ENVIRONMENTAL BIOCHEMISTRY - Textbook - V. M. Isaienko 2005

Chapter 9. RADIORESISTANCE OF ORGANISMS

9.1. General characteristics of radioactivity, sources, and doses of ionizing radiation

Chemical element atoms are conventionally designated as ^A_ZX, where X is the element symbol, A is the mass number, and Z is the atomic number. Atoms with identical atomic numbers but different mass numbers are called isotopes.

There are unstable isotopes whose nuclei transform into stable ones after one or more radioactive decays. Such isotopes are referred to radionuclides (or radioisotopes) because the decay of their nuclei is accompanied by radioactive emission, which is ionizing in nature.

Currently, approximately 1,700 isotopes are known, of which about 270 are stable, while the remaining 1,430 are radioactive.

Radiation energy is typically expressed in electron-volts (eV). 1 eV is the energy acquired by a particle with a unit electrical charge (the charge of an electron) moving through an accelerating electric field between two points with an electrical potential difference of 1 V. In the SI system, the unit of energy is the Joule (J). It has been established that 1 eV ≈ 1.60219 • 10^-19 J. There are also multiples corresponding to 10³ eV (keV) and 10⁶ eV (MeV).

A key characteristic of radioisotopes is their half-life (t_1/2), which is the time required for the quantity of a given radioisotope to decrease by half. The value of t_1/2 varies within a wide range. For instance, for lead-204 (²⁰⁴ ₈₂Pb) it is 10¹⁹ years, whereas for polonium-212 (²⁰⁴ ₈₄Pb) it is 3 • 10^-7 s.

Radioisotopes are also characterized by activity, which represents the number of disintegrations per unit time. In the SI system, the unit of radioisotope activity is the becquerel (Bq), defined as 1 disintegration per second, while the non-system unit is the curie (Ci): 1 Ci = 3.7 • 10¹⁰ Bq.

Radioactivity observed in naturally occurring radioisotopes is termed natural radioactivity. Artificial radioactivity refers to the radioactivity of man-made radioisotopes produced through nuclear reactions.

Natural Sources of ionizing radiation include: 1) radioisotopes present in the Earth's crust since the planet's formation, along with their decay products; 2) cosmic rays (galactic and solar radiation) penetrating the atmosphere to the Earth's surface; 3) radioisotopes formed directly through the interaction of cosmic radiation with atoms and molecules of the atmosphere and the Earth's crust; and 4) ultraviolet rays, which constitute a component of solar radiation.

Artificial sources of ionizing radiation stem from human activity. They are categorized into those that pollute and those that do not pollute the environment with radioisotopes. The first group includes: 1) nuclear weapons fallout; 2) industrial nuclear explosions; 3) nuclear power PLANTS AND THEIR fuel cycle facilities (uranium mining and enrichment, spent nuclear fuel reprocessing, and The transport of nuclear fuel and waste); 4) Industrial processes that increase the concentration of radioisotopes on the Earth's surface (extraction and Processing of metal ores, mineral fertilizers, coal, natural gas, oil, etc.); and 5) the use and processing of radioisotopes in medicine, industry, and scientific research.

Anthropogenic non-polluting sources of ionizing radiation include: 1) sealed sources in various installations and devices used in medicine, industry, and science; and 2) consumer goods containing sealed radioactive substances or generating ionizing radiation (radioluminescent Materials, electronic and electrical equipment, static eliminators, gas and aerosol detectors, alloy products containing uranium and thorium, etc.).

A significant threat is posed by radioactive releases resulting from accidents at nuclear power plants and facilities involved in the mining, enrichment, and processing of nuclear fuel. The largest such accident, classified as a global ecological catastrophe, was the Chornobyl NPP disaster on April 26, 1986. Contaminated areas exceeded 380 thousand hectares in Ukraine, 725 thousand hectares in the Russian Federation, and 1,350 thousand hectares in the Republic of Belarus.

The main Types of ionizing radiation are: 1) α-radiation (a stream of helium atom nuclei, ⁴₂He²⁺); 2) β-radiation (a stream of electrons or positrons); and 3) γ-radiation (short-wave electromagnetic radiation with a wavelength <0.01 nm, arising from Changes in the energy state of atomic nuclei produced by radioactive decay). There is also neutron, meson, muon (mu-mesonic) radiation, electron radiation (electrons resulting from the auto-ionization of excited atoms during energy redistribution within them), high-speed nuclei in accelerators, X-ray electromagnetic radiation (with a wavelength of 0.01–50 nm caused by the deceleration of electrons in certain metals), and Other types of radiation.

The biological effect of ionizing radiation on the environment is quantified using METABOLISM/2.html">THE CONCEPT OF a "dose." For photon radiation (ultraviolet, X-ray, and γ-radiation), the exposure dose is defined as The ratio of the total charge of all ions of the same sign produced in air by secondary electrons and positrons generated in a volume element upon their complete stopping, to the mass of the air in that volume. The SI unit of exposure dose is the coulomb per kilogram (C/kg). The non-system unit is the roentgen (R): 1 R = 2.58 • 10^-4 C/kg.

The absorbed dose is the ratio of the mean energy imparted by ionizing radiation to matter in a volume element to the mass of the matter in that volume. The SI unit of absorbed dose is the gray (Gy): 1 Gy = 1 J/kg = 100 rad.

The primary characteristic of The Effect of ionizing radiation on living organisms is the equivalent dose. Its Introduction is based on the fact that radiation-induced effects depend not only on The amount of energy absorbed, but also on the type and energy of the radiation. The equivalent dose is defined as the product of the absorbed dose and the radiation weighting factor (W_R). This factor indicates how many times higher the radiation hazard is for a given type and energy of radiation compared to reference radiation at the same absorbed dose. Reference radiation is taken as X-rays with a maximum photon energy of 250 keV. The value of W_R varies from 1 to 20 depending on the type and energy of radiation. For example, for X-rays, γ-rays, β-rays, and muons of all energies, W_R equals 1, whereas for α-particles with energy below 10 MeV, neutrons with energy from 100 keV to 2 MeV, and heavy recoil nuclei, it is 20.

In the SI system, the unit of equivalent dose is the sievert (Sv): 1 Sv = 1 Gy • W_R = 100 rem.

The fundamental parameter characterizing The transfer of ionizing radiation energy to matter is linear energy transfer (LET)—the ratio of the energy imparted to the length of the particle's track (the path of radiation in matter). The SI unit of linear energy transfer is [LET] = 1 J/m. The non-system unit of LET is eV/nm.

A measure of the biological effect of ionizing radiation is the lethal dose (LD₁₀₀), expressed in units of absorbed or equivalent dose, at which 100% of irradiated organisms (Cells) perish within a specific timeframe that varies across organisms. Conventionally, Radioresistance is also characterized by the median lethal dose LD₅₀ (where 50% of irradiated organisms perish within a species-specific time), LD₇₀ (70% mortality), and LD₉₀ (90% mortality). The concept of D₀ is also utilized. If the dose-response curve (Cell survival as a function of dose) is a straight line without a "shoulder" in the low-dose region, D₀ corresponds to D₃₇, i.e., the dose at which 37% of irradiated cells survive. If the dose-response curve exhibits an exponential relationship with a "shoulder," then D₀ = D₃₇ only in the exponential region.

The doses D₀, LD₅₀, LD₇₀, LD₉₀, and LD₁₀₀ are used as indices of radioresistance in irradiated organisms.

Radioactive elements affect living organisms both as chemical elements (in the form of ions or within molecules) and as sources of ionizing radiation. Accidents at nuclear fuel and energy complexes, as well as the testing or use of nuclear and thermonuclear weapons, can lead to a significant local increase in the environmental concentration of radioisotopes as chemical elements. However, their levels do not exceed the concentrations of non-radioactive chemical compounds in the natural environment. Therefore, as chemical elements, radioisotopes and the molecules containing them do not exhibit significant biological effects. In this case, their primary impact on the vital activity of organisms is caused by radiation.

Radioisotopes affect organisms in two ways: 1) as a source of external irradiation; and 2) as a source of internal (incorporated) irradiation, if they enter the Organism and accumulate in Tissues and cells. If radioisotopes are not absorbed, they affect the digestive tract simply by passing through it.

The specific contribution of internal and external irradiation depends on the size of the object absorbing the radionuclides and the coefficient of their accumulation by tissues and cells. This must be taken into account when determining ionizing radiation doses and their potential impact on organisms.