Radiation dose

The main characteristic of the interaction of ionizing radiation with the medium is the ionization effect. A quantitative measure based on the amount of ionization of dry air at normal atmospheric pressure, which is quite easy to measure, is called the exposure dose .

The exposure dose is the ratio of the total electric charge of ions of the same sign, formed after the electrons and positrons are completely braked in air, released or generated by photons in an elementary volume of air, to the mass of air in this volume.

In the international system of units (SI), the unit of measurement of the exposure dose is the pendant divided by kilogram (C / kg). The off-system unit is X - ray (P). 1 C / kg = 3876 R.

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 can not be easily determined because of the complexity and diversity of the processes occurring in this process. 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 of this, the concept of absorbed dose arose. It shows how much radiation energy is absorbed per unit mass of the irradiated substance and is determined by the ratio of the absorbed energy of ionizing radiation to the mass of the absorbing substance.

The unit of measurement of the absorbed dose in the SI system is gray (Gy). 1 Gy is a dose at which a mass of 1 kg transfers ionizing radiation energy of 1 joule . The non-systematic unit of absorbed dose is rad . 1 Gy = 100 rad.

The study of the individual effects of irradiation of living tissues showed that at the same absorbed doses, different types of radiation produce an unequal biological effect on the body . This is due to the fact that a heavier particle (for example, a proton ) produces more ions per unit path length 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 concept of an equivalent dose is introduced. The equivalent dose is calculated by multiplying the absorbed dose by a special coefficient - a weighting radiation coefficient , taking into account the relative biological effectiveness of various types of radiation.

The SI unit of equivalent dose is sievert (Sv). The value of 1 Sv is equal to the equivalent dose of any type of radiation absorbed in 1 kg of biological tissue and creates the same biological effect as the absorbed dose of 1 Gy of photon radiation. A non-systematic unit of measurement of the equivalent dose is rem (before 1954 - the biological equivalent of X-ray, after 1954 - the biological equivalent of Rad ^[1] ). 1 Sv = 100 rem.

The effective dose (E) is the value used as a measure of the risk of the long-term effects of irradiation of the entire human body and its individual organs and tissues, taking into account their radiosensitivity. It represents the sum of the products of the equivalent dose in organs and tissues by the corresponding weighting factors.

Some organs and tissues of a person are more sensitive to the action of radiation than others: for example, with the same equivalent dose, cancer in the lungs is more likely than in the thyroid gland , and irradiation of the sex glands is especially dangerous due to the risk of genetic damage. Therefore, the radiation doses of different organs and tissues should be considered with a different coefficient, which is called the tissue weighting coefficient . Multiplying the value of the equivalent dose by the corresponding weighing coefficient and summing over all tissues and organs , we obtain an effective dose that reflects the total effect for the body . Weighing coefficients are established empirically and calculated so that their sum for the whole organism is one.

The effective dose units are the same as the equivalent dose units. It is also measured in sieverts or rems .

The expected effective dose E (τ) is the dose of internal exposure from radionuclides received in the human body ^{[2] [3]} . The time of human exposure to such radionuclides is determined by their half-lives and biological retention in the body and can be many months or even years ^[4] . For regulatory purposes, the full period of dose accumulation is set to 50 years for an adult or, if a dose for children is being assessed, before reaching 70 years. When assessing the annual dose, the expected effective dose is combined with the effective dose from external exposure for the same period ^[5] .

Effective and equivalent doses are standardized values, that is, values ​​that are a measure of damage (harm) from the effects of ionizing radiation on a person. Unfortunately, they cannot be directly measured. Therefore, operational dosimetric quantities are introduced into practice that are uniquely determined through the physical characteristics of the radiation field at a point as close as possible to normalized ones. The main operating variable is the ambient dose equivalent (synonyms - equivalent to ambient dose, ambient dose).

Ambient dose equivalent H * (d) is the dose equivalent that was created in the ICPE spherical phantom (International Commission on Radiation Units) at a depth d (mm) from the surface in diameter parallel to the radiation direction, in a radiation field identical to that considered in composition, fluence and energy distribution, but unidirectional and homogeneous, that is, the ambient dose equivalent of H * (d), is the dose that a person would receive if he were at the place where the measurement is taken. The unit of ambient dose equivalent is sievert (Sv).

By counting the individual effective doses received by individuals, you can come to a collective dose - the sum of the individual effective doses in this group of people for a given period of time. The collective dose can be calculated for the population of an individual 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 measurement of the collective dose is man-sievert ( person- sv.), The off-system unit is man-sir (person-rem). The collective dose can accumulate over a long time, not even of one generation, but covering subsequent generations.

In addition, the following doses are distinguished:

• threshold - the dose below which the manifestations of this radiation effect are not noted.
• preventable - the predicted dose due to a radiation accident that can be prevented by protective measures.
• doubling - a dose that increases 2 times (or 100%) the level of spontaneous mutations . Doubling the dose is inversely proportional to the relative mutational risk .
• minimally lethal - the minimum dose of radiation that causes the death of all irradiated objects.

Millisievert (mSv) is often used as a measure of dose in medical diagnostic procedures ( fluoroscopy , X-ray computed tomography , etc.).

According to the decree of the chief state sanitary doctor of Russia No. 11 dated April 21, 2006 “On limiting the exposure of the population when conducting radiological medical examinations”, clause 3.2, it is necessary “to ensure compliance with the annual effective dose of 1 m Sv during preventive medical radiological examinations including during the medical examination. " The global average dose from x-ray studies accumulated per capita per year is 0.4 mSv, but in countries with a high level of access to medical care (more than one doctor per 1000 population), this indicator rises to 1.2 mSv ^[6] . Irradiation from other man-made sources is much less: 0.005 mSv from radionuclides left over from atmospheric nuclear tests , 0.002 mSv from the Chernobyl disaster , 0.0002 mSv from nuclear power.

The world average dose from natural sources accumulated per capita per year is 2.4 m Sv, with a spread of 1 to 10 m Sv ^[6] . The main components:

• 0.4 mSv from cosmic rays (from 0.3 to 1.0 mSv, depending on the height above sea level);
• 0.5 mSv from external gamma radiation (from 0.3 to 0.6 mSv, depending on the radionuclide composition of the environment - soil, building materials, etc.);
• 1.2 mSv of internal exposure from inhaled atmospheric radionuclides, mainly radon (from 0.2 to 10 mSv, depending on the local concentration of radon in the air);
• 0.3 mSv of internal exposure from incorporated radionuclides (from 0.2 to 0.8 mSv, depending on the radionuclide composition of food and water).

With a single uniform exposure of the whole body and the failure to provide specialized medical care, death as a result of acute radiation sickness occurs in 50% of cases ^[7] :

• at a dose of the order of 3-5 Gy due to damage to the bone marrow within 30-60 days;
• 10 ± 5 Gy due to damage to the gastrointestinal tract and lungs within 10-20 days;
• > 15 Gy due to damage to the nervous system for 1-5 days.

Dose rate (radiation intensity) - increment of the corresponding dose under the influence of a given radiation per unit time. It has the dimension of the corresponding dose (absorbed, exposure , etc.) divided by the unit of time. It is allowed to use various special units (for example: Sv / hour, rem / min, mSv / year , etc.).

• Dosimeter
• Low radiation doses
• Radioactivity
• Dose threshold
• Kerma air
• Particle fluence

Sources

• ICRP. Publication 103 of the ICRP. 2007 Recommendations of the International Commission on Radiation Protection . - 2009. - 344 p.
• I.B. Keirim Marcus. Equidosimetry. - M .: Atomizdat, 1980 .-- 191 p.