The detailed study of the (3-ray spectra discussed below has led to the conclusion that the 7-rays are characteristic radiations emitted from the radioactive nucleus in its rearrangement after the ejection of the (3-particle. These radiations from the nucleus in their passage through the atom excite the characteristic radia tions of the external electronic system. Thus the total 7-radia tion of a radioactive atom consists not only of the 7-rays from the nucleus, but also of the characteristic X-rays of the atom.
The first examination of the spectrum of the 7-rays was made by Rutherford and Andrade in 1914 by reflection of the rays from a thin crystal of rock-salt, using a fine glass tube filled with radon as a source of 7-rays. A complicated spectrum of bright lines was observed corresponding to the 7-rays emitted by the 0-ray products, radium B and radium C. Lines were observed cor responding to the ordinary X-ray spectrum of the elements radium B and radium C, and also a number of other lines of much shorter wave-length. These results have been extended by the recent work of Frilley, who by the use of intense sources of radiation has been able to observe still shorter waves, corresponding in energy to about 700,000 volts. The angle of reflection of such short waves from rock-salt is only about io minutes of arc, and it does not seem feasible by this method to measure the wave-length of the still shorter waves which are undoubtedly present. This difficulty has been surmounted by the use of an entirely different method which is specially applicable to rays of high frequency and energy. A brief discussion of this method and its application follows: When the (-rays from a product like radium B or radium C are bent by a magnetic field and fall on a photographic plate, a kind of magnetic spectrum is obtained. Superimposed on the continuous spectrum due to particles of all velocities (between certain limits) certain sharp lines are observed, each of which represents a definite group of 0-rays which are emitted at the same speed. The velocity corresponding to each line in the spec trum has been determined for a number of 0-ray products by Hahn and Lise Meitner. The magnetic spectrum of radium B and radium C was examined in detail by Rutherford and Robin son, and more than 50 lines were observed, representing 0-par ticles projected over a wide range of velocity. The appearance of these lines in the spectrum appears to be connected with the emission of 7-rays and is believed to be due to the conversion of the energy of the 7-ray of definite frequency into the energy of an electron according to the quantum relation. When a thin
layer of absorbing material is placed over the source, the primary 0-rays diminish in velocity and the lines become broad and dif fuse. At the same time, however, new groups of j3-rays are formed by the conversion of 7-rays into /3-rays in passing through the absorbing material, and these give well-marked bands on the photographic plate, occupying very nearly the same position as those due to the primary a-rays before absorption. A study of the slight shift in the position of the band when the 7-rays traverse elements of different atomic weights was made by Ellis. The results can be interpreted in the following way: A 7-ray in traversing the absorbing screen interacts with one of the electrons in an atom and occasionally the energy E of the 7-ray is trans ferred to the electron. The energy of the electron after escape from the atom is given by E–w where w is the work required to move the electron out of the atom. Since on the quantum relation the energy E of a 7-ray is given by hv, where v is the frequency of the 7-radiation and h the well known constant of Planck; it follows that by = E–w. Since the energy, E–w, of the escaping electron can be deduced by its deflection in a magnetic field, and w is known from X-ray data, the frequency v of the radiation is directly determined. On this view the origin of many of the lines in the primary, magnetic spectrum of the ft-rays is at once re vealed. A 7-ray in escaping from the radioactive atom oc casionally interacts with one of the outer electrons in the atom from which it originates, and communicates its energy to it. This electron may belong to any of the well known K, L, M, etc., levels in the atom, but the value of the energy w abstracted from the escaping electron depends on the level. It is greatest for the K level and rapidly diminishes for the L, M, etc., levels. In con sidering a large number of disintegrating atoms, there is a certain probability that a particular 7-ray will be converted in one of these levels and consequently a number of lines will appear in the magnetic spectrum corre' ponding to the conversion of a single 7-ray in the various electronic levels in the atom. Usually, the line due to conversion in the K level is much the strongest, but, for an intense 7-ray, the lines due to conversion K, L, M and N levels can be observed. By observations of this kind, it is thus possible to determine the frequency of the 7-rays by analysis of the magnetic spectrum of the 13-rays.