The Continuous X-Ray Spectrum

Abstract

It has long been known that the x-rays emitted from an x-ray tube contain radiation characteristic of the material of the x-ray target superimposed upon a continuous spectrum. As early as 1896 Winkelmann and Straubel had noticed a scattered radiation which was characteristic of fluor spar; and later the characteristic x-rays scattered by different elements were studied systematically in a series of classic researches by Barkla. However, the actual separation of different wave-lengths into a spectrum was not possible until Friedrich, Knipping and Laue showed that crystals act as three dimensional diffraction gratings. Proceeding rapidly on the basis of this discovery W. L. Bragg and W. H. Bragg developed the single crystal ionization x-ray spectrometer which consisted of an x-ray tube source, a crystal with one face acting as a two dimensional reflecting grating and an ionization chamber. Suitable slits were provided together with a means of rotating crystal and ionization chamber through known angles. The first x-ray spectrum was obtained with this instrument1 revealing characteristic radiation to be made up of spectral lines characteristic of the target material (see Fig. 1). These lines were superimposed on an apparently heterogeneous continuous background of radiation. It is with this continuous x-ray spectrum that we shall be concerned. Duane and Hunt [1] applied the Bragg type ionization spectrometer to a study of the x-rays emitted by a tungsten target x-ray tube having a constant potential of about 37000 volts from a battery source applied across it. The K characteristic tungsten lines are not excited at this voltage and the L lines are of too long a wave-length to appear. The result obtained is shown in Fig. 2, indicating clearly that a constant potential applied to an x-ray tube does not produce homogeneous radiation but a heterogeneous continuous spectrum.