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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