Measuring X-ray Wavelengths
Development of the x-ray tube greatly speeded up the detailed study of x rays, the origin and nature of which had finally been discovered by 1912, with the help of the suggestion by the German scientist Max von Laue (1879-1960) that x rays could be diffracted by three-dimensional crystals and thus must be electromagnetic radiation similar to visible light. This new approach was necessary because the wavelengths of x rays are so small that the diffraction gratings used for visible light will not work because the lines on the grating cannot be made with small enough spacings. A "natural" three-dimensional grating in the form of a single crystal of a material such as sodium chloride (salt) or calcite works very well since the spacing of the atoms in the crystal is roughly the same as the x-ray wavelengths of interest. Intrigued by von Laue's discovery of x-ray diffraction, English physicist W. L. Bragg (1890-1971), working with his father W. H. Bragg (1862-1942), began a series of experiments that culminated in the invention of the xray spectrometer in 1913. This device allowed the Braggs to examine the structure of certain crystals, laying the foundation for the science of x-ray crystallography. Ultimately, the development of the x-ray spectrometer led to important advances in atomic physics and an improved understanding of the periodic table.
In 1913, the English physicist H. G. J. Moseley (1887-1915) used a Bragg-type spectrometer to look at the characteristic x rays from many of the elements. Taking advantage of the regular decrease in wavelength of characteristic x rays as one looks at successively heavier elements, he discovered that it was possible to tell one element from another by looking at the characteristic x rays. He found that elements should be listed in the periodic table in terms of their atomic number, not the atomic weight as had previously been done. He determined, for example, that cobalt should come before nickel even though cobalt had a larger atomic weight. He was also able to predict the existence of several elements, such as scandium and promethium, which were then unknown but later discovered.