The British scientist Ernest Rutherford was one of the first to show that radioactivity could consist of three distinct types of radiation. The three types differ in their response to an electric field. A source of radioactivity is passed through a hole to form a beam. The beam is directed at a fluorescent screen which is illuminated when it is struck. In the absence of an electric field, all the radioactivity beams strike the target at a single point. When an electric field is applied, the initial beam is separated into three components. One of these is deflected upward by the electric field, indicating that is negatively charged. These radioactive emissions are called beta rays. A part of the beam is deflected downward indicating that these particles are positively charged. These radioactive emissions are called alpha rays. Notice that the alpha particles are deflected less than the beta particles by the same electric field. This occurs because the alpha particles are more massive, but the amount of deflection is also determined by the relative energies of the different types of radiation. The portion of the original beam that is un-deflected is due to radiation that is not charged. These emissions are referred to as gamma rays. Other experiments showed later that the beta rays are actually high speed electrons. The alpha particles proved to be helium nuclei. The gamma rays are high energy photons, analogous to x-rays.
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