be understood that the type of radiation dose measurement and its indicator depend on the sum of a number of factors, speaking of cosmic rays, it also depends on the latitude of the geomagnetic species and the position of the eleven-year cycle of solar activity.
The energy range of ionizing particles is in the range of indicators from a couple of hundred electron volts and reaches values of 10 15-20 electron volts. The length of the run and the ability to penetrate can vary greatly, ranging from a few micrometers to thousands or more kilometers.
Introduction to exposure dose
The ionization effect is considered to be the main characteristic of the form of interaction between radiation and the medium. In the initial period of the formation of radiation dosimetry, radiation was mainly studied, the electromagnetic waves of which lay within the limits between ultraviolet and gamma radiation, due to the fact that it is widespread in the air. Therefore, the level of air ionization served as a quantitative measure of radiation for the field. Such a measure became the basis for creating an exposure dose determined by the ionization of air under conditions of normal atmospheric pressure, while the air itself must be dry.
The exposure absorbed dose of radiation serves as a means of determining the ionizing possibilities of radiation of X-rays and gamma rays, shows the radiated energy, which, having undergone transformation, has become the kinetic energy of charged particles in a fraction of the air mass of the atmosphere.
The unit of absorbed radiation dose for the exposure type is the coulomb, the SI component, divided by kg (C/kg). Type of non-systemic unit of measurement - roentgen (P). One pendant/kg corresponds to 3876 roentgens.
Absorbed amount
The absorbed radiation dose, as a clear definition, has become necessary for a person due to the variety of possible forms of exposure of one or another radiation to the tissues of living beings and even inanimate structures. Expanding, the known range of ionizing types of radiation showed that the degree of influence and impact can be very diverse and is not subject to the usual definition. Only a specific amount of absorbed radiation energy of the ionizing type can give rise to chemical and physical changes in tissues and substances exposed to radiation. The very number needed to trigger such changes depends on the type of radiation. The absorbed dose of i-nia arose precisely for this reason. In fact, this is an energy quantity that has undergone absorption by a unit of matter and corresponds to the ratio of the ionizing type energy that was absorbed and the mass of the subject or object that absorbs radiation.
The absorbed dose is measured using the unit gray (Gy) - an integral part of the C system. One gray is the amount of dose capable of transmitting one joule of ionizing radiation to 1 kilogram of mass. Rad is a non-systemic unit of measurement, in terms of value 1 Gy corresponds to 100 rad.
Absorbed dose in biology
Artificial irradiation of tissues of animal and plant origin has clearly demonstrated that different types of radiation, being in the same absorbed dose, can affect the body and all biological and chemical processes occurring in it in different ways. This is due to the difference in the number of ions created by lighter and heavier particles. For the same path along the tissue, a proton can create more ions than an electron. The denser the particles are collected as a result of ionization, the stronger will be the destructive effect of radiation on the body, under conditions of the same absorbed dose. It is in accordance with this phenomenon, the difference in the strength of the effects of different types of radiation on tissues, that the designation of the equivalent dose of radiation was put into use. Absorbed Radiation is the amount of radiation received by the body, calculated by multiplying the absorbed dose and a specific factor called the Relative Biological Efficiency Ratio (RBE). But it is also often referred to as the quality factor.
The units of absorbed dose of the equivalent type of radiation are measured in SI, namely sieverts (Sv). One Sv is equal to the corresponding dose of any radiation that is absorbed by one kilogram of tissue of biological origin and causes an effect equal to the effect of 1 Gy of photon-type radiation. Rem - is used as an off-system measuring indicator of the biological (equivalent) absorbed dose. 1 Sv corresponds to one hundred rems.
Effective Dose Form
The effective dose is an indicator of magnitude, which is used as a measure of the risk of long-term effects of human exposure, its individual parts of the body, from tissues to organs. This takes into account its individual radiosensitivity. The absorbed dose of radiation is equal to the product of the biological dose in parts of the body by a certain weighting factor.
Different human tissues and organs have different radiation susceptibility. Some organs may be more likely than others to develop cancer at the same absorbed dose equivalent value, for example, the thyroid is less likely to develop cancer than the lungs. Therefore, a person uses the created radiation risk coefficient. CRC is a means for determining the dose of i-tion affecting organs or tissues. The total indicator of the degree of influence on the body of an effective dose is calculated by multiplying the number corresponding to the biological dose by the CRC of a particular organ, tissue.
The concept of collective dose
There is a concept of group absorption dose, which is the sum of the individual set of effective dose values in a particular group of subjects over a certain time period. Calculations can be made for any settlements, up to states or entire continents. To do this, multiply the average effective dose and the total number of subjects exposed to radiation. This absorbed dose is measured using the man-sievert (man-Sv.).
In addition to the above forms of absorbed doses, there are also: commitment, threshold, collective, preventable, maximum allowable, biological dose of gamma-neutron type radiation, lethal-minimum.
The strength of the dose and units of measurement
The indicator of the intensity of exposure is the substitution of a specific dose under the influence of a certain radiation for a temporary measuring unit. This value is characterized by the difference in the dose (equivalent, absorbed, etc.) divided by the unit of time. There are many custom built units.
The absorbed dose of radiation is determined by a formula suitable for a particular radiation and the type of absorbed amount of radiation (biological, absorbed, exposure, etc.). There are many ways to calculate them, based on different mathematical principles, and different units of measurement are used. Examples of units of measurement are:
- Integral view - gray kilogram in SI, outside the system is measured in rad grams.
- The equivalent form is sievert in SI, outside the system it is measured in rems.
- Exposure type - pendant-kilogram in SI, outside the system is measured - in roentgens.
There are other units of measurement corresponding to other forms of absorbed radiation dose.
conclusions
Analyzing these articles, we can conclude that there are many types, both of the ionizing radiation itself, and the forms of its effect on substances of animate and inanimate nature. All of them are measured, as a rule, in the SI system of units, and each type corresponds to a certain system and non-system measuring unit. Their source can be the most diverse, both natural and artificial, and the radiation itself plays an important biological role.