Flexi Says: Radioactive decay is the process in which unstable nuclei of radioactive atoms become stable by emitting charged particles and energy. There are three types of radioactive decay: alpha decay, beta decay, and gamma decay. Alpha and beta decay change one element into another. Gamma decay does not. All types of radiation are harmful. Gamma rays are the most dangerous type of radiation. They can travel farther and penetrate materials more deeply than can the charged particles emitted during alpha and beta decay. Gamma rays can be stopped only by several centimeters of lead or several meters of concrete. It’s no surprise that they can penetrate and damage cells deep inside the body.