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2.2.5} 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):
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8
BEAM 50.E+00 PROTON In our example, the beam starting point is given by:
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8
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 cordinates: 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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