The electric and magnetic moments of the neutron

Abstract

It is well known that the free neutron decays spontaneously into a proton, an electron and an antineutrino, that it has a spin of 1/2 h and a negative magnetic moment, but very careful measurements have failed as yet to reveal any evidence for a finite electric charge or dipole moment. This paper contains a brief discussion of early work and more detail of recent experiments at the Institut Laue-Langevin (I.L.L.), Grenoble, which have shown that the neutron charge is probably less than 4 x 10 -20 electron charges (Bayreuth-Munich group), the neutron electric dipole moment (e.d.m.) is less than 1.5 x 10 -24 cm times the electron charge (Oak Ridge-Harvard-Sussex group), and the ratio of the neutron and proton magnetic moments is equal to — 0.68497 947(17), the uncertainty being only 0.25/10 6 (Harvard-Sussex-Oak Ridge group). The main features of the Leningrad experiments with ultra-cold neutrons, which have reduced the neutron electric dipole length to 7.5 x 10 -25 cm, are reported, with some details of the performance of the ultra-cold neutron magnetic resonance spectrometer now working at I.L.L. and the way it will be used to look for a neutron e.d.m. The paper concludes with some comments on the importance of the neutron moments to the development of the theory of fundamental particles.