The Magnetic Scattering of Neutrons

Abstract

The interaction of neutrons with ferromagnetic materials has been investigated through a number of experiments. Because of the dependence of the interaction upon the spin orientation of the neutron relative to the magnetic field of the atom, and the effect of this spin dependent perturbation on the nuclear scattering, a beam of neutrons transmitted through or scattered from a magnetized iron plate becomes polarized by selective scattering.This resultant neutron polarization has been studied most throughly through the increase in intensity of a neutron beam transmitted through a single iron plate when magnetized. The dependence of observed polarization on thickness of iron has been shown to agree within the limits of error with the theories of Bloch and Schwinger. The dependence of magnetic interaction on neutron energy was studied by the use of an effective "howitzer" for the production of low energy neutrons. The interaction increased rapidly for lower energy neutron energies, as would be expected, since the form factor for the interaction increases for longer neutron wave-lengths. The effective cross sections of iron for neutrons of spin +\textonehalf{} and -\textonehalf{} have been determined to be 13.7\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}24}$ ${\mathrm{cm}}^{2}$ and 10.3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}24}$ ${\mathrm{cm}}^{2}$ for \ensuremath{\sim}300\ifmmode^\circ\else\textdegree\fi{}K neutrons, and 14.1\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}24}$ ${\mathrm{cm}}^{2}$ and 9.9\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}24}$ ${\mathrm{cm}}^{2}$ for \ensuremath{\sim}120\ifmmode^\circ\else\textdegree\fi{}K neutrons. Considering the uncertainty in the values to be expected theoretically, these cross sections are probably in reasonable agreement with the theories.The intensity of neutrons scattered from a single iron plate has been shown to decrease when the plate is magnetized. Neutron polarization has also been studied through the change in neutron intensity when a beam of neutrons partially polarized by transmission through one plate of magnetized iron is scattered from or transmitted through a second iron plate magnetized parallel or antiparallel to the first. (Polarizer-analyzer action.)These experiments show definitely the existence of non-adiabatic transitions of the magnetic spin quantum numbers of the neutron in rotating or precessing magnetic fields whose frequency is of the same order as the Larmor precession frequency, $\frac{g\ensuremath{\mu}H}{h}$, for the neutron.The sign and the approximate magnitude of the neutron's magnetic moment have been determined by an experiment which depended on measuring the probability of nonadiabatic transitions in a controlled precessing magnetic field. This experiment was dependent only on the neutron properties in free space. The neutron magnetic moment has been shown to be negative in sign, and to be 2\ifmmode\pm\else\textpm\fi{}1 nuclear magnetons in magnitude.