Although "radiation" can actually mean anything from the heat emitted by a flame to the light from a lightbulb, I think what you're really interested in are the rays and particles associated with radioactivity. Like most scientific discoveries, several people contributed to the eventual discovery and understanding of the phenomenon.

Although X-Rays were discovered by Wilhelm Röntgen in 1895, neither he nor his contemporary scientists associated them with the naturally-occuring emissions from certain minerals, such as uranium. The person who made this connection, and did the first serious study of the phenomenon, was Henri Becquerel.

Becquerel found that radiation was emitted by phosphorescent uranium mineral salts during experiments he conducted in 1896. But it wasn't until two years later when similar emissions were discovered in thorium (by both Gerhard Schmidt and by Marie Curie) that Becquerel made the realization that radioactivity was a general property of certain classes of elements. He was also the first to discover both the medical danger and medical usefulness of radioactivity when he discoverd he had been burned by a piece of radium he had carried in his pocket.

You can read more about these discoveries at:

• Radioactivity: http://www.britannica.com/bcom/eb/article/idxref/3/0,5716,157422,00.ht ml

• Becquerel: http://www.britannica.com/bcom/eb/article/9/0,5716,14239+2+14051,00.ht ml

• Radiation in cancer care: http://www.radiation-o ncology.com/radonc.html

                                                        -Adams Douglas
                                                         Senior Developer
                                                         Infonex, Inc.