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Early History of Radioactivity

The more or less accidental series of events that led to the discovery of radioactivity depended on two especially significant factors :  (1) the mysterious X rays discovered about one year earlier by W.C.Roentgen produced fluorescence (the term phosphorescence was preferred at that time) in the glass walls of X-ray tubes and in some other materials; and (2) Henri Becquerel had inherited an interest in phosphorescence from both his father and grandfather. The father, Edmund Becquerel (1820-1891), had actually studied phosphorescence of uranium salts, and about 1880 Henri Becquerel prepared potassium uranyl sulfate and noted its pronounced phosphorescence excited by ultraviolet light. Thus in 1895 and 1896, when several scientist were seeking the connection between X rays and phosphorescence and were looking for penetrating radiation from phosphorescent substances, it was natural for Becquerel to experiment along this line with the potassium uranyl sulfate.

It was on February 24, 1896, that Henri Becquerel reported his first results: after exposure to brigth sunlight, crystal of the uranyl double sulfate emitted a radiation that blackened a photographic plate after penetrating black paper, glass, and other substances. During the next few months he continued the experiments, obtaining more and more puzzling results. The effect was a strong with weak ligth as with bright sunligth; it was found in complete darkness and even for crystal prepared and always kept in the dark. The penetrating radiation was emitted by other uranyl and also uranous salts, by solution of uranium salts, and even by what was believed to be metallic uranium, and in each case with an intensity proportional to the uranium content. Proceeding by analogy with a known property of X rays, Becquerel observed that the penetrating rays from uranium would discharge an electroscope. All these results were obtained in the early part of 1896. Although Becquerel and others continued investigations for several rays, the knowledge gained in this phase of the science was summarized in 1898, when Pierre and Marrie Sklodovska Curie concluded that the uranium rays were an atomic phenomenon characteristic of the element, and not related to its chemical or physical state, and introduced the name “radioactivity” for the phenomenon.

Much new information appeared during the year 1898, mostly through the work of the Curies. Examination of other elements led to the discovery, independently by Mme. Curie and G.C. Schmidt, that compounds of thorium emitted rays similar to those from uranium. A very important observation was that some natural uranium ores were even more radioactive than pure uranium, and more activethan a chemically similar “ore” prepared synthetically. The chemical decomposition and fractionation of such ores constitued the first exercise in radiochemistry and led immediately to the discovery of polonium as a new substances observed only through its intense radioactivity and of radium, a highly radioactive substance recognized as a new element and soon identified spectroscopically. The Curies and their co-workers had found radium in the barium fraction separated chemically from pitcblende (a dark almost black ore containing about 75 percent U3O8), and they learned that it could be concentrated from the barium by repeated fractional crystallization of the chlorides, the radium salt remaining preferentially in the mother liquor. By 1902, Mme. Curie reported the isolation of 100 mg of radium chloride spectroscopically free from barium and gave 225 as the aproximate atomic weigth of the element. (The work had started with about two tons of pitchblende, and the radium isolated represented about a 25 percent yield). Still later Mme. Curie redertemined the atomic weigth  to be 226,5 (within 0,2 percent of the present best value) and also prepared radium metal by electrolysis of the fused salts.

Becquerel in his experiments had shown that uranium, in the dark and not supplied with energy in any known way, continued for years to emit rays in undiminished intensity. E. Rutherford had made some rough estimates of the energy associated with the radioactive rays; the source of this energy was quite unknown. With concentrated radium samples the Curies made measurements of the resulting heating effect, which they found to be about 100 cal/h per gram of radium. The evidence for so large a store of energy not only caused a controversy among the scientist of that time but also helped to create a great popular interest in radium and radioactivity.

(Friedandler, G., Kennedy, J.W., Macias, E.S., and Miller, J.M. 1981. Nuclear and Radiochemistry 3rd Edition. Toronto : John Wiley and Sons, Inc ; 1-2)

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