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 and Compton scattering, involve an interaction between an orbital electron and the photon, which results in the ejection of the electron. For photons whose energy exceeds 1.02 MeV, interaction with the absorber nucleus by the pair production mechanism, which is the direct conversion of energy into mass according to Einstein's theory, results in the transformation of the photon's energy into two particles: a negatively charged electron and a positively charged electron or positron. In each of these types of interactions, high‐speed electrons that carry a part of the photon's energy are produced. These energetic electrons, which are called primary ionizing particles, then transfer their energy by ionizing collisions with the orbital electrons of the absorbing media. The primary ionizing particles are the means by which energy is transferred from photons to the absorbing media.
5 DOSIMETRY
The magnitude of radiation effect is directly related to the concentration of absorbed energy in tissue (or in any other medium that is being irradiated). Other factors that must be considered in assessing the biological effect of radiation include the type of radiation (alpha, beta, or gamma) and the radiosensitivity of the irradiated tissue.
The first dosimetric quantity used in radiation safety, the roentgen (R) as originally defined, is actually a measure of exposure to X‐rays. The amount of energy absorbed from a given roentgen exposure depends on the nature of the absorber. Soft tissue absorbs about 96 ergs g−1 for 1 R of X‐ray exposure. Because of the dependence of absorbed dose on the nature of the absorber, the traditional dosimetric unit, the rad, was introduced. The rad is defined as that dose in which 100 ergs of energy are absorbed per gram of absorber.
Radiation safety technicians often make the assumption that 1 R of X‐rays is approximately equal to 1 rad. Regulators typically allow this assumption as doses will be slightly overestimated.
For purposes of radiation safety control, it is customary to use the subunit millirad (mrad), which represents 0.001 rad. The rad is applied to all types of radiation dosimetry – to external radiation as well as to internally deposited radionuclides and to doses from alpha, beta, and gamma radiations. The traditional radiation unit is being replaced by the SI system unit, called the gray, symbolized by Gy. One gray is defined as that dose in which 1 J of energy is absorbed in 1 kg of the absorbing medium
Since 1 J = 107 ergs and 1 kg = 1000 g,
For radiation safety purposes and for regulatory purposes, a dose unit is used that is normalized for the type of radiation and for the relative radiosensitivity of the irradiated tissue. The relative radiosensitivity is important when dealing with nonuniformly irradiated tissues and organs. The normalizing factor that accounts for the type of radiation is called by the US Nuclear Regulatory Commission (USNRC) the quality factor, Q, and by the International Commission on Radiological Protection (ICRP) the radiation weighting factor, wR. Note that the recommendations of ICRP 26 and ICRP 30 are currently the basis for the USNRC regulations, and other weighting factors are included in this chapter for reference. Relative radiosensitivity of irradiated tissue is considered by the tissue weighting factor, wT, which ranges from 1 for uniform whole body radiation to 0.03 for the thyroid. The traditional dose equivalent unit is called the rem, H. For whole body irradiation, the rem is defined as the product of the quality factor and the dose:
In the SI system, the unit for the dose equivalent is called the sievert, Sv, and is defined by
Values used for the normalizing factor for several radiations are listed in Table 1.
In the belief that the probability of a stochastic effect should be the same whether the whole body is uniformly irradiated or whether the radiation is nonuniformly distributed, ICRP 30 introduced the concept