THE EGS5 CODE SYSTEM

parameters which may be needed. The action requested is then performed and control returns to
the beginning of the loop. This continues until the control input has been exhausted.
The EGS5 code itself contains, among many other subprograms, four user-callable subroutines,
BLOCK SET, PEGS5, HATCH, and SHOWER. These routines call other subroutines in the EGS5 code,
some of which call two user-written subroutines, HOWFAR and AUSGAB, which define the geometry
and output (scoring), respectively. The user communicates with EGS5 by means of the subroutines
described above accessing variables contained in various COMMON blocks.
To use EGS5, the user must write a MAIN program and the subroutines HOWFAR and AUSGAB.
Typically, MAIN performs any initialization needed for the geometry routine HOWFAR and then sets
the values of COMMON block variables which specify such things as the names of the media to be
used, the desired cutoff energies, the unit of distance (e.g., centimeters, radiation lengths, etc.) and
so on. MAIN must also call the new initialization routine BLOCK SET, which is used primarily to
initialize data which is not defined in BLOCK DATA subprograms. At this point, MAIN may call the
new PEGS subroutine PEGS5 to prepare material data for the simulation (this may be skipped if
an existing data set is available). A call to the HATCH subroutine then “hatches” EGS5 by doing
some once-only integrity checks and initializations and by reading from a data set the material
data prepared by PEGS5 for the media requested. With the initialization completed, simulation
of the shower may begin with MAIN calling subroutine SHOWER when desired. Each call to SHOWER
results in the generation of a single EGS5 particle history, with the arguments to SHOWER specifying
the parameters (energy, position, direction, etc.) of an individual initial particle. This permits
the user the freedom to use any source distribution desired. The flow of control and data when a
user-written MAIN program is linked with EGS5 is illustrated in Figure 2.1. Detailed information
needed to write user programs is provided in the EGS5 User Manual (Appendix B of this report)
As a beginning introduction, however, the reader may find it useful to first study the series of short
tutorials provided in Chapter 3 of this document.
Both EGS5 and PEGS use MeV for the unit of energy, and all EGS5 variables reference the total
energy (i.e., kinetic plus rest mass) associated with particles. EGS5 primarily employs the CGS
system of units, but PEGS scales its outputs in units of radiation lengths for subsequent (internal)
use by EGS5. By default, the centimeter is the basic working unit of distance used in EGS5, but
other transport distance unit may be chosen by setting the variable DUNIT before calling HATCH (see
Appendix B for details).
Table 2.1 defines some of the mathematical and program symbols for entities in EGS and
PEGS, as well as other symbols used in this discussion. The first column gives the item number,
the second column shows the mathematical symbol used for the entities, and the third column
shows the FORTRAN name for the same thing in the EGS or PEGS code. A ‘P’, ‘E’, or both in
column four shows whether the item is used in PEGS, EGS, or both. 4 The fifth column contains
the definition, explanation, or name of the item.
A number of physical, mathematical, and derived constants are used by the codes. We have
4 Even though PEGS is integrated into EGS, PEGS and EGS have retained the majority of their prior distinctiveness,
sharing only a handful of variables.
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