Nucleus

The general arguments give us information as to the kind of particles which may be expected to exist within nuclei, at any rate within very heavy nuclei.

A line of argument as to the structure of the radioactive nuclei has been put forward by C. D. Ellis. He has made extensive measurements on the energies of the 0-rays liberated from heavy metals by the action of 7-rays from a radioactive substance. (See PHOTOELECTRICITY.) These electrons may be freed from the K or the L levels of the atom (see X-RAYS, NATURE OF Mom) and the study of the X-ray spectra enables us to find the work required to take an electron from any one of these levels to the surface. A comparison of the spectra of the 0-rays excited by the same 7-rays in different metals enables us to attribute the origin of certain observed j3-rays to certain levels; if we correct the observed energy by adding the work of release, we obtain the same energy for a given gray, no matter from which element it has come, or, otherwise expressed, a 13-ray spectrum is found which is independent of the element and expresses the energy initially communicated to the (3-particle by the incident y-ray. The quantum theory leads us to believe that the (3-particle in these circumstances takes the whole of the energy of the y-ray which releases it. These observations of the secondary 0-ray spectrum therefore serve to reveal the wave length of the inci dent 7-rays, since the relation between wave length and energy is given by the simple quantum relation.

Ellis has thus measured 7-rays of much greater energy, or short wave-length, than those measured by Rutherford and An drade. Considering the complete 7-ray spectrum of an element, say Radium B, he has found that the different 7-ray energies can be accounted for by assuming a small number of energy levels in the nucleus, quantum transitions between suitably selected pairs of which give the frequency of the 7-rays in the same way as quantum transition between the levels of energy in the outer parts of the atom give rise to spectral lines. (See QUANTUM THEORY.) We have, then, this evidence for energy levels within the radio active nuclei, but we do not know to what mechanism these energy levels are to be attributed. It has been supposed that they

could be best explained by electron orbits within the nucleus, cor responding to the quantised electron orbits in the outer parts of the atom so much invoked in the explanation of optical line spectra, but Kuhn has criticised this view, and suggested that it is more satisfactory to attribute the energy levels to the suitably quantised movements of massive positive particles.

This part of the subject is in its infancy, and we can say little with any degree of certainty. There is good evidence for a struc ture of protons and electrons; for a subdivision of the nucleus into energy levels, originating in circulating particles ; for the exist ence of a-particles, and of a-particles neutralised by two elec trons, as components of the radioactive nuclei. Beyond this speculation has an untrammelled range.

The Disruption of the Nucleus.

The nuclei of radioactive atoms, which are all heavy and correspondingly complex, are unstable and disrupt spontaneously, giving out either an a- or a 0-particle, with a corresponding change in the chemical nature of the atom. The ejection of an a-particle decreases the mass by 4 units and the atomic number by 2 units : the ejection of a 13 -particle makes practically no difference in the mass, but in creases the atomic number by I unit. (See RADIOACTIVITY, ISO TOPES.) The lighter elements (with the possible exception of potassium and rubidium) are all stable, and, even after the dis covery of radioactivity and the later theory of the nuclear atom, were supposed to be proof against any agencies available in the laboratory. It was known that the outer electrons could be removed by irradiation, or by the impact of particles of sufficient energy, but as long as the nucleus is intact the atom will repair itself by taking up electrons again at the first opportunity. The atom can be permanently changed in kind only by adding or subtracting particles from the nucleus, and the expression "break ing the atom" is generally used in popular parlance to indicate such a tampering with the nucleus.