POLARIZAti¶

Summary

Defines the polarisation of a photon beam or source and activates transport of polarised photons.

|WHAT(1)| ≤ 1.0:: x-axis cosine of the beam polarisation vector (electric vector in case of photons)
> 1.0:: resets the default (no polarisation)

This value can be overridden in user routine SOURCE by assigning a value to variable UBMPOL
Default: = -2.0 (no polarisation)
WHAT(2) =: y-axis cosine of the beam polarisation vector

This value can be overridden in user routine SOURCE by assigning a value to variable VBMPOL
Default: = 0.0
WHAT(3) =: z-axis cosine of the beam polarisation vector

This value can be overridden in user routine SOURCE by assigning a value to variable WBMPOL
Default: = 0.0
WHAT(4) :: flag for relative direction of beam and polarisation
≥ 1.0:: the polarisation is orthogonal to the direction of the primary photons
< 1.0:: resets the default (the polarisation is not orthogonal to the direction of the primaries)

This value can be overridden in user routine SOURCE by assigning a value to the logical variable LPPERP
Default: = 0.0 (the polarisation is not orthogonal to direction of the primaries)
WHAT(5) =: polarisation fraction (see explanation in WHAT(6) below)
< 0.0:: resets to default = 1.0
> 1.0:: resets to default = 1.0

This value can be overridden in user routine SOURCE by assigning a value to variable POLFRA
Default: = 1.0 (fully polarised in the direction described by WHAT(1) – WHAT(3))
WHAT(6) :: flag for interpreting WHAT(5):
≤ 0.0:: a fraction WHAT(5) of beam particles are linearly polarised in the direction described by WHAT(1) – (3) and the remaining fraction (1.0 - WHAT(5)) are not polarised
≥ 1.0:: a fraction WHAT(5) of beam particles are linearly polarised in the direction described by WHAT(1) – (3) and the remaining fraction (1.0 - WHAT(5)) are polarised in the direction orthogonal to both the beam and that described by WHAT(1) – (3)

This value can be overridden in user routine SOURCE by assigning a value to the logical variable LPFRAC
Default: = 0.0 (only a fraction WHAT(5) of the photons is polarised as indicated by WHAT(1) – WHAT(3), and the remaining fraction is not polarised)
SDUM: : not used
Default: (option POLARIZAti not given): photons are not assumed to be polarised

Notes

Polarisation direction defined by option POLARIZAti is meaningful only if the beam direction is along the positive z-axis, unless a command BEAMAXES is issued to establish a beam reference frame different from the geometry frame (see p. BEAMAXES).
The program takes care of properly normalising the cosines unless they are badly unnormalised (in the latter case the code would reset to no polarisation). If WHAT(4)≥ 1.0, the code makes sure that the two vectors are orthogonal within the minimum possible rounding errors.
What polarisation means is dependent on the physics implemented in the code: for the moment the only polarisation dependent effects are Compton, Rayleigh and photoelectric effect for photons, where of course the polarisation vector represents the electric field direction and must be normal to the beam direction.

Example:

* Synchrotron radiation beam with m_e/E mrad x,y divergence (produced by a 3 GeV
* electron beam). The actual spectrum is provided by a a user-written source
* (E_max = 500 keV). Photons are fully polarised in the horizontal (y) direction
* and the polarisation is orthogonal to the direction of the primary photons
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8
DEFAULTS                                                              EM-CASCA
BEAM        -500.E-6       0.0 1.7033E-4       0.0       0.0       1.0PHOTON
SOURCE           0.0       0.0       0.0       0.0       0.0       0.0
POLARIZA         0.0       1.0       0.0       1.0       1.0       0.0