X-Ray Spectroscopy

Spectrometric Methods.

The arrangement for the spec troscopic analysis of an X-ray beam is shown diagrammatically in rays and more especially with the question as to how this radiation varies with the material on the anticathode.

As to the X-ray tube and its working there are two main ways of producing the cathode rays used for bombardment. In the first type (3a) the tube has a vacuum of o•oi to o•ooi mm. Hg. The high tension in this case gives rise to ionization of gas remaining in the tube and the positive ions formed are thrown against the cathode (of aluminium) and on their collision set free a number of electrons. The electrons travel in the opposite direction— from the cathode to the anticathode—and bombardment of the anticathode gives rise to X-rays characteristic of the substance of the anticathode. In the second type (3b Coolidge-tube) the vacuum of the tube is much lower, so that no current passes fig. 2. The different monochromatic rays which constitute the beam coming from the X-ray source (or slit) are reflected by the crystal at different angles and directions according to the Bragg law (I) as the angles may vary from 11 to only such rays are reflected whose wave-lengths have values between Al and X2 where nX2=2dsim1'2.

On the photographic plate therefore there is found, after ex posure and developing, a spectrum ranging from to X2. The region of wave-lengths can be varied by turning the crystal.

through the tube when the high-potential is applied to the tube. In this case the electrons for the cathode rays are supplied by heating to a high temperature a filament (mostly of tungsten) which is placed in the centre of the cathode.

For

analysing purposes it is often necessary to change the substance on the anticathode and the tube must be built in such a way that the anticathode is easily detachable. This can be accomplished by mounting the anticathode in a special joint. A tube especially designed for spectroscopic purposes which permits rapid change of both cathode and anticathode is shown in fig. 4. The tube itself is built of metal with water cooling to permit a ordinary optics. Such spectra are obtained when a sheet of thin foil of some substance is placed between the X-ray source and the photographic plate. Instead of a continuous blackening from the white X-radiation the plate shows one or more sharp edges where there is a rapid change in the blackening; these edges are called "absorption-edges." The wave-lengths of these absorption-edges

show an intimate relation to the characteristic line-spectrum of the substance used as absorbing screen. Such absorption-spectra were first obtained by Duc Maurice de Broglie and given right interpretation by Sir William Bragg and M. Siegbahn. Fig. 6 is a reproduction of a spectral-plate showing these three kinds of spectra.

The Continuous Spectrum.

As already mentioned the "white" or continuous radiation from an X-ray tube covers a rather wide region of wave-lengths. The diagrams of fig. 7 give an idea of the distribution of the energy of the different wave lengths at voltages from 20 to so kilovolts. The curves of dis tribution always start at a definite minimum wave-length (Xm?n). The value of this wave-length decreases with increasing voltage.

It has been found empirically and verified by many investiga tors that this wave-length or its corresponding frequency very high output of energy. Further a small window of aluminium or goldbeater's skin makes it possible to study the radiation of longer wave-lengths which are absorbed by the glass walls of an ordinary tube.

Another way of exciting X-ray spectra is to irradiate the sub stance with an intense beam of X-rays. In this case the substance emits "secondary" rays which, with a few exceptions, are identical with the X-radiation sent out by the same substance used as an anticathode in an X-ray tube. This method does not however give the same intensity as the former.

Different Kinds of X-ray-spectra.

The X-ray spectrum emitted from an arbitrary substance is built up of two kinds of radiation, one of which shows a continuous distribution over a wide range of wave-lengths, the other consisting of a few mono chromatic rays overlapping the former (see fig. 5). The first mentioned radiation corresponds to the "white" light of ordinary optics, whereas the second is analogous to the line-spectra. The continuous spectrum contains the greater part of the energy of radiation and is therefore the most important part for medical and many other applications of the X-rays. This part of the X-ray spectrum shows qualitatively no dependence on the radi ating substance, the intensity only being different from various anticathodes.