The emission of the series is, then, a statistical phenomenon resulting from events which occur in a large number of perturbed atoms and the intensity of a line measures the probability of the particular combination of levels to which it corresponds.
Consider for an instant the higher terms of a series whose initial levels belong to the outer regions of the atom. They will have frequencies approaching the critical frequency of their final levels and their existence will depend on the real or virtual state of the outer initial levels. The series will thus be more numerous for the elements of high atomic weight, in which the levels occu pied by electrons are themselves more numerous. External actions, chemical action, forces of cohesion, etc., which modify the sur face levels, will likewise influence the higher terms of the series.
In fine it is by the lines close to the critical discontinuity that the influences of outside agents will be manifest. These lines may thus share with the lines of the luminous spectrum certain properties which will be absent in the lines arising from inner levels, the latter being practically independent of outside influences.
With the help of the preceding considerations we may describe in a summary and general way the properties of X-rays from the point of view of their emission and of their absorption by matter.
2. When the atom has absorbed a primary radiation which has removed an electron by the action of the photoelectric effect (see below). The atom then returns to its normal state with emission of rays of greater wave length than those of the exciting radiation; this is the most general case of secondary radiation. On irradiat ing a body with X-rays of frequency greater than of its K absorption discontinuity it is caused to emit the complete series of its normal X-rays.
3. When the atom is ionised as a result of a collision with fast electrons, it is then necessary, as shown by the experiments of Duane and Hunt and of Webster, for the energy of the exciting electron to be greater than the quantum by of the discontinuity which characterises the level whence the atomic electrons are removed; it is this process which takes place in the X-ray tubes.
A tube which has been evacuated, contains an anticathode of the element the rays of which it is desired to obtain. If the tube con tains a little residual gas, electrons of the cathode stream strike the anticathode and by their collision cause the emission of X-rays; their number, however, is difficult to regulate and their speed ill-defined. A better result is obtained by employing a tube with an incandescent cathode of the Coolidge type. (See RoNTGEN RAYS : Applications.) Such an incandescent filament serving as a cathode emits electrons of which the speed is a function of the applied voltage and the number a function of the temperature of the filament, i.e., on the magnitude of the heating current. We thus have a known number of electrons of known energy.
The characteristic rays taken as a whole (i.e., the K series, L series, M series, etc.) do not constitute the total radiation emitted from a target when it is subjected to the bombardment of a stream of electrons. There is, in addition, a continuous spectrum, more pronounced in the case of the elements of high atomic number, which is quite different in nature from the characteristic rays. The commencement of this spectrum, on the high frequency side, is abrupt and corresponds to the energy of the exciting cathode rays, as given by the quantum relation. That is to say, the highest frequency emitted, hp..., is connected with the potential V applied to the tube by the formula hv..„= eV where e represents the electronic charge and h Planck's constant. The examination of the distribution of the intensities as a function of the wave-length in the continuous spectrum shows that after this abrupt beginning, the intensity increases with increase of wave-length and passes through a pronounced maximum. There is a certain analogy with the well known curve which represents the emission of radiation by a black body; a fact which has not failed to attract attention. When the radiation is measured in various azimuths with respect to the exciting pencil of cathode rays, the position of the beginning of the spectrum remains in variable, but the intensity distribution is found to be modified. In accordance with theoretical predictions, the proportion of the high frequencies is much greater in the direction of the cathode rays.