Biological effect and radiation paradox
Radiation sharply increases the chemical activity of atoms in living matter. This triggers pathogenesis at the cellular level, causing severe morphological changes up to cell death. The main threat of exposure lies in delayed genetic and somatic consequences, as well as the development of radiation sickness (acute or chronic).
The key enigma of radiobiology is the radiobiological paradox. It lies in the colossal discrepancy between the negligible amount of absorbed physical energy and the catastrophic reaction of the living organism.
Example: A dose of 1000 rem is lethal to any mammal. However, in terms of thermal energy equivalent, this is only enough to heat the body by 0.001 °C (less than a cup of hot tea provides), and in mechanical equivalent, only enough to lift an eyelid. An important pattern: the more complexly organized a living being is, the lower its threshold for a lethal dose.
Nature and classification of radiation
According to its physical nature, ionizing radiation is divided into two groups:
- Corpuscular radiation:
- Alpha particles (helium nuclei: 2 protons and 2 neutrons). Possess the highest ionizing capacity (tens of thousands of ion pairs per 1 cm), but minimal penetration (microns in tissues, centimeters in air).
- Beta particles (electrons and positrons). Range in air reaches several meters, ionizes weaker (hundreds of ion pairs per 1 cm).
- Neutrons.
- Electromagnetic (photon) radiation:
- Gamma radiation (arises during nuclear transformations and annihilation).
- X-rays (generated when electrons decelerate on the tube anode).
- Photon radiations are distinguished by enormous penetrating power (range in hundreds of meters), but weak ionization (units or tens of ion pairs per 1 cm).
Radiation doses and units of measurement
In radiation hygiene, several types of doses are used to assess both physical energy and biological effect.
| Type of dose | Essence of the indicator | SI unit | Non-SI unit |
|---|---|---|---|
| Absorbed | Energy imparted to a mass of matter | Gray (1 Gy = 1 J/kg) | Rad (1 Gy = 100 rad) |
| Exposure | Measure of air ionization by photon IR | C/kg | Roentgen (R) |
| Equivalent | Absorbed dose × radiation quality factor | Sievert (Sv) | Rem (1 Sv = 100 rem) |
| Effective | Sum of equivalent doses × radiosensitivity of organs | Sievert (Sv) | - |
The quality factor takes into account the different biological effectiveness of radiations. For example, for X-rays and gamma radiation it is equal to 1, and for alpha particles it is 20. The effective dose is used to assess the risk of delayed consequences.
Radioactivity and hygienic standards
Radioactivity is the spontaneous transformation of atomic nuclei with a change in their mass, charge, and emission of radiation. Substances with such properties are called radionuclides. Their most important characteristic is the half-life (the time during which half of the nuclei decay).
Source activity (number of transformations per second) is measured in Becquerels (Bq) or Curies (Ci).
Strict limits are established in radiation safety:
- Maximum permissible dose (MPD) — the maximum equivalent dose allowed per year during occupational exposure.
- Annual limit on intake (ALI) — the permissible level of radionuclides entering the body per year, which over 50 years will create a dose not exceeding the standard.
Briefly: answers to topical questions
Which human organs and tissues belong to the first group of critical organs according to radiosensitivity?
- Hemopoietic and lymphoid system — the most radiosensitive critical organs, damage to which determines the course and outcome of acute radiation sickness.
- Gonads — radiosensitivity is no less high than that of the red bone marrow; radiation necrosis and apoptosis of the gonad epithelium develop.
- Gastrointestinal tract — highly sensitive due to the high frequency of mitoses in the cells of the intestinal mucosa.
Class and exam questions
What is the essence of the radiobiological paradox?
In the striking discrepancy between the negligible energy of absorbed radiation and the severe reactions of the living organism (up to death).
How does equivalent dose differ from effective dose?
The equivalent dose takes into account the damaging capacity of a specific type of radiation. The effective dose additionally accounts for the radiosensitivity of various organs and tissues to assess the risk of delayed consequences.
What does the activity of a radionuclide depend on?
From its half-life: the faster the spontaneous transformation of nuclei occurs, the higher the activity of the source per unit of time.
What else to study on this topic
- Mechanisms of natural background radiation formation
- Features of internal and external exposure of the population
- Natural radionuclides (uranium-238, thorium-232, potassium-40) and their significance
- Isotopes formed during nuclear power plant accidents (Strontium-90, Cesium-137)
- Qualitative physical characteristics: linear ionization density and range length
Master the entire topic in the "Hygiene" course
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- Complete study guide on "Dosimetry of Ionizing Radiation" and 530+ more pages of the subject
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Other topics in "Chapter 7. Basics of radiation hygiene"
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