Wilhelm Conrad Roentgen , 1845-1923, German physicist. His notable research in many fields of physics, especially thermology, mechanics, and electricity, has been overshadowed by his discovery (1895) of a short-wave ray, the Roentgen ray, or X ray, for which he received the first Nobel Prize in Physics (1901). He taught at several German universities, including those at Würzburg (1888-99) and Munich (1899-1920).

Bibliography: See biography by W. R. Nitske (1971).

radioactive elements The phenomenon of radioactivity was not discovered until the late nineteenth century—no doubt because we have no sense-organs for detecting it. Radioactivity cannot be felt, smelled, seen, or heard directly and is detectable only with the aid of mechanical or electronic devices. In about 1880 Henri Becquerel, a Parisian physicist, experimented with the luminescence of crystals of uranyl double sulphate, which is produced by exposure to ultraviolet light. He used his observations and those of the German physicist Wilhelm Conrad Roentgen to demonstrate that these crystals emit an invisible radiation that penetrates black paper and fogs a photographic plate. This work was carried forward by the Polish scientist Marie Sklodowska (later Curie) and Pierre Curie, who discovered that thorium was also an active emitter of penetrating radiation. They then discovered two further elements that emitted radiation, which they named polonium and radium. It was Marie Curie who proposed the term ‘radioactivity’ on the basis of the emissions of radium. In 1903 the Curies shared the Nobel Prize for physics with Henri Becquerel for the discovery of radioactivity. After Pierre's death, Marie continued her work on the chemistry of radium. In 1911 she received the Nobel Prize in chemistry in recognition of her successful efforts to isolate pure radium. She continued her work on the possible uses of radioactive elements in medicine, but died in 1934 of leukaemia, caused by the radioactive elements that she had studied.

Radioactive decay

The ionizing radiation emitted by radium aroused the curiosity of Ernest Rutherford at the Cavendish Laboratory at Cambridge University. After moving to McGill University in Canada, Rutherford did further work and reported that the radiation emitted by radioactive substances consists of three different components, which he named alpha, beta, and gamma radiation. The alpha component was eventually shown to consist of helium nuclei, and the beta rays were identified as electrons. Only the gamma rays proved to be electromagnetic radiation similar to the X-rays discovered by Roentgen. Rutherford, in collaboration with Frederick Soddy, studied the radioactivity of thorium. This led them to formulate the theory of radioactive decay. They suggested that the atoms of a radioactive element disintegrate spontaneously to form atoms of another element. They proposed that the disintegration is accompanied by the emission of alpha and beta particles and that the intensity of the radiation is proportional to the number of radioactive atoms (N) present. They showed that the change in the number of radioactive atoms present (dN) with change in time (dt) is equal to the decay constant (λ), which represents the probability that an atom will decay in unit time, multiplied by N. Rutherford moved to Manchester University in 1907 and in 1908 received the Nobel Prize for physics in recognition of his work on radioactivity. In Manchester he made further fundamental discoveries on the structure and composition of atoms. The results of his experiments indicated that atoms have very small, positively charged nuclei and that the nucleus is surrounded by orbiting electrons. Rutherford named the positively charged particle of the nucleus the proton in 1919, and a year later he speculated that the nuclei of atoms might contain a neutral particle. The hypothetical particle, the neutron, was discovered later on the basis of experiments by Sir James Chadwick and his co-workers. Rutherford's model of the atom was further refined by the Danish physicist Niels Bohr using the principles of quantum mechanics developed by Max Planck and Albert Einstein, and later by Shcrödinger and his co-workers using wave mechanics.

As the radioactive decay series of uranium and thorium was worked out, it emerged that there were several kinds of these elements, which decayed at different rates. It was also found that the atomic weights of the elements are not whole numbers, as had been previously believed, and that lead produced by the decay of uranium had an atomic weight that was different from that of natural lead. These observations led Soddy to suggest that a position occupied by a particular element in the periodic table could accommodate more than one kind of atom. He introduced the term isotopes (‘same place’ in Greek) for the differing forms of the same element. This led to the discovery that neon, for example, is composed of three kinds of atoms which have atomic weights of 20, 21, and 22. This advance relied primarily on the development of the mass spectrograph by F. W. Aston, a young chemist at the Cavendish Laboratory. Aston devoted his life to building increasingly precise mass spectrographs, with which he discovered 212 of the 287 naturally occurring isotopes. He also measured the masses of these isotopes and calculated the atomic weights of elements on the basis of the masses and relative abundances of their naturally occurring isotopes. For this work he was awarded the Nobel Prize in chemistry in 1922.

Geochronology

Alfred Nier became interested in problems of geochronology and the age of the Earth while working with K. T. Bainbridge at Harvard University. In collaboration with G. P. Baxter he began measuring the isotopic composition of lead and evolved dating methods based on the decay of uranium and thorium to lead. The mass spectrometers designed by Nier at the University of Minnesota enabled many other scientists to participate in isotopic research and contributed to the phenomenal growth of isotope geoscience until the end of the twentieth century. As geochronology developed, scientists also became aware that radioactive decay is an exothermic process, in other words, one that gives out heat. The rate of heat production in rocks was described by John Joly in a book entitled Radioactivity and geology in 1908. In this book Joly also discussed the measurement of the radioactivity of rocks and the origin of pleochroic haloes.

The ages of minerals and rocks containing uranium were first determined by Rutherford and Boltwood. Rutherford proposed in 1905 that the age of a uranium mineral could be measured by the amount of helium that had accumulated in it. The greatest ages that he obtained were about 500 million years, which provided positive evidence that Lord Kelvin's estimate of the age of the Earth, 40 million years, was in error. In 1907 Boltwood published the first age determinations of three uraninite speciments based on their uranium– lead (U/Pb) ratios. His dates ranged from 410 to 535 million years. The next fifty years saw rapid advancement in our understanding of radioactive decay and how it can be used to determine the ages of rocks. It was Patterson at the California Institute of Technology who in 1956 first established the age of the Earth and meteorites to be 4.54 Ga (billion years) by the use of lead isotope (²⁰⁷Pb, ²⁰⁶Pb, and ²⁰⁴Pb) systematics. Other radiometric dating methods that were exploited in the latter part of the twentieth century included potassium– argon (K–Ar), potassium–calcium (K–Ca), rubidium– strontium (Rb–Sr), samarium–neodymium (Sm–Nd), lutetium–hafnium (Lu–Hf), rhenium–osmium (Re–Os), and thorium-lead (Th–Pb), together with cosmogenic radionucleides such as beryllium-10 (¹⁰Be) and aluminium-26 (²⁶Al). The study of radioactive isotopes has given Earth scientists insights into the age and evolution of the Solar System, the planets, the Earth, and its moon.

K. Vala Ragnarsdottir

Bibliography

Faure, G. (1986) Principles of isotope geology (2nd edn). John Wiley and Sons, New York.