POLARIZAti

    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

     
Notes:
         1) 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.

         2) 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

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