Lesson_5.2 - Lesson 5.2 Natural Radioactivity The Discovery...

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Lesson 5.2 Natural Radioactivity The Discovery of Radioactivity Radioactivity was discovered by Henry Becquerel in 1896 as a result of an accidental event. Becquerel was investigating the properties of fluorescent salts. He placed some fluorescent material, namely uranium salt on a photographic plate wrapped very thickly into black paper and exposed it to strong sunshine. The film showed the expected blackening, and Becquerel explained it as the result of fluorescent radiations producing X-rays. Becquerel published his results and his - wrong - conclusions spread widely. In the spring of 1896 he continued the experiments. One day it was cloudy and so Becquerel placed the above described arrangement into a dark cabinet. A few days later he was curious and developed the film. He was astonished to see that it was totally black. In this way it turned out that the relationship between fluorescent light and X-rays does not hold. He performed further experiments systematically and determined that these radiations that blackened the photographic plate came from uranium, and that these radiations ionize air, just the same way as X-rays do. This phenomenon is characteristic to some elements. Becquerel asked his assistant Marie Curie to go on with the experiments. Mr and Mrs Curie worked steadily and in 1898 discovered other radioactive elements such as polonium and radium. Although they studied these radioactive materials widely, they had no idea where the energy of these radiations came from. Experiments conducted by Rutherford in 1898 showed that the radiation emitted by uranium is not homogeneous, but made up of different types of radiations. One type of radiation was found to be highly ionizing and has very low penetration ability. It could be stopped by a thin sheet of paper. A second type of radiation was found to have fairly high penetration ability, but produced only few ions along its path. He named the radiations alpha and beta. Later Becquerel proved that beta radiation is corpuscular and the particles are electrons. On the other hand, for the identification of alpha radiation much longer time had to elapse: Rutherford showed in 1909 that alpha particles are identical to helium nuclei consisting of 2 protons and 2 neutrons. In 1900 Villard discovered a third type of radiation of great penetrating ability and it was named the gamma radiation. Later experiments showed that gamma radiations are very high energy electromagnetic radiations, similar to X-rays but of much higher frequency and energy. You know by now that nucleons are held together by strong nuclear forces. The tug-of-war between the attractive force of the strong nuclear force and the repulsive electrostatic force between protons has interesting implications for the stability of a nucleus. The stability of a nucleus is largely determined by the ratio of protons to neutrons in the nucleus. Atoms with very low atomic numbers have about the same number of neutrons and protons. As Z gets larger,
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This note was uploaded on 05/01/2011 for the course PHY 2049 taught by Professor George during the Spring '11 term at Edison State College.

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Lesson_5.2 - Lesson 5.2 Natural Radioactivity The Discovery...

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