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)
|WHAT(1)| > 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 = -2.0 (no polarisation)
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 = -2.0 (no polarisation)
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 (polarisation not orthogonal to
the direction of 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 the
direction of the primaries)
WHAT(5) = polarisation fraction (see explanation in WHAT(6) below)
< 0.0: resets the default = 1.0
> 1.0: resets the 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(1,2,3)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(1,2,3) and
the remaining fraction (1 - WHAT(5)) are not polarised
>= 1.0 : a fraction WHAT(5) of beam particles are linearly
polarised in the direction described by WHAT(1,2,3) and
the remaining fraction (1 - WHAT(5)) are polarised in
the direction orthogonal to both the beam and
that described by WHAT(1,2,3)
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(1,2,3), and the remaining
fraction is not polarised)
SDUM : not used
Default (option POLARIZAti not given): photons are not assumed to be
polarised
Note: 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 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
