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Essentials of Nuclear Medicine Physics, Instrumentation, and Radiation Biology. Rachel A. Powsner
Читать онлайн.Название Essentials of Nuclear Medicine Physics, Instrumentation, and Radiation Biology
Год выпуска 0
isbn 9781119621010
Автор произведения Rachel A. Powsner
Жанр Медицина
Издательство John Wiley & Sons Limited
7 Rank the following binding energies from greatest to least:Electron binding energy for outer shell electrons.Nuclear binding energy.Electron binding energy for inner shell electrons.
8 True or false: The term metastable refers to an intermediate state of nuclear decay lasting longer than 10–12 seconds prior to undergoing isomeric transition.
9 Which of the following is true regarding beta decay of a specific radioisotope:The energy of the emitted beta particle is always the same.The energy of the emitted antineutrino is always the same.The summed energy of the emitted beta particle and antineutrino is always the same.
10 Which unit of measurement for radioactivity is defined as one radioactive decay per second?Bequerel.Millicurie.Megabequerel.
11 10 mCi equals how many MBq?2.7 MBq.37 MBq.270 MBq.370 MBq.
12 Lighter nuclides (Z < 83) with an excess of neutrons tend to decay by:Gamma emission.Beta minus decay.Isomeric transition.Positron emission.Alpha emission.
13 Which of the following statements are true?An alpha particle is the same thing as a helium nucleus.Neutrinos have the same charge as an electron.X‐rays always have lower energies than gamma rays.The terms “activity” and “count rate” are the same—they express a measurement of photons per second.
14 When orbital electrons move from an outer shell to an inner‐shell, which of the following is not trueCharacteristic X‐rays can be emitted.Auger electrons can be emitted.The atom becomes more stable.A mixture of gamma rays and internal conversion electrons can be emitted.
Answers
1 (c)
2 (1) d. (2) None of the above; usually used as a geological term. (3) None of the above; in nuclear medicine it refers to an element whose nucleus is in an unstable (excited) state. (4) (c). (5) (a).
3 (b) and (c) are true. (a) is false; technetium does not have a stable form; 99Tc has a T1/2 of 2.1 × 105 year.
4 (e).
5 (d).
6 6.3 mCi.
7 Nuclear binding energy, electron binding energy for inner shell electrons, electron binding energy for outer shell electrons.
8 True.
9 (3).
10 Bequerel.
11 370 MBq.
12 (2) Beta minus decay.
13 (1) only.
14 (d).
CHAPTER 2 Interaction of Radiation with Matter
When radiation strikes matter, both the nature of the radiation and the composition of the matter affect what happens. The process begins with the transfer of radiation energy to the atoms and molecules, heating the matter or even modifying its structure.
If all the energy of a bombarding particle or photon is transferred, the radiation will appear to have been stopped within the irradiated matter. Conversely, if the energy is not completely deposited in the matter, the remaining energy will emerge as though the matter were transparent or at least translucent. This said, we will now introduce some of the physical phenomena involved as radiation interacts with matter, and in particular we shall consider, separately at first, the interactions in matter of both photons (gamma rays and X‐rays) and charged particles (alpha and beta particles).
Interaction of photons with matter
As they pass through matter, photons interact with atoms. The type of interaction is a function of the energy of the photons and the atomic number (Z) of elements composing the matter.
Types of photon interactions in matter
In the practice of nuclear medicine, where gamma rays with energies between 50 keV and 550 keV are used, Compton scattering is the dominant type of interaction in materials with lower atomic numbers, such as human tissue (Z = 7.5). The photoelectric effect is the dominant type of interaction in materials with higher atomic numbers, such as lead (Z = 82). A third type of interaction of photons with matter, pair production, only occurs with very high photon energies (greater than 1020 keV) and is therefore not important in clinical nuclear medicine. Figure 2.1 depicts the predominant type of interaction for various combinations of incident photons and absorber atomic numbers.
Compton scattering
In Compton scattering the incident photon transfers part of its energy to an outer shell or (essentially) “free” electron, ejecting it from the atom. Upon ejection this electron is called the Compton electron. The photon, which has lost energy in the interaction, is scattered (Figure 2.2) at an angle that depends on the amount of energy transferred from the photon to the electron. The scattering angle can range from nearly 0° to 180°. Figure 2.3 illustrates scattering angles of 135° and 45°.
Photoelectric effect
An incident photon may also transfer its energy to an orbital (generally inner‐shell) electron. This process is called the photoelectric effect and the ejected electron is called a photoelectron (Figure 2.4). This electron leaves the atom with an energy equal to the energy of the incident gamma ray diminished by the binding energy of the electron. An outer‐shell electron then fills the inner‐shell vacancy and the excess energy is emitted as an X‐ray.
Table 2.1 lists the predominant photon interactions in some common materials.
Figure 2.1 Predominant type of interaction for various combinations of incident photons and absorber atomic numbers.
Figure 2.2 Compton scattering.
Attenuation of photons in matter
As the result of the interactions between photons and matter, the intensity of the beam (stream of photons), that is, the number of photons remaining in the beam, decreases as the beam passes through matter (Figure 2.5). This loss of photons is called attenuation; the matter through which the beam passes is referred to as the attenuator.