History and Development of X‐Ray Fluorescence Spectrometry

Abstract

X-ray fluorescence spectrometry provides a means of identification of an element, by measurement of its characteristic X-ray emission wavelength or energy. The method allows the quantification of a given element by first measuring the emitted characteristic line intensity and then relating this intensity to elemental concentration. While the roots of the method go back to the early part of this century [1,2,3], it is only during the last 25 years or so that the technique has gained major significance as a routine means of elemental analysis. The first use of the X-ray spectrometric method dates back to the classic work of Henry Moseley in 1912 (see Section 1.3). In Moseley's original X-ray spectrometer, the source of primary radiation was a cold cathode tube in which the source of electrons was residual air in the tube itself, with the specimen for analysis forming the target of the tube. Radiation produced from the specimen then passed through a thin gold window onto an analyzing crystal, where it was diffracted to the detector. At about the same period in time, after the application of the ability to image through opaque objects had demonstrated significant utility, Barkla observed [4] that when elements were irradiated with primary X-rays, the secondary radiation varied in penetration power (as measured by absorbing the rays with foils). Figure 5.1 shows Barkla's classic experiment in which he demonstrated that the total attenuation of the incident X-ray beam was different depending on whether the absorber was placed either between the source and the scatterer, or between the scatterer and detector. From this observation, Barkla concluded that the scatterer was not only scattering, but it was also modifying the beam. He further concluded that hard primary radiation could produce soft secondary radiation. It was Barkla who coined the term characteristic rays. He also observed that there were two components to this radiation, a harder and a softer one (specified by their ability to penetrate the foils). He chose to name them K and L rays, respectively, starting in the middle of the alphabet, as he assumed that harder and softer radiation would be discovered.One of the major problems in the use of electrons for the excitation of characteristic X-radiation is that the process of conversion of electron energy 75 X-Ray Fluorescence Spectrometry, Second Edition