Radioactivity is not something that is perceived immediately and directly it can be measured with appropriate special devices and units. Some atoms (radioactive atoms and radioelements) are unstable due to their structure. Radioactive atoms have the ability to undergo spontaneous radioactive fission, the nuclei are divided into small particles, this is radioactive decay. This process is accompanied by the release of alpha and beta particles, as well as gamma rays. Each type of radiation has its own specific level of energy, which we cannot feel.
Such radiation can penetrate living tissue and, like other substances, release part of its energy. Each radioactive element has its own half-life. This is the period during which half of the active (unstable) nuclei of atoms of a given radioactive substance decay. It can be very short (a few hours), but more often it extends from several days (Iodine 131) to several thousand years (Plutonium 239). Due to radioactive decay, the intensity of radiation from radioactive elements decreases over time.