During в+-decay, positrons are emitted. The positron is the antiparticle of the elec­tron, and therefore it is unstable. Its half time is the time of thermalization, which means that the time required for the velocity of the positron decreases to zero. It is about 10-10 s. If the positron encounters an electron in this interval, the two parti­cles (electron and positron) transform to electromagnetic radiation, gamma photons. The process is called “annihilation.” The rest mass of the positron (в — particle) is 0.51 MeV, equal to the rest mass of the electron, so 2 X 0.51 MeV energy is emitted in the annihilation process. Usually, two gamma photons with 0.51 MeV energies are emitted at an angle of 180°. The probability of the formation of two photons is about 90%. (This process is applied in the PET (Section 12.6)). In about 10% of the annihilation process, only one photon with 1.02 MeV is formed.

Interaction with the molecules of matter
Cherenkov radiation

Figure 5.13 Summary of interaction of beta particles with matter.

In some cases, three photons are emitted, and the total energy of them is also 1.02 MeV. The positive beta decay can be detected easily through the detection of the gamma photons with 0.51 MeV.

It is interesting to mention here that before the total thermalization, the positron can interact with an electron, constructing a short-life light element, positronium, whose nucleus is the positron. Positronium can be treated as an atom with an atomic number of zero.

Positronium has two forms: ortho — and para-positronium, depending on the spins of the positron and electron. In ortho-positronium, the spins are parallel; the lifetime in a vacuum is 1.4 X 10_7 s. In para-positronium, the spins are antiparallel; the life­time in a vacuum is 1.25 X 10_10 s. In other media, the chemical reactions (addi­tion, substitution, oxidation, and reduction) decrease the lifetime; thus, the kinetics of chemical reactions can be studied by measuring the lifetime of positronium.