There are two types of ß decay; ß+ and ß- decay. An excess of neutrons in an atom's nucleus will make it unstable, and a neutron is converted into a proton to change this ratio. During this process, a ß particle is released, and it has the same mass and charge as an electron. The resulting atom and the ß particle have a total mass which is less than the mass of the original atom, and one would think that the ß particles should have the energy equivalent to the mass lost (E = mc2). But ß particles aren't mono-energetic, and have a broad energy spectrum from zero to the maximum energy predicted.
Radioactivity was first discovered in 1896 by the French scientist Henri Becquerel, while working on phosphorescent materials. These materials glow in the dark after exposure to light, and he thought that the glow produced in cathode ray tubes by X-rays might be connected with phosphorescence. He wrapped a photographic plate in black paper and placed various phosphorescent minerals on it. All results were negative until he used uranium salts. The result with these compounds was a deep blackening of the plate. These radiations were called Becquerel Rays.
Radioactivity in small doses is a useful process that can be harnessed by man. For example, nuclear reactors exploit radioactivity to generate heat. Phosphorescent materials sometimes include small quantities of radioactive atoms. During pharmaceutical testing, drugs are sometimes laced with radioactive atoms so that they can be more easily traced as they move throughout the body. In large doses, radioactivity is extremely dangerous. In the Ukraine, a nuclear reactor meltdown incident that occurred during the Cold War era continues to have deleterious effects on the local population to this very day. Many weapons have been designed and tested which use radioactivity to kill people in large numbers.