Читать книгу Patty's Industrial Hygiene, Physical and Biological Agents - Группа авторов - Страница 26

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)