Experiments with high velocity positive ions

Abstract

It would appear to be very important to develop an additional line of attack on problems of the atomic nucleus. The greater part of our information on the structure of the nucleus has come from experiments with α-particles and if we can supplement these with sources of positive ions accelerated by high potentials we should have an experimental weapon which would have many advantages over the α-particle. It would, in the first place, be much greater in intensity than α-particle sources, since one microampere of positive ions is equivalent, so far as numbers of particles is concerned, to 180 grams of radium equivalent. It would in addition have the advantage of being free from penetrating β and γ rays which are a complication in many experiments, whilst the velocity would be variable at will. The main difficulty in obtaining such sources lies of course in the production and application of the very high potentials necessary to accelerate the particles if velocities approaching that of the α-particle are to be obtained. For example, α-particle from polonium have an energy corresponding to 5.2 million electron volts and a potential of 2.6 million volts would be required to give a helium nucleus an equal amount of energy. We have therefore to decide what is the minimum acceleration voltage at which we can usefully work, since the experimental difficulties increase very rapidly with increasing voltage. In making this decision we are naturally guided by the recent theoretical work of Gamow on the “ Theory of Artificial Disintegration.” On Gamow’s theory the probability of an α-particle of velocity v entering a nucleus of atomic number Z, after coming within the effective radius of the nucleus, is W = e -16π e 2Z/ hv J k , Where J k is a function varying slowly with v and Z. It is clear, therefore, that for particles of equal energy the lighter particle has the greater chance of penetration into the nucleus, so that we should choose protons as our source of positive ions for this reason.