Although our knowledge of atomic nuclei has expanded greatly in the last few years we are still in the lamentable position of having no certain information concerning the nature of the interaction between neutron and proton. Heisenberg’s suggestion (1932), in the form as modified by Majorana (1933), that this interaction is of an exchange nature has been much used for the discussion of the binding energies of the heavy nuclei. It has the great advantage of providing a simple explanation of the proportionality of nuclear binding energies to the number of nuclear particles, but there is no direct experimental evidence that it is correct. The simplest way in which one can hope to test any assumed form of interaction is from observation of the collisions between neutrons and protons. Unfortunately the relative velocities of the colliding particles which would give the most decisive test are not easily realized in practice, but the recent advances in the technique of nuclear disintegration have provided a source of neutrons of homogeneous velocity which may be used to give results of a higher precision and definiteness than hitherto. It is our purpose in this paper to describe the results to be expected in such experiments on the assumption of either exchange or ordinary forces, for a variety of forms of interaction. The known binding energies of the nuclei H
2
, H
3
, He
3
, He
4
enable us to restrict considerably the magnitude and range of the forces assumed, and we take account of this in all cases. It is found that there is a fair expectation of positive results from accurate experiments on the angular distribution of protons projected by neutrons of 2½ mV energy provided that the form of the interaction does not approach too closely that of the spherical potential “well”, i. e. provided there is an appreciable “tail” to the potential energy curve at large distances of separation. The calculations have not been restricted to neutrons of this particular energy but have been carried out for energies up to 20 mV. This is important in order to examine what conclusions, if any, can be derived from the experiments already carried out with neutrons, having a wide range of velocities.
The nature of the interaction between neutron and proton from scattering experiments
