Positron decay is a type of beta decay , also sometimes called beta-plus-decay (β + decay), positron emission , or positron emission . In β + decay, one of the protons of the nucleus is converted by weak interaction into a neutron , positron, and electron neutrino . Many isotopes emit positrons, including carbon-11 , nitrogen-13 , oxygen-15 , fluorine-18 , iodine-121 . For example, the following equation considers the conversion through β + decay of carbon-11 to boron-11 with the emission of a positron e + and an electron neutrino ν e :
The process of positron decay always competes with electron capture , which has an energy priority, but as soon as the energy difference disappears, the branching coefficient of the reaction shifts toward positron decay. In order for positron decay to occur, the difference between the masses of the decaying and daughter atoms Q β must exceed the doubled mass of the electron (i.e. Q β > 2 m e = 2 × 511 keV = 1022 keV). At the same time, electron capture can occur with any positive mass difference.
The spectrum of kinetic energy of positrons emitted by the nucleus in positron decay is continuous and lies in the range from 0 to E max = Q β - 2 m e . The energy of emitted neutrinos lies in the same range. The sum of the kinetic energies of the positron and neutrino is E max . A positron annihilates almost instantly with one of the electrons of a substance surrounding a decaying atom, emitting two annihilation gamma rays with an energy of 511 keV and an oppositely directed pulse. Detection of such gamma rays makes it easy to reconstruct the annihilation point; therefore, isotopes experiencing positron decay are used in positron emission tomography .
See also
- Proton decay
- Double positron decay