Читать книгу Patty's Industrial Hygiene, Physical and Biological Agents - Группа авторов - Страница 29

X‐rays and gamma rays are electromagnetic radiations that occupy the high‐energy end of the continuous electromagnetic spectrum that includes radio waves, microwaves, infrared rays, visible light, and ultraviolet radiation. X‐rays and gamma rays are qualitatively the same; they differ only in their manner of origin. Accordingly, the two terms may be used interchangeably in the context of radiation safety. Gamma rays are very penetrating; they pass through matter fairly easily, and can travel long distances in air. All electromagnetic radiations travel through space at the same speed, 3 × 10⁸ m s⁻¹. Gamma rays and X‐rays are sufficiently energetic to generate ions by knocking electrons out of atoms, while the other portions of the electromagnetic spectrum are not energetic enough to generate ions. Accordingly, X‐rays and gamma rays are called ionizing radiation, while the other parts of the electromagnetic spectrum are called nonionizing radiation.

According to Maxwell's theory of electromagnetism, a changing electric field is always associated with a changing magnetic field, and a changing magnetic field is always associated with a changing electric field. The intensity of these associated changing electric and magnetic fields as a function of time can be represented by a sine wave, thus leading to the wave model for electromagnetic radiation, Figure 4.

In a vacuum, the wave length, λ, and the frequency, f, of these waves are related to their speed, c, by

(6)

Wavelength is frequently expressed in angstrom units (1 Å unit = 10⁻¹⁰ m). According to the wave model, the energy carried by the waves is proportional to the square of the amplitude of the electric and magnetic field strengths.

The wave model of electromagnetic radiation is useful for explaining many, but not all physical phenomena, and is the basis for understanding the effects of nonionizing electromagnetic energy. Phenomena that are not amenable to explanation by the wave theory are explainable by the quantum theory. According to the quantum theory, electromagnetic radiation behaves as if it consists of particles of energy, called photons, which travel through space at the speed of light (3 × 10⁸ m s⁻¹). Each particle contains a discrete quantity, or “quantum” of electromagnetic energy. Ionizing radiation includes photons whose energy exceeds 12 eV. The photon's energy content is proportional to the frequency of the radiation when the radiation is considered as a wave, and is given by

(7)

where h is Planck's constant and has a value of 6.626 × 10⁻³⁴ J s, and the frequency, f, is in hertz.

FIGURE 4 Schematic representation of an electromagnetic wave.