Experiments on the protons produced in the artificial disintegration of the nitrogen nucleus

Abstract

It is now familiar that the events relative to the disintegration of radioactive bodies, to the scattering of alpha-particles and to artificial disintegration can be explained most simply by supposing the nucleus to have a potential field similar to a volcano and crater, whose walls can be penetrated by the alpha-waves. Using this idea to describe artificial disintegration where an alpha-particle hitting a nucleus causes a proton to be thrown out, we are led to the question of the fate of the alpha-particle—does it enter the crater and stay inside or does it re-emerge or does it cause the ejection of the proton from outside ? For the element nitrogen we have the direct experimental answer from Blackett’s work, where the tracks of the disintegration process are seen in an expansion chamber: all the observations show only three tracks, a fact which suggests that the alpha-particle stays inside the disintegrated nucleus; other less direct evidence renders it likely that this capture of the alpha-particle always takes place. The alpha-particle can make its entry either by surmounting the top of the potential barrier and falling inside; or by passing through the barrier. The calculation of the probability of this latter occurrence shows that except very near the top of the barrier it is extremely rare unless the energy of the incident particle coincides with an unoccupied level of energy inside, when the probability of entry rises enormously. This resonance was first shown to occur for the element aluminium by Pose. The object of the following experiments was to see whether the alpha-particle entered by resonance or over the top of the barrier; and if in the former way to fix the energy of the internal resonance level; if in the latter, to find the height of the potential barrier. In addition it was intended to investigate whether the protons produced from nitrogen are strictly homogeneous (that is, whether they have the same energy, a spread of energies, or two discrete energy values) by finding the absorption curve at smaller values than had previously been attempted. Such experiments might give confirmation to the suggestion put forward by Blackett and Lees,f that there is a selective absorption of the alpha-particles at about 2-6 cm. range.