 At the turn of the 20th century, radioactivity was discovered by the French scientist, Henry Baccaral. Using uranium salts, he was able to blacken a photographic plate. Here's a photograph of the plate. For the research by Baccaral, Ernest Rutherford, Madame Curie, and others, discovered three types of radiation. Here's how they did it. A radiation source shines on a lead plate with a small hole in it to create a beam. The beam is directed at a fluorescent screen. The screen flashes when it is struck. Without any electric field present, the beam illuminates a single point on the screen. But when an electric field is applied, the beam is separated into three components. One is deflected upward by the electric field, indicating that it consists of negatively charged particles. These were named beta rays. One is deflected downward, but not as far as the beta rays were deflected upward, indicating that it consists of positively charged particles that are more massive than the beta rays. These were named alpha rays. The radiation that continued to hit the center was not affected by the electric field and therefore has no charge. These emissions were named gamma rays. Radiation research has been going on in nuclear labs around the world ever since those days. What we know now is that alpha radiation comes from an unstable nucleus that disintegrates into a lighter nucleus and ejects an alpha particle, which is always two protons and two neutrons. That's a helium nucleus. This is called alpha decay and it decreases the radiating atoms atomic mass number by four and it decreases the atoms atomic number by two, changing its nature from one element to another. We see that alpha decay changes uranium-235 into thorium-231 and uranium-238 into thorium-234. Beta radiation comes from an unstable nucleus that ejects a neutrino and an electron, turning one neutron into a proton and upping the atomic number by one while leaving the atomic mass number unchanged. This is called beta decay. Here we see that beta decay changes thorium-231 into protactinium-231 and thorium-234 into protactinium-234. Gamma rays are extremely energetic photons and constitute the most dangerous form of radiation. They are produced when an excited nucleus returns to its ground state, much like when an excited electron around a nucleus produces a photon when it drops to its ground state. Only gamma ray photons have a million times more energy.