Beta decay

In beta decay, a radioactive nucleus emits either an electron or a positron; both are collectively called beta particles, with the positron being the electron's antiparticle - identical mass, but opposite (positive) charge.

Beta-minus ($\beta^-$) decay. Here an electron ($e^-$) is emitted, the nucleus's atomic number increases by one while its mass number stays the same:

At the level of individual nucleons, this happens because one neutron inside nucleus $X$ converts into a proton, emitting an electron and an antineutrino: $n\to p+e^-+\bar{\nu}$. A concrete example is carbon-14 decaying to nitrogen-14: $^{14}_{6}C\to{}^{14}_{7}N+e^-+\bar{\nu}$.

Beta-plus ($\beta^+$) decay. Here a positron ($e^+$) is emitted, and the atomic number decreases by one while the mass number stays the same:

Here a proton inside $X$ converts into a neutron, emitting a positron and a neutrino: $p\to n+e^++\nu$. A concrete example is sodium-22 decaying to neon-22: $^{22}_{11}Na\to{}^{22}_{10}Ne+e^++\nu$. Note that a single free proton, isolated outside any nucleus, cannot undergo $\beta^+$ decay by itself (energy conservation forbids it, since a free neutron is heavier than a free proton); by contrast, an isolated free neutron can and does undergo $\beta^-$ decay on its own, with a half-life of about 13 minutes. It is important to note that the emitted electron or positron was never sitting inside the nucleus beforehand - it is created at the instant of the neutron-to-proton (or proton-to-neutron) conversion. …