Physics and Radioactivity After the Discovery of Polonium and Radium

Physics and Radioactivity after the Discovery of Polonium and Radium by Pierre Radvanyi 1900, at the École Normale in Paris, Paul Villard discov- ered a third type of radiation that is very penetrating and analogous to X-rays, which will be later termed γ hysics and chemistry were quite interwoven in radiation. the early history of radioac- At the end of 1898, Rutherford Ptivity. In fact, the man con- became a professor of physics at McGill sidered to be the father of nuclear University in Montreal, Canada, where chemistry, Ernest Rutherford, was he began studying the radioactivity of a physicist by training and title. In thorium compounds. He observed, in 1908, he was awarded the Nobel 1899, a strange phenomenon: the con- Prize in Chemistry. tinuous production by thorium of what seemed to be a radioactive vapor The young Rutherford arrived or gas which he called “emanation.” in England from New Zealand in This emanation left on all bodies with 1895 with a scholarship and began which it came in contact an “excited working with Joseph J. Thomson radioactivity,” later called the “active at Cambridge on the ionization of deposit.” (Rutherford 1900). In 1900, gases. After the discovery of polo- in Germany, Ernst Dorn observed a nium, but before the discovery of similar emanation from radium. radium by the Curies, Rutherford Perplexed by the nature of emana- studied the Becquerel rays, the radi- tion, Rutherford asks Frédéric Soddy, ation emitted by uranium. He found Ernest Rutherford (1871–1937). a young chemist just arrived from that this radiation was complex and Oxford, to work with him on the prob- consisted of “at least two distinct types . one which lem. To them it appears to be an inert gas. At the will be termed for convenience the α radiation, and beginning of 1902, on the basis of new experi- the other . which will be termed the ß radiation.” In ments, they reach the conclusion that there exists an intermediate substance, which they call thorium X (called today radium 224), formed continu- ously in thorium, and giving rise to the emanation (today radon 220). They generalize that radioactivity is thus the spontaneous transmutation of an element into another by the emission of radiation. At first, Pierre Curie does not believe in the “material existence” of emanation. However, when Rutherford and Soddy succeed in liquefying emanation passing through liquid air, Pierre Curie gives in and accepts the interpretation of Rutherford and Many of the symbols used in the three natural, or classical, series (i.e., the Soddy. At the beginning of 1903, uranium, thorium, and actinium series) were assigned before the nature of Pierre Curie and Albert Laborde the isotopes was understood and now are obsolete. For example, in the observe that radium continu- thorium series, thoron (Th) is now called radon–220, and thorium D (ThD) is now called lead–208 (1996, Encyclopedia Britannica, Inc.). 32 CHEMISTRY International January-February 2011 January 2011.indd 32 1/3/2011 3:46:46 PM ously gives out heat: in one hour radium is able to melt more than its own weight of ice. In their leading paper of 1903 (Rutherford 1903), Rutherford and Soddy explain radioactive change, put forth the exponential law of radioactive decay, and define the radioactive constant. The two young scientists also provide the first tentative sketch of radioactive series; such a series should begin with a very long-lived radio element and end with a stable element. Measuring the kinetic energy of an alpha- particle and estimating the number of alpha-particles emitted, they compare the energy of radioactive change in one gram of radium to the energy liberated in a chemical reaction such as the union of hydrogen Potential barrier around a uranium nucleus presented and oxygen to form one gram of water. They conclude to an alpha particle. The central well is due to the average nuclear attraction of all the nucleons and that “the energy of radioactive change must therefore the hill is due to the electric repulsion of the protons. be at least twenty-thousand times, and may be a mil- Alpha particles with energy E trapped inside the lion times, as great as the energy of any molecular nuclear well may still escape to become alpha rays, by change.” In addition, they state that “The maintenance quantum mechanically tunnelling through the barrier. of solar energy, for example, no longer presents any fundamental difficulty.” wanted to count them by an “electric” method and Their findings soon allow scientists to determine the constructs, together with his young German co-worker age of rock and mineral samples. Between 1905–1907, Hans Geiger, the first particle counter in 1908. In order the American physicist Bertram B. Boltwood, following to ascertain the properties of the alpha-particles, he Rutherford’s suggestions, makes the first significant asks Geiger and an English-New Zealand student, E. measurements of the age of minerals by comparing Marsden, to study their scattering through thin metal- their lead (ultimate product of the radioactive series) lic foils. In 1909, the two physicists observe that some and uranium content: he finds ages on the order of alpha-particles are scattered backwards by thin plati- billions of years. Boltwood also discovers ionium (tho- num or gold foils (Geiger 1909). rium 230), the long-lived parent element of radium. It takes Rutherford one and a half years to under- From this point on, many laboratories worldwide— stand this result. In 1911, he concludes that the atom Paris, Montreal, Manchester, Vienna, Berlin—endeavor contains a very small “nucleus” where almost all its to complete the radioactive series (e.g., U 235, the mass is concentrated; the nucleus should carry the long-lived parent of the actinium series, is not identi- positive charges he theorizes, whereas it is surrounded fied until 1929 with the help of mass spectroscopy). by negatively charged electrons (Rutherford 1911). The consequences of this discovery for physics are Alpha-Particles and the Discovery of substantial. A Dutch amateur physicist, Antonius van the Nucleus den Broek, suggests that the Mendeleev serial number corresponds to the charge of the nucleus; so for each At that time of Rutherford’s early work on radiation, of these numbers there exists a distinct element. This it was strongly suspected that alpha-particles were is verified experimentally with the help of X-ray spec- swift helium atoms. After becoming a professor of troscopy by Henry Moseley in 1913. On the basis of the physics in Manchester in 1907, Rutherford spent much Rutherford atom, using Planck’s quantification rules, time obtaining decisive experimental proof that these the young Danish theoretician Niels Bohr calculates a particles carry two unit electric charges. To do so, he new model of the atom (Bohr 1913). Radioactivity, he wished to count the alphas one by one. The scintil- asserts, is a property of the nucleus. lation method, developed by W. Crookes, J. Elster, The number of new radioelements, in the limited and H. Geitel, allowed just that. However, Rutherford higher part of the Mendeleev table, become larger CHEMISTRY International January-February 2011 33 January 2011.indd 33 1/3/2011 3:46:49 PM After the Discovery of Polonium and Radium and larger, and some appear to be chemically identical There remains a puzzle: Why do ß-rays form con- (e.g., radium D and lead). To explain this phenomenon, tinuous spectra? A heated discussion takes place Soddy proposes in 1911 the existence of “isotopes,” between Meitner, Chadwick, and Ellis. Finally, Ellis and radioelements of the same chemical species that W.A. Wooster show in 1927, in a careful calorimetry have different atomic weights. Such experiment, that the mean energy isotopes should then also exist for liberated in the ß-decay of radium E is nonradioactive elements he proposes. only about one third of the maximum The so-called “displacement laws” energy of its ß-spectrum. Physicists for α- and ß-decay are formulated in are abashed: where is the rest of the 1913, independently by K. Fajans, G. v. available energy going? Niels Bohr Hevesy, A.S. Russell, and Soddy. is ready to give-up on the idea of Meanwhile, at the Radium Institute energy conservation in individual in Vienna, Victor Hess wishes to under- nuclear events. However, in 1930 in stand the background always pres- Zurich, Wolfgang Pauli comes up with ent in radioactivity measurements. In an unexpected explanation: in ß-decay the course of balloon ascents dur- two particles are emitted and not just ing 1911–1912, he discovers the exis- one. The electron is emitted together tence of radiation from outer space, with a yet unknown particle, which is later called “cosmic radiation.” The electrically neutral and a negligibly first observation of a “nuclear reac- small mass. This new particle will be Enrico Fermi (1901–1954). tion” is made by Rutherford, still in called a “neutrino.” However, the first Manchester, in 1919, on nitrogen nuclei bombarded by direct experimental observation of neutrinos will not alpha-particles; this reaction gives rise to the emission be made until 1953–1956. of protons. This is the beginning of nuclear physics. Pauli’s proposal finds general acceptance. On the This same year, Rutherford becomes director of the basis of this hypothesis, at the end of 1933 in Rome, Cavendish Laboratory at Cambridge. Enrico Fermi formulates his theory of ß-decay: electrons and neutrinos (antineutrino) are not present Further Progress in the Study of inside the nucleus; they are emitted at the instant of Radioactivity their creation (Fermi 1934). A new type of interaction is postulated that will later be called “weak interaction.” Rutherford and others have shown that the α-rays In 1928, a Russian-born young theoretician, George emitted by radioactive substances are monoenergetic.

Load more