The photoelectric effect of X-rays can also be demonstrated by the "Cloud method" of C. T. R. Wilson. In a gas through which a beam of X-ray passes, the sinuous paths of the electrons ejected from certain atoms can be seen and photographed. In this case the phenomenon is visible for a single atom and a large number of interesting observations can be made, such, for example, as an estimation of the initial directions of the velocities of the ejected electrons.
It is thus seen that the conductivity created in the gas by X-rays results from the pairs of ions separated by the photo electrons along their trajectory and is not a direct effect of the incident rays. Observations of the details of the phenomenon have led to the discovery that the secondary X radiation is often absorbed by the emitting atom itself and photoelectrons emerge directly (Auger effect). Several phenomena moreover, seem defi nitely to indicate that the radiation has a greater tendency to be absorbed by the actual atom which has emitted it than by others, and it would be of great interest to know the laws of this internal absorption.
Wilson's method enables us equally well to determine the initial direction of the velocity of the ejected electrons and to make a comparison, either with the direction of the incident beam, or with the state of the polarisation of the latter.
In spite of the difficulty of the measurements, the fact emerges from the experiments that the electron possesses a component of velocity in the direction of propagation of the rays, and that the magnitude of this component increases with the frequency as if a certain quantity of momentum passed from the wave to the cor puscle. It seems to be equally well established that the elec trons have a tendency to emerge following the direction of the electric vector of the polarised wave.
The initial velocities do not however possess a unique direction but are distributed more or less closely about a mean value; the theories of Bubb, and of Auger and Perrin, especially the latter, are interesting attempts to explain the experimental results.
length a thousand times shorter, such as X-rays, it would be neces sary to employ lines correspondingly more closely drawn (that is, if it was desired to follow exactly the same method, for we shall see that the artifice of "grazing incidence" enables optical gratings to be used in the study of X-rays). It is for this reason that the natural gratings afforded by crystals with their regular alignments of molecules suggested themselves.
Crystals are in fact regarded as composed of piles of equi distant planes, in which the atoms, or molecules, are distributed in a regular manner and form the intersections of a lattice system. To take a simple case, for example potassium chloride, if we suppose that the atoms of chlorine and potassium are placed at the intersections of a cubic lattice each atom will act as a diffract ing centre. The effect of the whole, due to the interferences of all the elementary waves issuing from each atom, can be easily calculated.
The German physicist, von Laue, and his collaborators, Friedrich and Knipping, first succeeded in 1912 in obtaining in this way the phenomena of crystal diffraction. Von Laue gave the theory of it and Sir William Bragg showed that the diffracted rays could be considered as reflected regularly at the different planes of the crys tal lattice provided that their wave-length satisfied the relation: where n is any integer, d the spacing between the lattice planes parallel to the plane considered, and 0 the angle made by the incident ray with this plane.
It is thus seen that a heterogeneous beam of X-rays, on passing through a crystalline medium, will give a series of beams whose positions can easily be predicted beforehand, and which will fur nish a diagram of spots on a photographic plate placed perpen dicular to the incident beam. The arrangement of these spots reflects the symmetry of the crystal employed.
If, on the other hand, we imagine a single crystalline face and cause the angle of incidence 0 to vary, we shall obtain reflected rays which will correspond to the different wave lengths present in the initial beam and will consequently furnish a true spectrum thereof.