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