5  Polarisation in McStas

McStas provides a set of components for simulating neutron beam polarisation and the optical elements that manipulate it: polarising monochromators, mirrors and guides, magnetic-field regions (including spin flippers and spin-echo coils), and polarisation-sensitive monitors. This chapter originated as an appendix written by P. Christiansen documenting the first polarisation components (2006); it is now promoted to a full chapter and brought up to date with the current McStas 3.x component set and the pol-lib field/precession framework that underlies it.

We first describe the polarisation vector and how it is related to the neutron wave-function (section 5.1) and then the physics of the scattering processes relevant to polarisation (section 5.2). Section 5.3 describes in detail how McStas propagates the polarisation vector through magnetic fields – the numerical precession algorithm and the field functions available – since this underlies every magnetic-field component in the library. Section 5.4 then documents the current polarisation components themselves, organised by role, and section 5.5 lists example and test instruments, including a worked spin-echo example.

We rely heavily on the books [Lov84; Wil88] for the underlying physics, where the detailed calculations can be found.

The notation used here (and in [Wil88]) is \(P\) (scalar), \(\PB \) (vector), \(\tP \) (unit-vector), \(\sigmaH \) (operator), and \(\sigmao \) (vector of operators).