--------------------------------------- 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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