Quantum aspects of neutron spin behavior in homogeneous magnetic field

When speaking about quantum aspects of the spin behavior, one usually keeps in mind the peculiarities of the spin projections. Quantum aspects of diff erent nature, related to the spin-dependent interaction of neutrons with ma gnetic fields, are considered. As the origin of the neutron spin precession is the birefregence of magnetic media, the precession phase planes are, ge nerally, not perpendicular to the neutron momenta (non-frontal precession). The spin-dependent inelastic interaction of a particle with magnetic field s may result in the nutation of its spin in a homogenous static field, the origin of such behavior of the spin being purely quantum mechanical. The conventional approach to the neutron polarization is shown to ensue fro m the exact solutions of the Schrodinger equation. Quantum aspects in descr iption of the neutron spin behavior play a role only in extreme cases. Quan tum behavior not described in the Larmor precession picture may come into p lay for low energy (ultracold) neutrons sufficiently strong fields. When no t the total momentum but its component less by orders of magnitude is effec tive, as the case in reflectometry, the quantum aspects may become essentia l even for thermal neutrons. They should be taken into account in the inter pretation of some polarized neutron experiments. The superposition of two fermion states, the phase difference between which changes with a frequency omega, is proved to be equivalent to a state with the spin rotating about an axis with a period 2 pi/omega, this rotation be ing uniform only when the two states are orthogonal. (C) 2001 Elsevier Scie nce B.V. All rights reserved.