Neutron Scattering

4 . Polarization analysis
W .
Schweika
4 .1 Introduction
Most typical neutron experiments are concerned only with the intensities related to a
specific change in momentum and energy of the neutron in a scattering process yielding
information about the structure and dvnamics of the considered svstem of interest . However,
since the neutron has a spin it interacts with the magnetic moment of the electron
shells, and the scattering amplitude depends also on the nuclear spins of a sample . There
are cases where one is able to distinguish magnetic from nuclear scattering, either from
the different form factor dependence or from a temperature variation across a magnetic
phase transition . Here we shall discuss the technique of using polarized neutrons and the
analysis of their final polarization after the scattering process . It is a powerful method
for distinguishing the varions possible scattering scattering contributions, - i. c . magnetic,
nuclear coherent, and nuclear spin-incoherent scattering -, and to separate them
from each other by experimental means without further assomptions . Its application to
neutron spin echo techniques will be discussed in chapter 11 .
The theoretical foundation for polarized neutron studies lias essentially been set by the
early works of Halpern and Johnson[1], Maleyev[2], and Blume[3] . A good introduction
is given in the classical work of Moon et al [7], sec also [4, 5] . This lecture will net treat
the full complexity of magnetic scattering (sec [2, 3]) . Emphasis will be given rather to
basic ideas following Refs . Schärpf [6] and Moon et al [7] .
4 .2 The motion of the neutron in magnetic fields and experimental devices
The essential characteristics of the motion of a neutron in a magnetic field is the precession
mode, which for simplicity can be considered in a classical treatment[6 ] . In tact,
even the quantum mechanical treatment, which introduces Pauli spin matrices into the
Schr5dinger equation, is effectively a classical treatment considering the origin of these
matrices . They result from the problem of mapping three dimensions onto two by intro-