---------------------------- The purpose of several FLUKA input options is to trigger calls to user routines (user routines are described in Chap. 13}). One of the most important ones is SOURCE, which makes FLUKA get the characteristics of its primary particles from subroutine SOURCE instead of from options BEAM and BEAMPOS. This option allows to pass to the subroutine several parameters, thus allowing to drive it from input without the need to re-compile it. Note that even when using a user-written source, it is still necessary to have in input a BEAM card indicating the maximum expected energy of a primary particle, so that the program can prepare appropriate cross section tables. If command SOURCE is present, but no SOURCE routine has been linked, the default one in the FLUKA library will be called, which leaves unchanged the particle type, energy, position etc. as defined by BEAM and BEAMPOS. Command USERWEIG can call 5 different user routines used to modify a scored quantity (at the time of scoring). The routines are: * FLUSCW is a function returning a multiplication factor for fluences. A typical application is to convert a fluence to dose equivalent. * COMSCW is a function returning a multiplication factor for star densities and doses. Common application: converting energy deposition to dose. * USRRNC is a subroutine providing a convenient user hook for scoring residual nuclei. * ENDSCP is a subroutine performing a displacement of the energy deposited in a particle step, for instance to account for an instrument drift. * FLDSCP is a subroutine performing a displacement (drift) of the track corresponding to a particle step. Complex magnetic fields can be defined or read from a map by a user routine MAGFLD. Calls to the routine are activated by command MGNFIELD. A collision file (also called a collision tape, or a phase space file) is a file on which FLUKA writes on request details of user-selected events: particle trajectories, energy deposition events, source particles, boundary crossings, physical interactions, etc. This task is performed by subroutine MGDRAW, which is called if option USERDUMP is requested in input. The default routine present in the FLUKA library can be driven as it is, without re-compilation, by setting some of the USERDUMP parameters, but can also be modified and re-compiled to adjust to specific needs of the user. A typical simple task is to draw particles trajectories. Another frequent application of USERDUMP is to perform a calculation in two steps, where the second step uses the collision file as a source. In principle it is also possible to use subroutine MGDRAW for scoring, for instance by interfacing it to some histogramming package, as it is customary in some other Monte Carlo programs. However, in general this is discouraged in FLUKA, unless the desired quantity cannot be scored via the standard FLUKA input commands, which is very rare. The FLUKA scoring options are indeed highly optimised and well checked against possible errors and artefacts. It is very unlikely that a user might be able to achieve in a short time the same level of reliability. In any case, user-written scoring via MGDRAW MUST be avoided in all runs where biasing is present, because to handle correctly the particle weights requires other FLUKA tools which are not available to the normal user. Three more input options activating calls to user routines are USRICALL, USROCALL and USRGCALL. The first two allow the user to issue a call respectively to an initialisation routine USRINI and to an output routine USROUT. The third one activates a call to a routine USRGLO, which performs a global initialisation before any other made by FLUKA.
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