Читать книгу Essentials of Nuclear Medicine Physics, Instrumentation, and Radiation Biology - Rachel A. Powsner - Страница 56

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.