DISCOVERY - A. H. Becquerel (1896)
During the course of his experiments in 1896, on the fluorescence of
uranium salts, Antoine Henri Becquerel left a wrapped photographic
plate in a drawer. On developing the plate Becquerel found that it had
been exposed. He concluded that as no light could have possibly
penetrated the wrapping, the plate must have been fogged by rays from a
sample of uranium salt which was stored in the same drawer.
For his discovery of spontaneous radioactivity Becquerel
was awarded half of the Nobel Prize for Physics in 1903, sharing the
other half with Pierre and Marie Curie for their further study and
experimentation of the Becquerel radiation.
Picture Source - http://www.umich.edu/~radinfo/images/beq.jpg