放射線の生物学的影響

Nuclear radiation, both particle and electromagnetic, is quantified in terms of activity and measured by radiation detectors. However, the biological effects of radiation exposure depend not only on the activity but also on the ionizing power, penetration ability, exposure time, and area exposed. Each type of radiation penetrates matter to a different extent. Alpha particles have the least penetration ability, as they are relatively massive; most are stopped by the outer layer of skin. However, when ingested, they directly contact internal tissues and are highly damaging. Charged particle radiation, such as alpha radiation, directly ionizes biomolecules within cells, whereas neutrons, gamma rays, and X-rays affect the cellular processes indirectly. For example, gamma radiation ionizes water in living tissue to produce a hydroxyl radical, which further ionizes biomolecules, thereby damaging cells. The damage is greater if many ionizations are induced in a small area.  The energy delivered by radiation to material is measured as the ‘absorbed dose’, the SI unit of which is the gray. The deposition of one joule of energy per kilogram of material corresponds to one gray. Longer exposure times result in more energy deposition, resulting in a higher dose. The same absorbed dose of different radiation types may cause different amounts of biological damage because of the variation in ionizing and penetration powers. When considering biological damage, the absorbed dose is multiplied by a radiation weighting factor to determine the ‘equivalent dose’. Its SI unit is the sievert.  Body tissues have varying sensitivities to ionizing radiation, represented in terms of tissue weighting factors. When the equivalent dose is greater in one area, the doses are adjusted by tissue weighting factors and summed to determine the ‘effective dose’ to the body overall. Accurately determining the effective dose requires choosing an appropriate radiation detector, as detectors vary in whether they measure dose or activity, the types of radiation that they detect, and whether they can tell those types of radiation apart.  A Geiger–Müller counter is the commonly known device used to measure activity from alpha, beta, X-ray, and gamma radiation. It can be modified to respond proportionally to the energy of the radiation, allowing it to measure dose from X-rays and gamma rays.