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Interactions of Charged Particles with Matter
Published in W. P. M. Mayles, A. E. Nahum, J.-C. Rosenwald, Handbook of Radiotherapy Physics, 2021
So far we have only considered the interactions of stable charged particles: negative electrons, positive protons or heavier ions. However, positively charged electrons (or positrons) may also be present in the medium. This can be due to β+ emission (see Section 2.2.4.2) or the photon-interaction process of pair or triplet production (see Section 4.3.3). Positrons are inherently unstable, and recombine with electrons through the process of electron-positron annihilation.
Dictionary
Published in Mario P. Iturralde, Dictionary and Handbook of Nuclear Medicine and Clinical Imaging, 1990
Positronium. A system consisting of a positron and an electron in which the positron is bound to the otherwise free electron. Except for the effect of the lower mass of the positron the system behaves exactly like a hydrogen atom. It has a mean life of about 10−7 s. It is destroyed by electron-positron annihilation.
Estimation of energy absorption buildup factors of some human tissues at energies relevant to brachytherapy and external beam radiotherapy
Published in International Journal of Radiation Biology, 2019
Figure 9(a) shows the variation of EABF with Zeq at 0.0201 MeV (lowest energy of Pd-103), 0.035 MeV (highest energy of I-125), 1 MeV and 5 MeV (6 MV energies) for 1 mfp. It is seen that the EABF is decreasing when Zeq increases at 0.0201 MeV and 0.035 MeV. This is expected as photoelectric absorption becomes higher as the Zeq increases. However, at 1 MeV and 5 MeV the EABF values seem to be almost constant and do not change significantly at 1 mfp. For higher penetration depths, the decreasing trend at lower energies remains the same as in the case for 1 mfp (Figure 9(b)). However, there are significant changes at 1 MeV and 5 MeV for 20 mfp when compared to the ones at 1 mfp. It is seen that there is a decrease in EABF with increasing Zeq values at 1 MeV for 20 mfp since the Compton scattering is still dominating the region. In contrast, there is an increase in EABF when the Zeq increases at 5 MeV (Figure 9(b)). It is interesting to note that at 5 MeV, the dominance of pair production (the threshold energy for pair production is 1.02 MeV) takes over the Compton scattering. After electron-positron pair production originated from the interaction of high energy photons with the nucleus, the electron-positron annihilation occurs giving rise to annihilation photons in the interacting material. This annihilation photons contribute EABF at higher penetration depths, e.g. 20 mfp (Figure 9(b)) and easily escapes the material at low penetration depths, e.g. 1 mfp, thus cannot buildup in the medium (Figure 9(a)). Since the pair production cross section has a Z2 dependence, the tissues with high Zeq, i.e. bone will have higher EABF values at 5 MeV for 20 mfp (Figure 9(b)).