Essentials of Nuclear Medicine Physics, Instrumentation, and Radiation Biology. Rachel A. Powsner

oppositely directed photons, each of 511 keV. This is referred to as an annihilation reaction (Figure 2.11). It is another example of the interchangeability of mass and energy described in Einstein’s equation: energy equals mass times the speed of light squared, or E = mc² (Figure 2.12).

      Bremsstrahlung

Small charged particles such as beta particles (electrons and positrons) may be deflected by nuclei as they pass through matter, which may be attributed to the positive charge of the atomic nuclei. As the electron or positron is deflected it also loses speed (decelerates) and this interaction generates X‐radiation known as bremsstrahlung (Figure 2.13), which in German means “braking radiation.” The energy of the X‐ray is equal to the loss of the kinetic energy of the incident electron or proton as it decelerates (see section on kinetic energy). The greater the speed of the incident particle, the greater the charge of the nucleus, and the closer the incident particle is to the nucleus the greater the energy of the generated X‐ray (Figure 2.14). Since the proximity of the incident particle to the nucleus is random, the X‐rays produced by a stream of particles will have a range of energies from near zero up to the kinetic energy of the incident particle (the maximum value is generated when the incident particle is completely stopped by the atomic nucleus). Figure 2.15 illustrates the bremsstrahlung X‐ray spectrum for ⁹⁰Yttrium which has a maximum X‐ray energy equal to the maximum beta particle energy of 2280 keV.

Figure 2.10 Particle range in an absorber.

Figure 2.11 Annihilation reaction.

Figure 2.12 Einstein’s theory of the equivalence of energy and mass.

Figure 2.13 Bremsstrahlung. Beta particles (β^–) and positrons (β⁺) that travel near the nucleus will be attracted or repelled by the positive charge of the nucleus generating X‐rays in the process.

Reference

      1 1. Shapiro, J. Radiation Protection. A Guide for Scientists, Regulators, and Physicians, 4th edn. Cambridge MA: Harvard University Press, 2002, pp. 42 and 53.

Figure 2.14 Bremsstrahlung X‐ray energies increase with increasing proximity of the beta particle to the atomic nucleus (illustrated), greater incident beta particle energy, and greater nucleus charge.

Figure 2.15 Bremsstrahlung X‐ray energies vary from near zero to the maximum incident beta particle energy.

Questions

      1 Which of the following is true of the interaction of charged particles with matter?Alpha particles have a higher LET than beta particles.The range of alpha particles is generally greater than beta particles.Alpha particles have a higher specific ionization than beta particles.

      2 True or false: Bremsstrahlung is X‐ray radiation emitted as a moving electron or positron decelerates and changes direction in close proximity to a nucleus.

      3 True or false: The photoelectric effect is the dominant type of photon interaction in tissue for radionuclides used in the practice of nuclear medicine.

      4 For each of the terms listed here, select the appropriate definition:HVL (half value layer).TVL (tenth value layer).μ (mu) (linear attenuation coefficient).Thickness of an attenuator that will reduce the intensity (number of photons) in a monoenergetic beam by 90%.Thickness of an attenuator that will reduce the intensity (number of photons) in a monoenergetic beam by 50%.0.693/HVL.

      5 Which of the following occur during photon interactions with matter (more than one answer may apply)?Excitation.Pair production.Ionization.Compton scattering.Bremsstrahlung.Photoelectric effect.Annihilation reaction.

      6 Which of the following occur during charged particle interactions with atoms (more than one answer may apply)?Excitation.Pair production.Ionization.Compton scattering.Bremsstrahlung.Photoelectric effect.Annihilation reaction.

      7 Which of the following are true about annihilation reaction?The conversion of the mass of the positron and electron into energy is an example of the interchangeability of mass and energy as described by Einstein’s famous equation E = mc2.Two oppositely opposed 450 keV photons are emitted as a result of the annihilation reaction.Both (a) and (b).

      8 Which of the following are referred to as nonpenetrating radiation?Positrons.Gamma photons.X‐rays.Alpha particles.Beta particles.

      9 Which term refers to the loss of photons from a beam of radiation as it passes through matter?Attenuation.Absorption.

      10 Which term is used to describe the transfer of energy from radiation to surrounding matter?Attenuation.Absorption.

      11 You shield a sample of 99mTc using a 1 mm‐thick sheet of lead. What fraction of the photons will be transmitted through the lead? The linear attenuation coefficient, μ, of lead for 140 keV photons is 23.1 cm–1.

Answers

      1 (a) and (c) are true, (b) is false; alpha particles have a shorter range than beta particles.

      2 True.

      3 False: Compton scattering is the dominant interaction.

      4 (a) 2, (b) 1, (c) 3.

      5 (b), (c), (d), (f).

      6 (a), (c), (e), (g).

      7 (a).

      8 (a), (d), (e).

      9 (a).

      10 (b).

      11 0.1 (transmitted fraction = Iout / Iin = e–μx = e–2.31 = 0.1).

CHAPTER 3 Formation of Radionuclides