(b. Amsterdam, Netherlands, 5 October 1889; d. Groningen, Netherlands, 12 February 1950)
Coster is not related to a contemporaneous Dutch writer with the same name. His first post was as teacher in a primary school. Outside support then allowed him to study physics from 1913 to 1961 at Leiden University, where he formed lifelong student ships with his teacher Ehrenfest and his fellow student Kramers. After obtaining his physics degree Coster studied and received a degree in electrical engineering at Delft Technological University because he judged it to have useful experimental applications. His doctoral thesis (Leiden, 1922), on the spectroscopy of X rays, was sponsored by Ehrenfest.
A fellowship enabled Coster to work on precision X-ray spectroscopy from 1920 to 1922 in Manne Siegbahn’s laboratory at Lund and from 1922 to 1923 with Niels Bohr in Copenhagen. Here he completely worked out the linking up of X-ray experimental data with Bohr’s theory of atomic structure and the periodic table of the elements. His crowning achievement was the discovery in 1923, with G. von Hevesy, of the element hafnium (named after Copenhagen and havingthe atomic number 72). They worked along lines suggested by Bohr, who expected the element to be a homologue of zirconium (atomic number 40) and rather different in chemical behavior from the rare earth metals (lanthanides, atomic numbers 57–71) that precede it and are very similar to each other because the “inner” shell of fourteen 4f-electrons is gradually filled. Hevesy concentrated the new element from zirconium compounds, in which it occurs naturally, as Bohr had suggested. The enrichment was checked by Coster, using the X-ray spectrum as indicator. This was rendered more difficult by a freak of nature that made the two strongest hafnium lines coincide almost exactly with the second-order reflections of the two zirconium Kα lines, thus camouflaging its presence in zirconium compounds.
Following his return to the Netherlands, Coster was assistant to Lorentz at the Teyler Laboratory in Haarlem until 1924, when he was called to the chair of experimental physics at Groningen University; he held the chair until 1949. With a rather limited budget he modernized and extended the physics laboratory, not only introducing X-ray work but also mounting a Rowland grating for studying band spectra, mostly of diatomic molecules. He attracted Dieke and Lochte-Holtgreven as skilled experimenters in this field and R. Kronig as a general modern theorist. Zernike was already doing classical work in interference theory and infrared spectroscopy. The X-ray work was carried on by a staff including Prins, Druyvesteyn, and van der Tuuk, as well as a steadily increasing stream of students (including Wolff, Veldkamp, and Knol) and visitors (including Hanawalt, Nitta, and Smoluchowski). Among the new results were the anomalous dispersion and scattering of X rays in the neighborhood of an absorption edge, leading to differences between the (111) and (ĪĪĪ) reflections of a zinc sulfide crystal. This effect was later used by Bijvoet to determine the absolute left-hand or right-hand configurations in crystals, which are difficult to obtain from other data.
Another major new point was the elucidation of the fine structure of absorption edges, extending some hundreds of electron volts from the main edge and resulting from the alternation of greater and lesser “ease” with which the crystal lattice accepts the ejected electrons. This explanation arose when the wave theory of the electron was in statu nascendi and Kronig (with Sir William Penney) applied it for the first time to the alternation of forbidden and allowed energy ranges in a one-dimensional model of a crystal. The discovery of a special kind of Auger effect, in which a secondary electron is ejected by a primary energy difference between two states having equal principal quantum numbers, also resulted from the collaboration of Coster and Kronig.
In band spectra and ultrasoft X-ray spectra, interesting new results were obtained. Appreciable work was done in neutron and nuclear physics. One gifted pupil, Hugo de Vries, became a specialist in biophysical applications of nuclear and nonnuclear physics, such as carbon-14 dating and the study of the nervous organs of fishes.
Coster was not only an energetic executive but also the moving force of his laboratory, eager to acquire mental pictures of the atomic processes and to explain them in simple terms. He was less interested in mathematical subtleties but never accepted a hazy presentation. The last years of his directorship and teaching were burdened by a progressive spinal disease, resulting ultimately in total paralysis.
Articles by Coster and his pupils can be found in Zeitschrift für Physik; Physica (The Hague); and Proceedings of the Royal Academy of Amsterdam.
Coster is discussed in Jaarboek van de K. Nederlandse Akademie van wetenschappen, gevestigd te Amsterdam (1951–1952), p. 198; and Nederlands tijdschrift voor natuurkunde, 15 (1949), 285.
J. A. Prins