THE EGS5 CODE SYSTEM

inside the shower call loop.
B.4.9 Output of Results (Step 9)
This step is self-explanatory, and is included only for the sake of completeness.
B.5 Specifications for HOWFAR
EGS calls user code HOWFAR when it reaches the point at which it has determined, because of stepsize
specifications and/or interaction probabilities, that it would like to transport the top particle
on the stack a straight line distance USTEP in the current media. All of the parameters of the
particle are available to the user via COMMON/STACK/ as described earlier. The user controls the
transport upon return to EGS by altering one or more of the following variables: USTEP, IDISC,
IRNEW, and DNEAR(NP). Except for DNEAR (which is in COMMON/STACK/), these are available to the
user via COMMON/EPCONT/. The ways in which these variables may be changed and the way EGS will
interpret these changes is discussed in detail below. (Note, flow diagrams for subroutines ELECTR
and PHOTON have been included in Appendix A of SLAC-R-730/KEK-2005-8 for the user who
requires a more complete understanding of what actually takes place during particle transport.)
IDISC If the user decides that the current particle should be discarded, then IDISC must be set
nonzero (the usual convention is to set IDISC=1).
A positive value for IDISC will cause the particle to be discarded immediately. A negative value
for IDISC will cause EGS to discard the particle when it completes the transport. EGS initializes
IDISC to zero, and if left zero no user requested discard will take place. For example, the easiest
way to define an infinite, homogeneous medium is with the HOWFAR routine:
subroutine howfar
return
end
In this case, particle transport will continue to take place until energy cutoffs are reached. However,
a common procedure is to set IDISC=1 whenever the particle reaches a discard region, e.g.outside
the problem geometry.
USTEP and IRNEW If immediate discard has not been requested, then the HOWFAR should check
to see whether transport by distance USTEP will cause a region boundary to be crossed. If no
boundary will be crossed, then USTEP and IRNEW may be left as they are. If a boundary will be
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