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7.2} Definition of the radiation source

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 The simplest particle source is pointlike, monoenergetic and monodirectional,
 that is, a "particle beam". Option BEAM, fully described later, is used to
 define the particle type and momentum (or energy). If desired, this option can
 also define an energy spread, a beam profile shape and an angular divergence.
 However, the two latter distributions are restricted to a beam directed in
 the positive z direction: to describe divergence and beam profile for an
 arbitrary beam direction it is necessary to define a beam reference frame by
 means of option BEAMAXES.

 The energy declared with BEAM is used by the program to initialise
 cross section tables and other energy-dependent arrays: therefore that command
 must always be present, even when a more complex source is described by means
 of a user routine.

 The particle starting point and direction are declared by means of option
 BEAMPOS. If BEAMPOS is not present, the beam particles are assumed to start
 from the origin of the coordinates 0., 0., 0. and to be directed along the z
 axis. It is important that the starting point be not on a boundary and not
 inside a blackhole region. In many cases, starting in vacuum upstream of the
 actual target can be convenient.

 Both BEAM and BEAMPOS commands can be placed anywhere in the input file, before
 the START command.

 Particle sources with more complicated features (with arbitrary distribution in
 energy, space, angle, time, and even with more than one type of particle) can
 be described by a user-written subroutine SOURCE. To call it, a command SOURCE
 must be present in input.

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