ABSTRACT
A positron is the antiparticle of an electron. It has the same mass but an opposite charge to that of the electron. Dirac introduced an idea of the positive electron in 1930 (Dirac, 1930: 361), and, soon after, Anderson found it, that was the positron, in 1932 during observation of radiations from the universe, as shown in Figure 7.1 (Anderson, 1933: 491). With this discovery came into existence the study of positrons. Many techniques, such as electron paramagnetic resonance (EPR)
7.1 Introduction .......................................................................................................................... 137 7.2Positron Annihilation in Insulators ...................................................................................... 139 7.3Positronium ........................................................................................................................... 139
7.3.1Ps in Liquids ............................................................................................................. 139 7.3.2Ps in Solids ............................................................................................................... 140
7.4 Experimental Techniques ..................................................................................................... 143 7.4.1Positron Source ......................................................................................................... 144 7.4.2Positron Annihilation Lifetime Measurement .......................................................... 144 7.4.3Doppler Broadening Measurement ........................................................................... 146 7.4.4Age-Momentum Correlation Measurement ............................................................. 146 7.4.5 Other Techniques ...................................................................................................... 147
7.5Positronium Formation ......................................................................................................... 148 7.6Radiation Chemistry Studies by Positron Annihilation ....................................................... 149
7.6.1Comparison of the Yields of Hydrated Electrons Observed by Pulse Radiolysis .... 149 7.6.2Comparison of Electron Mobility Experiments ....................................................... 149 7.6.3 Electron Thermalization in Water ............................................................................ 150 7.6.4Electric Field Effect (Blob Model of Ps Formation) ................................................. 152 7.6.5Slow Ps Formation in Spurs (Young-Age Broadening Effects) ................................ 153 7.6.6Ps Formation with Trapped Electrons ...................................................................... 154 7.6.7Reactions of o-Ps with Spur Species ........................................................................ 158 7.6.8 Positronium as a Probe of Free Radicals .................................................................. 160
7.7Positron Annihilation on Molecules ..................................................................................... 163 References ...................................................................................................................................... 165
and electron diffraction, and instruments, such as the electron microscope, are found based on the electron. Similarly, the positron microscope (Van House and Rich, 1988: 169; Oshima et al., 2009: 194104) and positron diffraction (Kawasuso and Okada, 1998: 2695; Fukaya et al., 2009: 193310) are found based on the positron. However, the biggest difference between the two is that free positrons do not exist in our world. Therefore, the number of positrons that can be obtained is quite limited. Recently, it has become possible to store a large amount of positrons and inject them into a sample. The formation of positronium (Ps) molecules is now possible (Cassidy et al., 2007: 062511). Ps is abound state of apositron and an electron. It may be considered that Ps is an isotope of the hydrogen atom.
