Читать книгу Patty's Industrial Hygiene, Physical and Biological Agents - Группа авторов - Страница 25

An alpha particle is a highly energetic nucleus of an ordinary helium atom, which consists of an assembly of two positively charged protons and two electrically neutral neutrons, and is designated by the symbol , that is emitted from the nucleus of certain radioactive isotopes. When an alpha particle is emitted, the alpha‐emitting parent is transformed into a daughter element whose atomic number is 2 less, and whose atomic mass number is 4 less than that of the parent. In the case of ²³⁸U, for example which is transformed to ²³⁴Th

Two important properties of alpha radiation are the high rate of energy transfer to the medium through which the alpha particles travel and, consequently, a very small penetration range into an absorbing medium. Although most alpha particles from radioisotopes have kinetic energies in the range of 3–7 MeV, they can penetrate only a few centimeters of air before exhausting all their kinetic energy. Tissue and other solid materials are almost opaque to alpha particles. A 5.3‐MeV alpha particle from ²¹⁰Po, for example whose range in air is about 4 cm, can penetrate only about 0.005 cm of soft tissue. This thickness is less than that of the dead outer layer of skin. For this reason, alpha radiation originating from alpha‐emitting radioisotopes outside the body is not considered hazardous. However, alpha radiation from an internally deposited radioisotope transfers its kinetic energy directly to viable tissue. The high concentration of absorbed energy, together with the microscopic distribution of the absorbed energy, makes internally absorbed alpha radiation more toxic per unit amount of absorbed energy than energy absorbed from the other radiations.