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

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СКАЧАТЬ increased radiation exposure and inhalation hazards to workers when the pipelines are cut and the scale is dispersed as a radioactive aerosol.

      Another source of TENORM is the phosphate industry. Phosphorous is usually associated with uranium because the two elements bind tenaciously together. Uranium concentrations of about 20–300 ppm are found in phosphate rock. Radiation from phosphate slag (which is used as an aggregate in road‐paving materials), phosphate fertilizers, and potash (which also contains relatively large amounts of ⁴⁰K), thus must be considered by the industrial hygienist when evaluating possible occupational health risks in these industries.

      Radioactivity is also found in relatively high concentrations in the fly ash and in the bottom ash of coal‐ and lignite‐burning boilers. Through this mechanism, many coal‐ and lignite‐fired electricity generating stations release more radioactivity to the environment than do nuclear power stations.

      Workers who are explicitly involved with the use and maintenance of these radiation sources are not the only ones of concern to the industrial hygienist. The sources, once produced, must be transported from the manufacturer to the consumer. Additionally, radioactive waste is generally generated during the manufacture of the sradioisotopes as well as after they have been used. Thus, one must also apply the appropriate radiation safety standards and procedures to those workers involved in the transport of the radioisotopes and in the handling of waste materials.

3 BASIC PHYSICAL PRINCIPLES

      3.1 Energy

      All radiation effects, whether beneficial or detrimental, as well as all radiation measuring devices, are based on the absorption of energy from the radiation source.

      In health physics and in atomic and nuclear physics, energy is measured in units of electron volts (eV). One electron volt represents the amount of kinetic energy possessed by an electron after it is accelerated by an electrical potential of 1 V. Commonly used multiples of the electron volt are the kiloelectron volt (keV), 1000 eV and the megaelectron volt (MeV), 10⁶ eV. One electron volt = 1.6 × 10⁻¹² erg or 1.6 × 10⁻¹⁹ J; 1 MeV = 1.6 × 10⁻⁶ erg or 1.6 × 10⁻¹³ J.

      3.2 Radioisotopes

      The Bohr atomic model describes the atom as a positively charged central nucleus surrounded by electrons that revolve around the nucleus in specific radii, and are bound to the nucleus by the attractive electrical force between the positively charged nucleus and the negatively charged electron. The nucleus contains positively charged protons and electrically neutral neutrons. The attractive nuclear force between neutrons and protons and between neutrons acts to overcome the repulsive forces among the positively charged protons.

      The atomic number, and hence the chemical nature of the element, is determined by the number of protons within the nucleus. While the number of protons may remain constant, most of the naturally occurring elements consist of nuclei that have differing numbers of neutrons. Nuclei of the same element that have different numbers of neutrons are called isotopes. Copper, for example whose atomic number is 29, is a mixture of two isotopes. About 69% of Cu atoms contain 34 neutrons, and hence has 63 particles (nucleons) within its nucleus, and about 31% have 36 neutrons, or 65 nucleons in the nucleus. The number of nucleons is called the atomic mass number. Symbolically, isotopes are written with the chemical symbol and the atomic mass number as a superscript to the left of the symbol: ⁶³Cu and ⁶⁵Cu in the case of copper 63 and copper 65, respectively. For an isotope to remain stable, the neutron/proton ratio must lie within a relatively narrow range. If the ratio is outside this range, the nucleus is unstable and achieves stability through a spontaneous nuclear transformation that corrects the neutron/proton ratio. Such an unstable isotope is called a radioisotope. Radioisotopes emit various combinations of particles and gamma rays that are specific to each isotope; this process is called radioactive transformation or radioactive decay. In most cases, the transformed nucleus is a different element, one whose neutron/proton ratio is closer to stability than the unstable parent's nucleus.

      All radioisotopes are uniquely identified by a set of three different characteristics:

       Type of radiation that is emitted

       Energy of the emitted radiation

       Rate at which the radioisotope decays. This rate is described either by the half‐life, or by the fraction that decays per unit time (the decay rate constant).

      3.3 Rate of Decay

      The half‐life is defined as the period of time during which one‐half of the radioisotope decays. After a second half‐life period, one‐half of the remaining isotope decays, leaving one‐fourth of the original activity, and after the next half‐life, only one‐eighth of the original activity is left. More than 99% of the original activity is gone after seven half‐lives. Half‐lives of radioisotopes range from microseconds to billions of years.

      The decay rate constant is defined as the instantaneous fractional decrease of the radioisotope. In the case of ²²⁶Ra, whose half‐life is 1600 years, the decay rate constant is 0.00043 per year. This means that it is decreasing at a rate of 0.043% per year. Radioisotopes with a short half‐life decay at a much faster rate. For example, ¹³¹I, with a half‐life of eight days, decays at a rate of 8.7% per day. Generally, the relationship between an initial amount of activity, A₀, and the amount of activity left after a time t, and whose decay rate constant is λ per unit time, is given by

      (1)

      The half‐life, T_1/2, is related to the decay rate constant by

      The half‐time, T_1/2, and the decay constant, λ, must be in the same time units when calculating with these decay equations.

      3.4 Quantity of Radioactivity

      Since radioactive isotopes are used because of their radioactivity, the unit for expressing the quantity of radioactive material is based on the disintegration rate. Thus, for example 3.7 × 10¹⁰ atoms in 1 g of ²²⁶Ra decay in 1 s, while 1.7 × 10¹⁴ atoms in 1 g of ²¹⁰Po decay in 1 s. One gram of ²¹⁰Po is much more radioactive than 1 g of ²²⁶Ra. Clearly, mass is not the most appropriate unit for specifying the quantity of a radioisotope. The traditional unit for the quantity of a radioisotope is Curie, symbolized by Ci. The Ci is defined as that quantity of a radioactive material in which 3.7 × 10¹⁰ atoms are transformed in 1 s. The commonly used subunits are the millicurie, mCi (10⁻³ Ci), microcurie, μCi (10⁻⁶ Ci), nanocurie, nCi (10⁻⁹ Ci), and picocurie, pCi (10⁻¹² Ci).

      In the SI system, the unit for the quantity of radioactivity is the becquerel (Bq). One becquerel is defined as that quantity of radioactive material in which one atom disintegrates in 1 s. Since the Bq is such a small quantity of radioactive material, the kilobecquerel (kBq) (10³ Bq), megabecquerel (MBq) (10⁶ Bq), and the terabecquerel (10¹² Bq) are frequently used. 1 MBq = 27 μCi.

      3.5 Specific Activity

      The concentration of radioactivity is called the specific activity. Specific activity may be СКАЧАТЬ