The line-spectra on the other hand are characteristic of the emitting substance. They correspond to the K- and L-radiation of Barkla, but analysis has shown that each of these radiations consists of a group of monochromatic spectral-lines.
In addition to the emission-spectrum of X-rays there is also an absorption-spectrum analogous to the absorption-spectra of the quantum theory as follows. When the potential is applied between the cathode and the anticathode an electron on passing from the former to the latter acquires the energy : eV. If the electron is suddenly brought to rest at the anticathode this energy is given off as an electromagnetic wave with the frequency p and containing the energy hp, which gives hp= eV which corresponds to the above equation (2) for the maximum frequency. Generally the electron before being completely stopped will suffer several collisions and will send out smaller amounts of energy or waves of smaller frequencies.
The total amount of energy, that is, the integral energy in cluded by the distribution-curves in fig. 7, has also been measured by many investigators. As the result of these researches it can be stated that the total energy is proportional to the square of the voltage on the tube.
Secondly it has been found (by G. W. C. Kaye and others) that for different elements used as anticathode the total radiation is proportional to the atomic number of the element.
Already in the early days of X-ray technique it was found ad vantageous to use heavy elements as anticathodic materials for the medical X-ray tubes, an observation which is in agreement with the last general law.
Two groups of spectral-lines were found. One of these groups was identified with the K-series of Barkla; the other, of longer wave-length and of more complex structure, corresponds to the L-series. Fig. 8 is a reproduction from Moseley's paper of spectra of successive elements from Ca (atomic number 20) to Zn (3o). It shows in the most beautiful way how regularly the spectra—in this case the K-group—repeat themselves from one element to the next. With increasing atomic number (here from 20 to 30) of the emitting element the line-group is displaced in regular steps towards the shorter wave-length. This rule is not confined only to these elements and this group, but is universally confirmed by all X-ray spectra.
In 1916 M. Siegbahn discovered a new series of still greater wave-lengths called the M-series. The existence of series outside the K- and L-group had already been suspected earlier by reasons of analogy. In the later development of X-ray spectroscopy some indications of still other series, N, 0, etc., have been traced, but further experimental work is needed to prove its existence.
A general scheme of the X-ray spectra is given in the diagram fig. 9 which contains the strongest lines of the three groups K, L and M at every third element from Na r) to U (92). The diagram shows how all three series are regularly displaced to longer wave-lengths as one proceeds from the heavier elements to the lighter. If the spectrum of some special element, say Tungsten, a material commonly used for the anticathode, is con sidered, it will be seen that there are big gaps between the three groups where no lines are to be found. This fact is of predominant importance in interpreting the spectra in their relation to the structure of the atom.
As to the structure of the different groups it may be mentioned that the K-group generally consists of 4 lines (a1a20132) the L group of more than 20 lines (the strongest lines designated by aia2, • • •, 'Y172 • • • ) and finally the M-group of about 20 lines.