Quantum spin-flip transitions are of great importance in the synchrotron radiation (SR) theory. For better understanding of the nature of this phenomenon, it is necessary to except the effects connected with the electric charge radiation from observation. This fact explains the suggested choice of the spin-flip radiation model in the form of radiation of the electric neutral Dirac-Pauli particle moving in the homogeneous magnetic field. It is known that in this case, the total radiation in the quantum theory is conditioned by spin-flip transitions. The idea is that spin-flip radiation is represented as a nonstationary process connected with spin precession. From this point of view, we shall shown how to construct a solution of the classical equation of spin precession in the BMT theory having the exact solution of the Dirac-Pauli equation. Thus, one will find the connection of the quantum spin-flip transitions with classical spin precession.According to the uncertainty principle, for the spin-flip transition with the characteristic frequency of SR [1] we obtainwhere µ is the magnetic moment, ρ is the radius of curvature of the relativistic electron trajectory in the homogeneous magnetic field H. The ratio of the transition time ∆t and the time of the radiation forming ∆t on the circular arc dl ≈ γρ is equal ∆t/∆t ≈ 2πγ −4 ≪ 1.That means that in the ultrarelativistic case, spin-flip transitions are practically noninertial, that is why the electron motion can be considered for the time ∆t as uniform and rectilinear.Let us consider the electric neutral Dirac-Pauli particle, which moves in the homogeneous magnetic field H = (0, 0, H). As is known [2], wave function of the above particle has the form Ψ(r, t) = L −3/2

A procedure based on the semiclassical approximation for high energy levels is developed to yield solutions to the classical equation of charge motion and to the Bargmann-Michel-Telegdi spin equation. To this end, exact solutions to the Klein-Gordon and the Dirac-Pauli equations are used. The essence of the procedure under review is that the quantum state of a charged particle in a homogeneous magnetic field is represented as a superposition of states corresponding to the neighboring energy levels. As a consequence, the expectation values of the momentum and spin operators with respect to the resulting nonstationary wave function (packet) strictly obey the classical equations of charge motion and spin precession.

The problem of the uniqueness in the introduction of spin operators in the synchrotron radiation (SR) theory is discussed. For this purpose we give the invariant spin projections on the basis of the spin projections in the rest frame. The spin equations are used to construct the integrals of motion in the presence of the external electromagnetic field.PACS 03.65.Sq -Semiclassical theories and applications.

The structural composition and the properties of the first quantum spin-orientation-dependent correction to synchrotron radiation power are discussed. On the basis of spin mass renormalization it is shown that, in the conventional sence, the Thomas precession is not a source of relativistic radiation. This conclusion is in agreement with well-known statements on the spin dependence of mass and purely kinematic origin of Thomas precession.Comment: 7 pages, LATEX, to be published in Nucl. Instr. and Meth. A (2001

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