This phenomenon has been described as if an excited atom always emitted its surplus energy first in the form of, radiation, but that, on occasion, this could be subsequently reabsorbed in the same atom. It is held by many authorities that it is rather artificial to imagine radiation to be emitted, apparently to exist independently of the atom, and then to be reabsorbed all inside the minute volume of the atom. It is maintained that it is simpler to state at once that an excited atom, that is an atom with greater energy than the normal, has at its disposal two distinct methods of getting rid of its extra energy, either by emitting a quantum of radiation, or by emitting an electron. There is no doubt that this is a difficult point, but a somewhat similar phenomenon occurs among the radioactive bodies, when, in the opinion of the writer, the evidence is strongly in favour of a true internal absorption of radiation.
The phenomenon of radioactivity is due to a disintegration of the central positively charged nucleus around which are the elec trons forming the rest of the atom. Frequently accompanying the actual disintegration, which involves the emission of a material particle from the nucleus, either the a- or 0-particle, there is also an emission from the nucleus of very high frequency radia tion called the y-rays. These are of the same type as X-rays, but of higher frequency. These 7-rays are quite frequently absorbed in the parent atom in their passage out from the nucleus through the electronic system, and give rise to high speed photoelectrons. As some radioactive bodies emit a considerable number of differ ent frequencies, each of which can be converted in either the K, L, Al, N, etc. levels, it will be seen that a great number of photo electric groups are possible. These may be investigated by the magnetic spectrum method, previously described in the case of X-rays, and illustrated by fig. 2. In this case, however, since the radioactive body is at one and the same time both the source of radiation and of the photoelectrons, it is merely necessary to place a wire coated with the radioactive material underneath the slit to obtain results.
The radioactive body used was Radium B, and the radiations have an average frequency corresponding to about 300,00o volts. The great complexity of the spectrum may be seen from this reproduction which only shows a part of the spectrum; to obtain all of it different magnetic fields would have to be used. The sharpness and narrowness of the lines is noteworthy. The reason for this is that, in this 7-ray case, all the photoelectrons are due to this internal absorption in the parent radioactive atom, and, since the radioactive material only forms the thinnest imaginable layer on the surface of the wire, all the electrons emerge with their full velocity. In the X-ray case, when there is external absorp
tion, photoelectrons are emitted throughout the body of the source, and, while those coming from the surface will have the greatest energy and will form the sharp edge of the line, those liberated inside the material can also get out, but will lose a portion of their energy in the process. They thus hit the photographic plate nearer the source and produce a long tail to the line.
This case of internal absorption is to some extent analogous to the compound photoelectric effect with X-rays, and here there is evidence that we may regard the •y-ray in all cases to be emitted but sometimes to be reabsorbed. On the one hand is the entire lack of any evidence of any other form of interaction between the nucleus and the electronic structures, which makes it doubtful to assume it in this case. On the other hand it has been shown by Ellis and Wdoster that this internal absorption appears to obey exactly the same relations as those found for normal external ab sorption. If this view be substantiated by future work, we have here an extremely interesting example of a photoelectric effect all occurring within the minute volume of an atom.
The Direction of Emission of the Photoelectron.—A mat ter of great interest is to ascertain how the initial direction of emission of the electron is associated with the direction of the electric force in the radiation. It is well established that radiation is some type of electromagnetic disturbance propagated through space in which electric and magnetic forces at right-angles are associated, both also being at right-angles to the direction of propagation. (See ELECTRICITY.) It is to be anticipated that it is the electric force which acts directly on the electron and wrests it from the atom, so that we should expect the electrons to have at least a large component of this motion in the direction of the electric force. The effect of the magnetic force on the electron, when once it is in motion, will be to throw it slightly forward. However, it is scarcely to be anticipated that all the electrons will come out at exactly the same angle to the electric force, since there are many subsidiary effects which will tend to introduce variations and produce a distribution over a range of angles instead of one unique direction.