SPECTROSCOPY, X-RAY. It is now a generally accepted fact that X-rays form a branch of ordinary optics. Proceeding from the visible light (8,00o-4,000 Angstrom units) to the electro magnetic waves of shorter wave-lengths we are able by recent spectroscopic methods to pass without any break through the ultra-violet region (the Schuman-Lyman-Millikan regions) to the realm of wave-lengths characterizing X-rays (extending from some hundredths to about one tenth of an Angstrom unit). On the short wave-length side of the X-ray region we pass into those wave-lengths known as the radioactive gamma-radiation and the still shorter wave-lengths of the "cosmic" rays.
That the X-rays produced in an ordi nary X-ray tube are complex and differ in quality when the vacuum of the tube, or primarily the tension on the tube, is varied was noted by Röntgen himself in his first papers dealing with the new radiation. It was shown early that the X-radiations became more penetrative as the tension of the tube was raised. The degree of penetration (usually measured in terms of the thickness of aluminium necessary to reduce their intensity by half) was used as a means of characterizing the quality of the radiation.
It was by this means also that C. G. Barkla was able to show that the different elements when excited so as to give off X-rays all have their own "characteristic" radiations. For instance for an element such as silver Barkla showed the existence of two "characteristic" radiations called the K- and L-radiation having very different penetrating power.
The K-radiation from this element is reduced to half its in tensity after passing through a sheet of aluminium about I mm. thick whereas the L-radiation is diminished in the same degree after it has passed through only 0.004 mm. of aluminium.
These two characteristic radiations, the K-series and the L series, were experimentally verified for a great number of ele ments. The measurements of their penetration in aluminium showed that their "hardness" increases regularly for both series when the atomic weight of the emitting element increases.
Corresponding to the emission of the characteristic fluorescent radiation of a certain element, Barkla and his collaborators, espe cially Sadler, showed the existence of a remarkable abnormality in the absorption of the X-rays. This was the first indication of what is now called the absorption-spectra in the field of X-rays, and which have been shown to be intimately connected with the emission spectra or characteristic radiation of the elements.
The knowledge which had so far been obtained regarding the characteristic X-radiation and absorption was extended to quite a new branch of spectro scopy after the fundamental discovery of Laue (1912). By this discovery experimental science obtained the tool that was necessary for exploring the fascinating subject, which X-ray spec tra of the elements turned out to be.
The diffraction patterns which Friedrich and Knipping obtained on allowing a fine beam of X-rays to pass through a crystal as From the position of a certain spectral-line on the photographic plate its wave-length is computed by equation (I). A number of different methods have been used for determining the angle ik which forms the main problem of the X-ray spectrometry.
To excite the X-ray spectrum of a substance a small piece of it is placed on the anticathode (3a, 3b) of an X-ray tube. After the tube has been exhausted to a suitable vacuum a high potential (io to i5o kilovolts) is applied to the electrodes of the tube. By this means the anticathode is bombarded by the electrons forming the cathode rays. The kinetic energy of the electrons imparted to the bombarded atoms is partially transformed into heat, light, etc., and partially to X-rays. We will here deal exclusively with the last mentioned suggested by Laue induced W. H. and W. L. Bragg to perform experiments, which formed the first step of the development of the X-ray spectroscopy. As the Braggs showed, a monochromatic X-ray beam is reflected by a cleavage face (or any other atomic plane) of a crystal according to the ordinary laws of optical re flection, i.e., the incident and the reflected beams are in the same plane and this plane is perpendicular to the reflecting face, and further the angles between these two beams and the reflecting face are equal. In addition to these laws the following condition must be fulfilled if reflection is to occur, namely: nX= 2dsin (I)
where X is the wave-length of the monochromatic radiation, d is the distance between two adjacent atomic layers parallel to the reflecting plane, 4 the angle between the beam and the plane and n indicates the "order" of the reflection. (Fig. I.) This equation, generally known as Bragg's law, forms the basis for measuring the wave-lengths of X-rays. It may be men tioned that this law needs a small correction, due to the fact that the wave-length is slightly different in a vacuum (or air) and in the crystal.