Nuclear physics
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Abstract
Nuclear physics started in 1894 with the discovery of the radioactivity of uranium by A. H. Becquerel. Marie and Pierre Curie investigated this phenomenon in detail: to their astonishment they found that raw uranium ore was far more radioactive than the refined uranium from the chemist’s store. By chemical methods, they could separate (and name) several new elements from the ore which were intensely radioactive: radium (Z588), polonium (Z584), a gas they called emanation (Z 586) (radon), and even a form of lead (Z582). Ernest Rutherford, at McGill University in Montreal, studied the radiation from these substances. He found a strongly ionizing component which he called a rays, and a weakly ionizing one, b rays, which were more penetrating than the a rays. In a magnetic field, the a rays showed positive charge, and a charge-to-mass ratio corresponding to 4He. The b rays had negative charge and were apparently electrons. Later, a still more penetrating, uncharged component was found, g rays. Rutherford and F. Soddy, in 1903, found that after emission of an a ray, an element of atomic number Z was transformed into another element, of atomic number Z22. (They did not yet have the concept of atomic number, but they knew from chemistry the place of an element in the periodic system.) After b-ray emission, Z was transformed into Z11, so the dream of alchemists had become true. It was known that thorium (Z590, A5232) also was radioactive, also decayed into radium, radon, polonium and lead, but obviously had different radioactive behavior from the decay products of uranium (Z592, A 5238). Thus there existed two or more forms of the same chemical element having different atomic weights and different radioactive properties (lifetimes) but the same chemical properties. Soddy called these isotopes. Rutherford continued his research at Manchester, and many mature collaborators came to him. H. Geiger and J. M. Nuttall, in 1911, found that the energy of the emitted a particles, measured by their range, was correlated with the lifetime of the parent substance: the lifetime decreased very rapidly (exponentially) with increasing a-particle energy. By an ingenious arrangement of two boxes inside each other, Rutherford proved that the a particles really were He atoms: they gave the He spectrum in an electric discharge. Rutherford in 1906 and Geiger in 1908 put thin solid foils in the path of a beam of a particles. On the far side of the foil, the beam was spread out in angle—not sur-