Many atomic nuclei emit a particle without any external effect and transform themselves into another nucleus.
These nuclei are radioactive, and the process is called radioactive decay.The emitted particles form the radioactive radiation.
The radioactive decay is a random, statistical process. Since the radioactive decay is a random process, it is impossible to predict the time when an individual nucleus of the radioactive material will decay.
However, in a macroscopic material containing a large number of nuclei the average time when the radioactivity of the material decreases to the half of its initial value can be determined. The decay probability of a nucleus is independent of the time since the nucleus already exists.
The transition (decay) process is characterised by the half-life. This is the time during which half of the initially existing nuclei decay. | 
