Understanding the nature of alpha, beta and gamma rays
This diagram, copied from an original by Marie Curie, shows the effect a magnetic field can have on different types of radiation. Magnetic fields curve the trajectory of particles carrying an electric charge. Alpha rays, curving to the right, are positively charged, the beta rays curving to the left are negatively charged, and the unaffected gamma rays are electrically neutral. Years later, after 1932, beta particles were observed being curved to the right – this would herald the discovery of the positron, and beta-positive radiation.
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In the years following the discovery of radioactivity, physicists devoted themselves to the study of the properties of the different types of radiations that were emitted by radioactive sources.
The three types that were discovered were classified according to their penetrative ability and electrical charge: and named ‘alpha’, ‘beta’ and ‘gamma’.
Ernest Rutherford identified the nature of alpha and beta radiations. He connected first alpha radiations to helium and later on identified them to helium nuclei after his discovery of the atom nucleus. He interpreted also the emission of beta particles as the emission of electrons discovered a few years before. In 1902, he was able to show that a change in the nature of the atom occurred during the emission of alpha and beta rays : atoms were transmuting.
In 1900, Paul Villard at the Ecole Normale found that gamma rays were simply high-energy photons, and of the same type as X-rays.
Despite being a random phenomenon, radioactivity is governed by a fundamental mathematical law of decay. While in Canada between 1901 and 1903, Ernest Rutherford and his young student Frederick Soddy derived the exponential law of a radioactive decay defined by its ‘half-life’. They proved that the ‘half-life’ was a characteristic property of such a decay, with each element having its own intrinsic half-life.
Physicists were also able to observe the existence of radioactive generations; the polonium and radium Pierre and Marie Cure extracted from pitchblende were direct descendants of the uranium it contained.
Radioactive generations provided valuable evidence for the idea that radioactivity accompanies atomic transmutation, one of the fundamental principles of subatomic physics.
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