ABSTRACT

Following the discovery of X-rays by Wilhelm Röntgen, at the University of Würtzburg in 1895 (for which in 1901 he was awarded the Nobel Prize), investigations of the properties of this radiation quickly led to new revelations in the physics of matter. Among the better-known scientists, von Laue and then the Braggs (William and Lawrence, father and son, who together won the 1915 Nobel Prize) used X-ray diffraction to investigate the structure of crystalline solids, deriving, amongst other things, a value for inter-atomic distances. Rutherford (winner of the 1908 Nobel prize), who had used various methods to deduce an early version of the nuclear model of the atom, inspired H.G-J.Moseley, working at Manchester University, to use crystal spectrometers to measure the wavelengths of the X-rays emitted by various elemental targets subject to cathode-ray (or, as we would say today, electron) bombardment. This work led to two seminal papers, published in 1913 and 1914, in which he demonstrated what we now know as Moseley’s law, which relates the wavelength λ of the principal Xray line to a new ordinal (which he named the ‘atomic number’, Z, approximately equal to half the atomic weight) by the relation λ=c/Z2, where c is a constant. During his measurements, he had noticed in certain cases when analysing supposedly pure elements that he actually observed X-rays precisely matching those emitted by other elements, and deduced, correctly, that they arose from impurities in his elemental targets. As an afterthought to his work, he added to his first paper the sentence ‘The prevalence of lines due to impurities suggests that this may prove a powerful method of chemical analysis’. Surely this was the first reference to the use of electron-induced X-ray emission as an analytical technique. Shortly after writing this, following the patriotic fervour of the time, Moseley enlisted in the British army, and lost his life in action at Gallipoli on 10 August 1915.