Patty's Industrial Hygiene, Physical and Biological Agents. Группа авторов

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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.

      4.2 Beta Particles

      Beta particles are ordinary electrons that are ejected with a high kinetic energy from the nucleus of certain radioisotopes, resulting from the change of a neutron into a proton and the ejected beta particle. Emission of a beta particle results in the transformation of the beta emitter into another element whose atomic number is one greater than that of the parent, and whose atomic mass number remains unchanged. In the case of strontium‐90 (⁹⁰Sr), for example a pure beta emitter, the daughter element is yttrium‐90 (⁹⁰Y), the next higher element after Sr in the periodic table. In this case, the reaction does not stop with the production of ⁹⁰Y. The yttrium daughter is also radioactive, and decays by beta emission to zirconium‐90 (⁹⁰Zr), which is stable. These transformations are written symbolically as

      In the radioactive decay of many radioisotopes, gamma rays accompany the beta particles; these radioisotopes are called beta–gamma emitters. Radionuclides that emit only beta particles without any accompanying gamma radiation are called pure beta emitters.

Beta particles emitted by any particular radioisotope have a spread of energies, or a spectral distribution, that extends from near zero to a maximum that is characteristic of that radioisotope, as illustrated in Figure 1. The maximum energies vary from one radioisotope to another, and span a range of energies extending from several kiloelectron volts to several megaelectron volts. The average energy beta from any particular radioisotope is, in most cases, approximately one‐third of the maximum energy.

FIGURE 1

      Beta energy spectrum for ³²P.

      Source: Based on data from Radiological Toolbox v. 1.0.0.

The depth of penetration, or the range of the beta radiation in matter, increases as the energy of the radiation increases. In air, very low‐energy betas have a range of several centimeters, while high‐energy betas travel about 3 m in air per MeV of energy. Figure 2 shows the relationship between range and energy for beta particles.

      The range of the beta radiation in Figure 2 is expressed in units of density thickness. Density thickness is related to linear thickness by

      (3)

      For example, a sheet of aluminum 1‐mm (0.1‐cm) thick, density = 2.7 g cm⁻³, has a density thickness of

      (4)

FIGURE 2

      Range–energy relationship for beta particles.

      From Ref. 2.

The concept of density thickness is useful because different materials are almost equivalent in their ability to stop beta radiation if their density thicknesses are equal. In this context, a sheet of graphite, whose density is 2.2 g cm⁻³, is equivalent to 0.1 cm Al if its linear thickness t is

      (5)

      4.2.1 Interaction with Matter

      The physical mechanisms by which charged particles, such as alpha and beta particles, transfer their kinetic energy to matter are relatively well understood. The most likely occurrence is a collision between the charged particle and one of the extranuclear orbital electrons in the energy‐absorbing matter with which the radiation СКАЧАТЬ