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7.1} Basic commands

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 Most FLUKA commands are optional, and if anyone of them is not used an
 appropriate set of defaults is provided. A few commands, however, are nearly
 always needed in order to provide a meaningful definition of the problem to be
 studied.

 In general, for a problem to be fully determined, the following elements need
 to be defined:
    1) the radiation source
    2) the geometrical layout
    3) the materials
    4) the requested results
    5) setting of parameters, accuracy, conditions, and in general technical
       directives to the program on how the calculation shall be performed

 Defaults are provided in FLUKA for all the above features, but those for
 items 1), 2) and 3) are unlikely to be useful: therefore the few commands used
 to define source, geometry and materials are practically always present in the
 input file.

 For what concerns item 4), the user has a choice of several options to request
 the estimation of various radiometric quantities. Of course, there is no much
 point in running the program without requesting any result, but in a phase of
 input preparation it is quite common to have a few runs without any scoring
 commands. A typical minimum input containing only specifications for the above
 items 1), 2) and 3) will still produce some useful information. Looking at the
 standard FLUKA output, the user can do several consistency checks, and can get
 some better insight into the problem from the final statistics and energy
 balance.

 The last part of problem definition, element 5) (setting) is important but is
 supported by very robust defaults. In many cases, the only user concern should
 consist in choosing the right set of defaults.  However, there are some
 applications which require explicit setting commands, for instance to request
 photonuclear reactions for electron accelerator shielding.


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