The latitude effect is due to the earth's magnetic field. Charged cosmic ray particles entering the earth's atmosphere in the equatorial region travel in a direction that is nearly perpendicular to the earth's magnetic field, whereas charged particles that enter the atmosphere at higher latitudes cross the magnetic lines of force at a glancing angle. The equatorial particles thus experience a much greater magnetic force, which turns them away from their initial path toward the earth, than the particles that enter at the high latitudes. The cosmic ray intensity is therefore lowest at the equator, and increases continuously as the latitude, both north and south, increases.
Terrestrial radiation comes mainly from three different groups of naturally occurring radioisotopes. Primordial radioisotopes that were present when the earth was formed are
238U, T1/2 = 4.5 × 109 years
235U, T1/2 = 7.1 × 108 years
232Th, T1/2 = 1.4 × 1010 years
40K, T1/2 = 1.3 × 109 years.
These radioactive elements are ubiquitous; average uranium and thorium concentrations in the soil are of the order of several parts per million. Natural potassium, which is abundant in the earth's crust, contains approximately 0.012% of 40K.
2.1.1 Uranium and Thorium Progeny
The primordial U and Th isotopes are the progenitors of three chains of sequentially decaying radioisotopes that have several common characteristics. Each series contains a radioisotope of the gaseous element radon, and each series terminates with a stable isotope of lead. Radon gas diffuses out of the ground, and the radon daughters, which are solids under ordinary circumstances, attach themselves to atmospheric dust particles. This naturally occurring airborne radioactivity must be accounted for when using the measured radioactivity in a dust sample to compute the concentration of an airborne radiocontaminant. Although the mean concentration of these naturally occurring radioisotopes is relatively low, the actual radioactivity concentration from this source varies according to the concentrations of uranium and thorium in the ground. Areas that have uranium concentrations much higher than the average thus may be expected to have relatively high atmospheric radon concentrations.
2.2 Anthropogenic Sources
X‐rays. Any electronic devices in which electrons fall through potentials of 10 kV or more produce X‐rays, and should be considered a potential source of exposure. Examples include medical diagnosis and therapy, industrial radiography for inspection, and X‐ray fluorescence and X‐ray diffraction analysis in research laboratories.
Accelerators. Intense beams of all radiation types are produced for medical radiation therapy, radioisotope production, neutron production for neutron activation analysis, and research.
Radioisotopes. This category includes sealed sources (for industrial uses such as gauging, inspection, food and drug sterilization, and medical radiation therapy), sealed neutron sources (for inspection, neutron activation analysis, and oil‐well logging), and unsealed radioisotopes (used in medical research, diagnosis, therapy, and generally in scientific and industrial research).
Nuclear reactors. Properly operating nuclear reactors produce negligible environmental radioactivity and exposure to the public. Nuclear reactors produce large quantities of neutrons in the fuel, resulting in the activation of many components in close proximity to the fuel. The fuel will contain both alpha‐ and beta‐emitting nuclides.
Technologically enhanced natural radioactivity. The concentration of radioactivity may be technologically increased to produce the so‐called TENORM (technologically enhanced naturally occurring radioactive material). Since this natural radioactivity is found in water and oil, in addition to rocks, coal, and soil, any scale that builds up in pipelines used for oil and gas production or for carrying water and brine in industrial facilities concentrates the radioactivity, especially radium. This can lead to significantly 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