Determinations of the frequencies of the main 7-rays given by a number of radioactive elements have been made by this method by Ellis, Meitner, Thibaud, Black and others. In the case of radium B and radium C, the frequencies so determined agree well with the direct measurements made by the crystal method, so that it is clear that this powerful method can be safely applied for waves of the highest frequency. So far, the highest frequency observed corresponds to over two million volts.
The penetrating 7-rays in general accompany a /3-ray dis integration, and it has proved of great importance to settle whether the 7-rays are emitted before or after the expulsion of the disintegration electron from the nucleus. This question has been attacked by a number of observers by different methods, ultimately leading to a definite proof that the 7-ray always fol lows the emission of the disintegration 0-particle, presumably after a short but unknown interval. For example, the 7-rays which are ascribed to the disintegration of radium B of nuclear charge 82 actually arise from the nucleus of charge 83 resulting from the expulsion of a 13-particle from radium B and are con verted in the electronic levels of an atom of charge 83.
Ellis has obtained some evidence that the main 7-rays from radium B and radium C can be interpreted by assuming a system of energy levels in the nucleus, analogous to the well known sys tem of energy levels in the electronic system of an atom which have been postulated for the explanation of optical and X-ray spectra, but the wave-lengths of the 7-rays are not yet known with sufficient precision to fix the levels with much certainty. The general evidence suggests that the 7-rays are not due to the i passage of an electron from one level to another in the nucleus, but to the movement of a more massive particle, possibly a proton, or helium nucleus.
In this connection, it may prove significant that 7-rays are emitted from some products which break up with the emission of a-particles but no j3-particles. Whatever may be the exact mode of origin of the 7-rays, it seems clear that the high frequency v-rays originate in the nucleus of the transformed atom and represent the characteristic modes of vibration of parts of the nuclear structure.
Radioactivity of Ordinary Matter.—Apart from the well known radioactive elements of high atomic weight, only two other elements have been shown to exhibit radioactivity to a detectable degree, viz., potassium and rubidium. Campbell showed that these elements emit only j3-rays and in amount small compared with uranium. This property appears to be atomic, but no evi dence has been obtained of any subsequent changes. If the 0 particle comes from the nucleus of the atom, potassium should be transformed into an isotope of calcium, and rubidium into an isotope of strontium. Hervey has partially separated the isotopes of potassium of atomic masses 39 and 41 by diffusion methods and concludes that the radioactive property is mainly concentrated in the heavier isotope.
Radium and thorium have been found to be distributed, but in very minute amount, in the surface rocks and soil of the earth. The emanation from the soil diffuses into the atmosphere and causes a small ionization which can be readily measured. A penetrating 7-radiation, no doubt due to the presence of radium and thorium in the earth's crust, has been observed near the earth's surface, but becomes very small over a lake or the sea.
Structure of Radioactive Nuclei.—On modern views, the atom consists of a central positively charged nucleus of very minute dimensions surrounded at a distance by a compensating distribution of electrons. (See Moms.) The ultimate constit uents of the nucleus are supposed to be protons and electrons, the former in excess, but probably combinations of protons with electrons exist as secondary units within the nucleus. It seems clear that radioactivity is a property of the nucleus and that the primary a- and j3-particles and the 7-rays have their origin in the nucleus. Some of the secondary 0-rays are due to the con version of the energy of 7-rays in passing through the outer electronic system of the atom while characteristic X-rays are also excited in the rearrangement of the outer atom. From con sideration of the scatterings of a-rays by heavy elements and the energy of emission of a-particles from radioactive atoms, it has been suggested that the nucleus is a complex structure, consist ing of a central, highly compact, inner nucleus surrounded at a distance by a number of satellites. Rutherford supposed these satellites to be electrically neutral and to be held in equilibrium by the enormous distorting forces arising from the intense electric field of the central nucleus. For some reason, one of the satellites circulating in a quantum orbit becomes unstable and escapes from the system, shedding two electrons as soon as the electric field falls to a certain critical value, and gaining further energy in its escape through the repulsive field. The final speed of ejection of the a-particle on this view depends on the quantum orbit from which it is liberated. It is suggested that some of the electrons circulate at high speeds close to the central nucleus and that the 7-rays arise not from the j3-electrons but from the fall of the neutral satellite from one quantum orbit to another. Enskog has suggested that the a-particles are held in equilibrium by magnetic attraction between the a-particles and central nucleus.
BIBLIOGRAPHY.-Mme. M. Curie, Traite de Radioactivite, 2 vol. (1910) ; Sir E. Rutherford, Radioactive Substances and their Radia tions (1913) ; S. Meyer and E. V. Schweidler, Radioaktivitat (5927) ; F. Soddy, Chemistry of the Radioelements, parts I. and II. (1914-15)• See also Hevesy and Paneth, A Manual of Radioactivity 0926) ; Fajans, Radioactivity (5922) ; A. S. Russell, Chemistry of the Radio active Substances (1922) . See also under "Radioactivity" in the Annual Reports of the Chemical Society. (E. Ru.)