In the description of the theory behind the component functionality we will use the usual symbols r for the position \((x,y,z)\) of the particle (unit m), and v for the particle velocity \((v_x, v_y, v_z)\) (unit m/s). Another essential quantity is the neutron wave vector \(\textbf {k} = m_\textrm {n} \textbf {v}/\hbar \) , where \(m_\textrm {n}\) is the neutron mass. k is usually given in Å\(^{-1}\), while neutron energies are given in meV. The neutron wavelength is the reciprocal wave vector, \(\lambda =2 \pi / k\). In general, vectors are denoted by boldface symbols.
Subscripts "i" and "f" denotes “initial” and “final”, respectively, and are used in connection with the neutron state before and after an interaction with the component in question.
The spin of the neutron is given a special treatment. Despite the fact that each physical neutron has a well defined spin value, the McStas spin vector s can have any length between zero (unpolarized beam) and unity (totally polarized beam). Further, all three cartesian components of the spin vector are present simultaneously, although this is physically not permitted by quantum mechanics. For further details about polarization handling, you may refer to the Polarisation chapter, 15.14.