THE EGS5 CODE SYSTEM

arrived at their values in a very mnemonic way by means of the PEGS subroutine called PMDCON(Physical, Mathematical, and Derived CONstants). These are items 1 through 16 in Table 2.1.As illustrated in items 17 through 27, cupped ( ˘ ) energy variables are in MeV, cupped distancevariables are in radiation lengths, and uncupped quantities denote CGS units.Materials are specified by giving density, ρ, numbers of elements, N e , atomic symbols of theelements, and the proportions of the atoms which are of each type, p i . PEGS maintains tablesof atomic numbers, Z i , and atomic weights, A i , for elements 1 through 100. For cases in whichmaterials are composed of single elements, PEGS has a table of default densities which it uses.For materials which have non-standard isotopes or densities, the values supplied by PEGS maybe overridden. As an alternative to specifying the atomic proportions by number p i , they may begiven in terms of weight, by ρ i . The parameters used to define media are summarized as items 28through 33 in Table 2.1.The PEGS subroutine, MIX, uses the material parameters described above to compute additionalmolecular parameters (items 34 through 53 in Table 2.1) useful in computing cross sections formixtures, as will be seen in subsequent sections.33