Experiments performed subsequently showed that radioactivity was
a nuclear phenomenon in which an unstable nucleus undergoes a decay This is referred to as radioactive decay Three types of radioactive decay
occur in nature :
(i) a-decay in which a helium nucleus 
4
2He  is emitted;
(ii) b-decay in which electrons or positrons (particles with the same mass
as electrons, but with a charge exactly opposite to that of electron)
are emitted;
(iii) g-decay in which high energy (hundreds of keV or more) photons are
emitted Each of these decay will be considered in subsequent sub-sections