Instead of using X-rays generated in the usual way, it is possi ble to employ the natural X-rays or 7-rays emitted by radioactive bodies during their disintegration. These are similar to X-rays, only in most cases the frequencies, and therefore the energy of the quantum, are much higher. It was by obtaining photographs by the use of these rays that it was first possible to demonstrate that the y-rays were sharply monochromatic, and constituted a true characteristic spectrum of the nucleus. This method is par ticularly interesting in this connection since it forms the only practicable way of measuring the wave-length of y-rays. The ordinary crystal methods of measuring the wave-length of X-rays are difficult to apply as the wave-length is so short, and before the photoelectric effect was understood only estimates could be made. By investigating the photoelectrons ejected from various metals by the y-rays, Ellis was able to ascertain the particular portion of the atom from which the electrons were being emitted, and then a simple measurement of their energies, coupled with our X-ray knowledge of the atom, immediately yielded the hv of the 7 -rays.
An entirely different method of investigation is rendered possi ble by C. T. R. Wilson's cloud track apparatus in which, by a sudden expansion, moist air enclosed in a suitable chamber is put in a condition of supersaturation. The degree of supersaturation is so chosen that in the absence of any nuclei no condensation occurs. Immediately after the expansion a flash of X-rays is allowed to pass through the chamber, and condensation occurs on the ions that are formed. If suitably illuminated these water drops can be photographed. The X-ray beam was limited to a narrow pencil down the centre of the chamber and five separate photo electrons were visible. Each of the tracks of these photoelectrons shows the history of a separate absorption of a quantum of energy by by the atom located at the head of the track.
The electrons are ejected with such a high velocity that they can penetrate a considerable distance through the gas. Their path is marked by the ions they form in their passage. In the case of electrons formed by X-rays of lower frequency the speed of ejec tion is less. This is shown in photographs which reveal the greater tortuosity of the paths and the greater density of the water drops along a track.
X-ray absorption and the small stopping power of hydrogen allowed the ejected electrons to traverse reasonably large distances and give good tracks.
It might be thought that the double tracks of the photo graph only show a normal photoelectric effect by two atoms so close together that the tracks appear to start from the same point, but these double tracks occur far more frequently than would be expected on a probability basis, and it is simplest to consider them to come from one atom by some slight modification of the usual process. It will be seen that the further facts support this hypothesis. If a series of such photographs are taken under ex actly the same conditions, with the one exception that the voltage on the X-rays (i.e., the frequency of the X-rays) is increased, it is then found that, while one member of a pair of tracks increases in length as predicted by Einstein's equation, the other shows no change. This must not be taken to show an exception to Einstein's equation but rather that this second track is due to some process in the atom, admittedly excited by the primary radiation, but yet not directly depending on its frequency. The first action of the radiation is to eject an electron from the atom in the normal pho toelectric way, and the second track occurs because the atom is left in what is called an "excited state" that is capable of further changes.
In the article ATOM it is explained how for numerous pur poses we can picture the electrons in the atom arranged in a series of shells or groups, each characterized by a definite energy. The process of radiation is held to be due to the removal of an elec tron from an inner group and the subsequent filling up of the gap by one of the electrons previously in an outer group. Now in the compound photoelectric effect the primary effect of the X-rays is to remove an electron from one of the groups of an argon atom, and in the majority of cases this will be from the innermost or K group. The argon atom is now said to be excited in the K group, and a subsequent transition of one of the outer electrons into this vacant place will lead to the emission of one quantum of the argon K radiation. Sometimes this radiation escapes from the atom, but it may be internally absorbed and never escape at all. In this case the argon atom will emit a second photoelectron whose energy, and therefore length of track in the gas, is deter mined by the frequency of the argon K radiation and not by that of the incident X-radiation. It will, therefore, not change when the frequency of the incident X-rays is changed. It is possible to secure a second and even a third repetition of this interesting process thereby giving rise to the emission of three or four photo electrons from one atom.