Chapter 16 X-ray Spectroscopy

Before the discovery of the diffraction of X-rays in crystals some very important studies had been made of the quality of these rays by measuring their absorption in different materials. Especially Barkla had been able, by this rather simple method, to find some fundamental properties of the X-rays when they were scattered by different elements. It was known that the radiation leaving an X-ray tube was heterogeneous and dependent on the material of the anti-cathode. After passing a series of e.g. aluminum foils the 'softer' components of the radiation were successively reduced and finally the beam consisted of a rather homogeneous 'hard' radiation. The hardness of this radiation depends on the voltage applied to the X-ray tube. When such a homogeneous X-ray beam hits a plate of some element three kinds of secondary radiations are emitted. One part is scattered radiation with the same quality as the incoming one. The other two parts are characteristic of the element of the secondary radiator. One of them is an X-radiation, the other consists of negative particles, electrons. By systematic studies of this characteristic X-radiation from different elements in the periodic system Barkla distinguished two series of homogeneous X-radiation which he called the K-and the L-fluorescent radiations. Both these radiations become stepwise harder when the atomic weight of the 'radiator' increases. For the same element the emitted K-radiation is about 300 times as penetrating as the L-radiation. A condition for the appearance of these fluorescent radiations is that the incoming beam is somewhat harder than the characteristic radiation of the element in question. The emission of the electronic component of the secondary radiation was shown to be closely linked to the K-and L-series. Several important facts about the characteristic radiations were revealed empirically in these early researches on X-ray spectra, which