When, after several hours of
exposure, the photographic plate was developed, it showed blackening
due to something that must have been emitted by the compound and
was able to penetrate both black paper and the silver 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