FLUKA: 2.3.5} Definition of the primary particles
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2.3.5} Definition of the primary particles
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All "events" or "histories" are initiated by primary particles, which in the
simplest case are monoenergetic, monodirectional and start from a single
point in space (pencil beam). The card BEAM defines the particle energy (or
momentum) while the card BEAMPOS controls their starting position and
direction. These two commands can be used also to define particle beams
having a simple angular or momentum distribution (Gaussian or rectangular),
or a simple transverse profile (Gaussian, rectangular or annular), or a
simple space distribution of starting points (spherical, cartesian or
cylindrical shell). Isotropic or semi-isotropic angular emission can be
described as a special case of an angular rectangular distribution.
For particle sources with more complex distributions in energy, space and
direction, the user must write, compile and link a special routine (SOURCE),
following the instructions given in Chap. 13}, and input a card SOURCE.
A summary of the input data concerning primary particles is printed in the
standard output under the title "Beam properties".
The beam definition for our example can be the following (monochromatic,
monodirectional proton beam of momentum 50 GeV/c):
BEAM 50.E+00 PROTON
In our example, the beam starting point is given by:
BEAMPOS 0.0 0.0 -50.0
In the cartesian geometry used by FLUKA, the previous card means that the
beam is injected at x, y, z coordinates: 0, 0, -50 cm and is directed along
the positive z axis. Of course, the choice of the point of injection, the
direction, etc., must be coherent with the geometrical description of the
set-up, as discussed in the following section.
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