Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves.

The fundamental law of radioactive decay is based on the fact that the decay i.e the transition of the parent nucleus to a daughter nucleus is purely statistical process. The disintegration (decay) probability is a fundamental property of an atomic nucleus and remains equal in time.

The SI unit of radioactivity is the Becquerel (Bq), which is defined as a decay rate of one disintegration per second (dps). Another unit Curie (Ci) can be defined as a decay rate of 3.7 x 10¹⁰dps.

Half life can be defined as the period of time in which half of the radioactivity has disappeared (half of the nuclei have disintegrated).

The atoms that are involved in radioactive decay are called Isotopes.

The Day 0 Activity is for inputting the radioactivity on the reference date.

There are three common types of radioactive decay: Alpha, Beta, and Gamma.

The reason alpha decay occurs is because the nucleus has too many protons which cause excessive repulsion.

Beta decay occurs when the neutron to proton ratio is too great in the nucleus and causes instability. In Beta decay, a neutron is turned into a proton and an electron. The electron is then emitted. There is also positron emission when the neutron to proton ratio is too small. The final type of Beta decay is known as Electron Capture and also occurs when the neutron to proton ratio in the nucleus is too small.

Gamma decay occurs because the nucleus is at too high energy. The nucleus falls down to a lower energy state and in the process, emits a high energy photon known as a Gamma particle.