Beta particles occur 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. The kinetic energy of beta particles has a continuous spectrum.
Beta minus decay
If the number of neutrons in a nucleus is in excess, 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. The antineutrino has no rest mass nor electric charge and does not interact readily with matter.
For the isotopes that undergo β- -decay, each nucleus emits an electron and an antineutrino. The mass number remains the same but the atomic number increases 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 protons 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 (β+ or beta plus particle) and a neutrino. Similar to an antineutrino, a neutrino has no electric charge nor rest mass.
In the case of the β+ decay, each decaying nucleus emits a positron and a neutrino, reducing its atomic number by one while the mass number stays the same.
A positron 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 (i.e., in nuclei with too few neutrons). Instead of conversion of a proton into a 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).
History and etymology
Enrico Fermi first theorized beta decay in 1933. In that year, in fact, he wrote his famous work: "Tentativo di una teoria dell'emissione dei raggi beta"; in it he transformed Pauli's qualitative hypothesis into a quantitative theory.
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