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A proton can be converted into a neutron and a positron, which is an electron with a positive, instead of negative, charge (Figure 1.16). The positron is also referred to as a positive beta particle or positive electron or anti‐electron. In positron decay, a neutrino is also emitted. In many ways, positron decay is the mirror image of beta decay: positive electron instead of negative electron, neutrino instead of antineutrino. Unlike the negative electron, the positron itself survives only briefly. It quickly encounters an electron (electrons are plentiful in matter), and both are annihilated (see Chapter 8, Figure 8.1). This is why it is considered an anti‐electron. Generally speaking, antiparticles react with the corresponding particle to annihilate both. During the annihilation reaction, the combined mass of the positron and electron is converted into two photons of energy equivalent to the mass destroyed, each with an energy of 511 keV or a total of 1.022 MeV. Following ejection of a positron from a nucleus the atom must also shed an orbital electron to keep the overall charge of the atom neutral. So, in essence, the atom is losing the mass equivalent of two electrons (remember positrons are basically positively charged electrons). Positron emission will only occur when the difference in mass between the parent (original) and daughter atoms is at minimum the mass of two electrons, which, as we will see in Chapter 2, Figure 2.12 is equal to 1.02 MeV of energy.

Figure 1.16 β ⁺ (positron) decay.