Paper 2.19: "Measurement of the Neutron Magnetic Moment," G. L. Green, N. F. Ramsey, W. Mampe, J. M. Pendlebury, K. Smith, W. B. Dress, P. D. Miller and P. Perrin, Phys. Rev. D20, 2139–2153 (1979)
Reprinted with permission from The Physical Review. Copyright 1979, The American Physical Society.
Although most of our spectroscopy with separated coherent oscillatory fields was with atoms or molecules, the method is equally effective for neutrons in an external magnetic field. As will be discussed in Section 3, most of our neutron experiments have been searches for a neutron electric dipole moment as tests of parity and time reversal symmetry. However, the same apparatus has also been suitable for precision measurements of the neutron magnetic moment. The first accurate measurements of the neutron magnetic moment by L. Alvarez and F. Bloch in 1940 used a neutron beam resonance method with a single oscillatory field.
In 1956, V. W. Cohen, N. Corngold and I [Phys. Rev. 104, 283–291 (1956)] measured the neutron magnetic moment, at the Brookhaven reactor, with neutron polarizing mirrors, a 1.5-m-long magnet and separated oscillatory fields. Our determination became the accepted value for the next 23 years.
In 1979 our group at Grenoble made a new neutron beam measurement with improvements such as the use of a liquid deuterium moderator to provide an intense beam of slow neutrons and the transmission of the neutrons through hollow tubes with total reflection of the neutrons on the inner walls of the tubes (Paper 2.19). The principal error in previous experiments had been the uncertainty as to whether the field averaged in discrete steps with a small NMR water probe was the same as for the moving neutrons. For this reason we used a flowing water magnetometer with the neutrons and water passing in succession through the same tube. After we made suitable corrections for various small effects [Metrologia 18, 93–94 (1982)], such as the diamagnetic susceptibility of water, we obtained the ratios of the magnetic moment of the free neutron to those of the electron, the Bohr magneton and the nuclear magneton to 0.25 parts per million.
