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

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СКАЧАТЬ out of the atom, and the atom is said to be ionized. Ionization is the process of separating an electron from an atom, thereby upsetting the electrical neutrality of the atom and producing a pair of electrically charged particles – the ejected electron, which is the negative ion, and the residual atom, which is a positive ion. Work must be done by the ionizing particle, the alpha or beta particle, to separate the electron from its atom. An alpha particle loses an average of about 35.5 eV of energy per ion pair produced in air or in soft tissue, while a beta particle loses an average of about 34 eV per ionizing collision. These “collisions” are in reality interactions between the electric fields associated with the ionizing particles and the electrons in the absorbing media. Because an alpha particle has a large mass, moves slowly, and is doubly charged, it has a high rate of ionizing collisions, and consequently a high rate of energy loss. This high rate of energy loss explains the very low penetration of alpha radiation into matter. The linear rate of ion production is called the specific ionization. In air, the specific ionization of alpha particles is of the order of 10⁴–10⁵ ion pairs per centimeter 5.

      Beta particles have only a single charge and travel at speeds near that of light. As a consequence, the specific ionization of beta particles is relatively low. While an alpha particle produces of the order of 50 000 ion pairs per centimeter in air, a beta particle in air produces only about 100 ion pairs per centimeter. This difference in the specific ionization between the two types of radiation is important in health physics for several reasons. First, it accounts for the higher penetrating power of beta particles than that of alpha radiation. For example, a 1.71‐MeV beta penetrates tissue to a depth of about 8 mm, while a 5.3‐MeV alpha, which has three times the energy, penetrates only about 0.005 cm of tissue. Second, it is utilized in the design of radiation‐measuring instruments sensitive to this difference in order to be able to distinguish between the two types of radiation.

      4.3 Bremsstrahlung

      Bremsstrahlung (from the German meaning “braking radiation”) is the production of X‐rays when a charged particle undergoes a sudden change in velocity. When a high‐speed electron collides with an atomic nucleus electric field, there is an abrupt change in the particle's velocity, and a fraction of the particle's kinetic energy is converted into X‐rays. This fraction is extremely small for low‐energy betas and for low atomic numbered absorbers, but it increases with increasing energy and with increasing atomic number. For this reason, beta shields are made of materials of low atomic number. In practice, beta‐shielding material of atomic number higher than 13 (Al) is seldom used.

Bremsstrahlung production is of importance in two cases, the first being when it is deliberately used to generate useful X‐rays. In this application, electrons are emitted from the cathode in a specially designed high vacuum diode, Figure 3, and are accelerated across a high voltage (∼100 kV in medical diagnostic X‐ray units). When electrons strike the high atomic numbered tungsten anode, approximately 1/1000th of their kinetic energy is converted into electromagnetic X‐ray energy. The intensity of the X‐rays increases as the beam of accelerated electrons increase and as the accelerating voltage increases; the penetrating power of the X‐rays depends only on the high voltage, and increases with increasing high voltage. To protect against unwanted bremsstrahlung, X‐ray tubes are enclosed in lead shields that have shuttered apertures through which the useful beam escapes.

      Second, bremsstrahlung X‐rays are an unwanted side effect of shielding betas or in an instrument or other device in which electrons are accelerated across high voltages, such as an electron microscope, a klystron microwave generator, or an electron beam metallurgical furnace. Since these devices are not intended to be used as an X‐ray source, these unwanted X‐rays can pose a serious hazard if the user or the industrial hygienist is unaware of their existence.

FIGURE 3 Stationary target X‐ray tube. X‐rays are formed via bremsstrahlung in the tungsten target. http://www.osha.gov/SLTC/radiationionizing/introtoionizing/ion3.gif.

      4.4 Electromagnetic Radiation

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.

      4.4.1 Interaction with Matter

      Ionizing photons (X‐rays and gamma rays) interact with absorbing media by several different competing mechanisms, depending on the quantum energy and the atomic number of the absorber. Two of these mechanisms, photoelectric absorption СКАЧАТЬ