Beta particles occurs with either negative or positive charge (β- or β+) and are known to be either electrons or positrons, respectively, therefore beta decay represents radioactive decay, in which a beta particle is emitted. Kinetic energy of beta particles has a continuous spectrum.
Beta minus decay
If the number of neutrons in a nucleus is higher than the number of neutrons in the stable nucleus, a neutron will undergo the following transformation: n --> p + β- + νe*, i.e., a neutron will be converted into a proton with the emission of a beta minus particle (electron) and an antineutrino.
For the isotopes that β- -decay, each nucleus emits an electron and an antineutrino while increasing its atomic number by one.
There are numerous examples of beta minus emitters in nature like 14C, 40K, 3H, 60Co etc. The example of importance in radiology is the decay of cobalt-60: 60Co --> 60Ni + β- + ν*.
Beta plus decay
If the number of neutrons in a nucleus is smaller than the number of neutrons in the stable nucleus, a proton will undergo the following transformation: p --> n + β+ + νe, i.e., a proton will be converted into a neutron with the emission of a positron (beta plus particle) and an electron neutrino.
In the case of the β+-decay, each decaying nucleus emits a positron and a neutrino, simultaneously reducing its atomic number by one unit.
A positron (β+) does not exist for a long period of time in the presence of matter. It then combines with an electron, with which it undergoes annihilation. The masses of both particles are then replaced by electromagnetic energy that is emitted from the annihilation in the form of two 511-keV gamma rays that are emitted in opposite directions.
There are no positron emitters in nature. They are produced in nuclear reactions. The most important positron emitters in medicine are: 11C, 15O, 18F, 30P etc.
Electron capture is concurrent to beta plus decay. Instead of conversion of a proton into neutron with a beta particle being emitted together with a neutrino, the proton captures an electron from the K shell: p + e --> n + ν.
The energy of the emitted beta particles is around 3 MeV, while their speed approximately corresponds to the speed of light.
Beta particles can penetrate matter. They lose energy in collisions with the atoms. There are actually two processes involved:
- a beta particle transfers a small fraction of its energy to the struck atom
- a beta particle is deflected from its original path by each collision and, since the change in the velocity leads to the emission of electromagnetic radiation, some of the energy is lost in the form of low-energy x-rays (Bremsstrahlung).
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