Special source: synchrotron radiation

Synchrotron radiation as a source can be specified via command SPECSOUR ​ and SDUM = SYNC–RAD, SYNC–RDN, SYNC–RAS, or SYNC–RDS.

Synchrotron radiation photons are assumed to be emitted by a particle (most commonly an electron or a positron, but any charged particle is allowed), moving along one or two circular arcs or helical paths. The emitting particle is not transported. The bending is assumed to be due to a magnetic field of intensity and direction specified by the user, but no magnet needs necessarily to be described in the geometry, and the magnetic field must not be declared with command MGNFIELD nor assigned to any region with command ASSIGNMAt.​ ​

The program samples energy and direction of the synchrotron radiation photons from the proper energy and angular distributions. Polarisation is implemented as a function of emitted photon energy. ​ The emitting particle can have any direction with respect to that of the magnetic field. Therefore, photon emission can occur along arcs (if the particle direction is perpendicular to the magnetic field) or helical paths in the more general case.

The user must specify several parameters:

  • the emitting particle type and energy (or momentum)

  • the particle direction, at the beginning of the first and possibly of the second arc or helical path

  • the magnetic field intensity (or alternatively the curvature radius of the particle trajectory)

  • the direction of the magnetic field

  • the lower limit of the photon energy spectrum

  • the length of one of the trajectory arcs or helical paths (the other path, if present, having the same length)

  • the starting points of the first path, and if present, of the second path

The SPECSOUR command for synchrotron radiation extends over two cards. The input parameters are:

First card:

WHAT(1) =

particle emitting the radiation

Default

= 3.0  (electron)

WHAT(2) >

0.0: emitting particle momentum (GeV/c

< 0.0:

kinetic energy of the emitting particle (GeV)

WHAT(3) >

0.0: curvature radius of the emitting particle trajectory (cm)

< 0.0:

absolute value of the bending magnetic field (T)

WHAT(4) =

lower limit of the photon energy spectrum (GeV)

Default

= 10\(^{-7}\)  GeV

WHAT(5) =

x-component of the magnetic field versor

WHAT(6) =

y-component of the magnetic field versor

SDUM
: SYNC–RAD if the z-component of the magnetic field versor is > 0.0,
SYNC–RDN if the z-component of the magnetic field versor is < 0.0
SYNC–RAS if the z-component of the magnetic field versor is > 0.0
and magnetic field of second arc (if defined) changes sign with respect to that of the first arc.
SYNC–RDS if the z-component of the magnetic field versor is < 0.0
and magnetic field of second arc (if defined) changes sign with respect to that of the first arc.

Continuation card:​

WHAT(1) =

length of the emission arc or helical path (cm).

Default

= 100.0  cm

WHAT(2) =

x-coordinate of the starting point of a possible second path of same length (see Note 1))

WHAT(3) =

y-coordinate of the starting point of the second path (see Note 1))

WHAT(4) =

z-coordinate of the starting point of the second path (see Note 1))

WHAT(5) =

x-component of the emitting particle direction versor at the beginning of the second path (see Note 1) and 2))

WHAT(6) =

y-component of the emitting particle direction versor at the beginning of the second path (see Note 1) and 2))

SDUM

= “ & ” in any position in columns 71–78 (or in last field if free format is used)

Second continuation card (optional)

WHAT(1)

= energy threshold (GeV), below which the produced photons must be treated as optical photons. The standard ElectroMagneticFluka (EMF) treatment cannot be applied to photons with energies below 100 eV. Photons with energy below the transport threshold (which cannot be lower than 100 eV) are immediately absorbed as soon as they reach a material different from vacuum. Lower energy photons can still be transported if treated as optical photons; in that case the user must define the relevant optical properties of the various materials using the OPT… options. By default this threshold is set to 0 GeV WHAT(2-6) = not used

Note

  1. The starting point of the first arc or helical path as well as the initial direction of the emitting particle must be defined in the BEAMPOS card.​

  2. The z-component of the emmitting particle direction versor for the 2nd arc takes the same sign as the one of the 1st arc.