The Discovery of Radioactivity

In 1896 Henri Becquerel was  using  naturally  fluorescent minerals to  study  the properties  of  x-rays, which  had  been  discovered  in  1895  by  Wilhelm  Roentgen.  He exposed potassium uranyl sulfate to sunlight and then placed it  on  photographic  plates wrapped in black paper, believing that the uranium absorbed the sun’s energy and then emitted it as x-rays. This hypothesis was disproved on the 26t h-27t h of February, when his experiment “failed” because it was overcast in Paris. For  some reason, Becquerel decided to develop his photographic plates anyway. To  his  surprise, the images were strong  and

clear, proving that the uranium emitted radiation without an external source of energy such as the sun. Becquerel had discovered radioactivity.

Becquerel used an apparatus similar to  show  that  the radiation he discovered could  not  be  x-rays.  X-rays are  neutral and  cannot be  bent in  a magnetic field. The new radiation was bent by the magnetic field so that the radiation must be charged and different than x-rays. When different radioactive substances were put in the magnetic field, they deflected in different directions or not at all, showing that there were three classes of radioactivity: negative, positive, and electrically neutral.

The term radioactivity was actually coined by Marie Curie, who together with her husband Pierre, began investigating the phenomenon recently discovered by Becquerel. The Curies extracted uranium from ore and to their surprise, found that the leftover ore showed more activity than the pure uranium. They concluded that the ore contained other radioactive elements. This led to the  discoveries of  the elements polonium and  radium. It took  four more years of processing tons of ore to isolate enough of each element to determine their
chemical properties.



Ernest Rutherford, who did many experiments studying the properties of radioactive decay, named these alpha, beta, and gamma particles, and classified them by their ability to penetrate matter. Rutherford used an apparatus similar to that depicted in Fig.  3-7. When the air from the chamber was removed, the alpha source made a spot on the photographic plate. When air was added, the spot disappeared. Thus, only a few centimeters of air were
enough to stop the alpha radiation.

Because alpha particles carry more electric charge,  are  more  massive, and  move slowly compared to beta and gamma particles, they interact much more easily with matter. Beta particles are much less massive and move  faster, but  are  still electrically charged. A sheet of aluminum one-millimeter thick or several meters of air will stop these electrons and positrons. Because gamma rays carry no electric charge, they can penetrate large distances through materials before interacting—several centimeters of lead or a meter of concrete is needed to stop most gamma rays.