What happens in beta decay?
Beta-particle emission leads to an increase in the number of protons in the nucleus and a simultaneous decrease in the number of neutrons. It is essentially a high-energy electron emission.
Below the stability belt the nuclei have lower neutron-to-proton ratios than those in the belt (for the same number of protons). To increase this ratio (and hence move up toward the belt of stability), these nuclei either emit a positron or undergo electron capture.
This process has the same net effect as electron capture.
There are three main nuclear fission (radioactivity) particles – alpha, beta, and gamma. Alpha particles are really just helium atoms ejected at high velocity. Beta particles are electrons or positrons, and a gamma particle is a high-energy photon.