11} Collision tape

 ********************************************************************************

 A "collision tape" is a file where quantities describing selected events
 are recorded in the course of a FLUKA run.

 This file is the standard output of the MGDRAW user routine, that can be
 customised by the user to get different and/or more complete output
 (see description of user routine MGDRAW in Chap. 13}).

 Note that "event" would be a more appropriate word than "collision",
 and "file" better than "tape". For historical reasons, however,
 the expression "collision tape" is used in Monte Carlo jargon rather
 than "event file". It is true that most interesting events are generally
 collision events (but also boundary crossings, decays, etc.), and that
 the large size of the file may require the use of a magnetic tape (or at
 least, that was often the case in the past).
 Recently, the expression "phase space file" has also been used.

 There are several reasons for which the user might decide to write a
 collision tape. Some examples are:

  * to perform a non-standard analysis or scoring. In general, this is
    not recommended because the available FLUKA scoring facilities are
    reliable, efficient and well tested. However, there may be special cases
    where a user-written scoring is necessary.
  * to save details of transport for a new independent analysis. In this
    case, however, the user must make sure that no phase-space region
    of interest be undersampled because of biasing options in the
    corresponding run. As a general rule, writing of a collision file is not
    recommended in non-analogue (biased) calculations.
  * to connect FLUKA to other radiation transport codes (now less
    likely than in the past, since FLUKA covers most energy ranges
    and transports most particles which can be of interest).
  * to split the transport problem in two or more sequential phases.
    A technique used in deep penetration calculations, which can be
    considered as an extension of splitting, consists in recording
    all particles crossing a given boundary (with their energy,
    weight, coordinates and direction cosines at the point of crossing),
    and to sample repeatedly source particles from that set in a
    successive run (Fas87). A special subroutine SOURCE (see 13} and
    option SOURCE) must be written for this purpose. The user must make
    sure that the final normalisation is done with respect to the total
    particle weight used in the first step of the procedure, and not
    to that of the second step. It is also recommended to assign blackhole to
    all materials immediately beyond the recording boundary, to avoid that
    backscattered particles be considered twice.
  * to perform some manipulation in an intermediate phase of Monte
    Carlo transport. An example is a variation of d), in which a user
    program interpolates some smooth analytical distribution through
    the recorded quantities, from which source particles are sampled
    in the next FLUKA run.
  * to trace suspected errors in transport
  * to connect to a graphical display program

 FLUKA allows to write a complete dump of each source particle, of each
 trajectory and of each energy deposition event, possibly under event-driven
 conditions specified by the user (see description of user routine MGDRAW
 in 13}).

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