The Electric Quadrupole Moment of the Deuteron and the Field Theory of Nuclear Forces

Abstract

THE discovery by Rabi and his collaborators1 that the deuteron in its ground state possesses an electric quadrupole moment is of considerable theoretical importance, since it clearly shows that the forces acting between a proton and a neutron must to a quite appreciable extent depend on the spatial orientations of the spins of the heavy particles. It is well known that the vector meson theory of nuclear forces formally provides such a directional coupling, though of so strongly singular a type that it can only be given a definite meaning by taking recourse to some arbitrary cutting-off prescription. Now, Bethe2 has recently examined the question how such a cutting-off should be performed, in a theory giving rise to charge-independent nuclear forces, in order to obtain the correct positions of the ground-level and the excited 1S-level of the deuteron. He was led to the conclusion that, while the way in which the cutting-off is performed is of small influence on the results, the value to be assumed for the cutting-off radius depends critically on the combination of charged and neutral meson fields adopted: if one uses the symmetrical combination of meson fields suggested by Kemmer3, the cutting-off radius should be chosen larger than the range of the nuclear forces, and a reasonable value of this radius can only be obtained if the meson field is assumed to be purely neutral. This last assumption amounts of course to giving up the remarkable connexions suggested by the symmetrical theory between the problem of nuclear forces and those of cosmic ray phenomena, beta decay and especially the magnetic moments of proton and neutron.