Atoms have a center called a nucleus made of packed protons and neutrons. Sometimes a nucleus has a bad mix of these parts and becomes completely unstable. To fix this severe problem, the nucleus goes through a process called radioactive decay.
During a common process called beta-minus decay, a neutron suddenly turns into a proton. When this change happens, the nucleus spits out a fast electron to balance the charge. This tiny ejected electron is exactly what we call a normal beta particle.
During a different process called beta-plus decay, a proton turns into a neutron instead. This unique change spits out a positron, which is an electron with a positive charge. Beta particles carry a huge amount of energy and speed as they fly away.
They can pass easily through paper but get stopped by a thin sheet of metal. If these fast particles hit living tissue, they can damage cells and cause health problems. Scientists use our knowledge of these particles to run medical scans and date ancient bones.