Particle emission from natural radioactive decay was the first observation of radioactivity. Wilhelm Röntgen had produced X rays in 1896, and a year later Henri Becquerel showed that naturally occurring uranium produced radiation spontaneously. While the radiation was thought initially to be similar to Röntgen's x rays, Rutherford showed that some types of radiation were more penetrating than others. He called the less penetrating radiation alpha (a) rays and the more penetrating ones beta (ß) rays. Soon after, it was shown that these radiations could be deflected by a magnetic field, i.e., they carried charge. It was clear that these were not electromagnetic rays and were, in fact, particles.
Gamma rays are electromagnetic radiation emitted from the nucleus after a spontaneous nuclear decay. This is usually associated with the emission of an alpha or beta particle although there are alternative decay schemes. X and y rays are indistinguishable after they are emitted from the atom and only differ in their site of origin. After the emission of a particle in a radioactive decay the nucleus can be left in an excited state and this excess energy is given off as a y ray, thus conserving energy.
Gamma ray emission is characteristic, and it is determined by the difference in energy levels between the initial and final state of the energy level transitions within the nucleus.
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