THE EGS5 CODE SYSTEM

and x 0 , x 1 , a, m s are parameters obtained by fits made to values of δ as a function of energywhich were explicitly calculated from atomic oscillator strengths for specific materials. There aretwo different sets of parameters available[164, 166] which are both based on the same program tocalculate δ [20]. The stopping power routines in PEGS4 were revised at a time when only the 1982data was available[166] and hence PEGS contains the parameters from this reference for the 73materials given in Table 2.3. The composition of the materials is given elsewhere (e.g., Table 5.5 ofBerger and Seltzer[20] or Table 7 of Seltzer and Berger[151]). These parameters differ from those inthe more extensive list of 278 materials presented in 1984 by Seltzer and Berger[152]. However, thisis the result of a different fitting procedure being used and does not lead to significantly differentvalues of δ [164].Since there are such a variety of density effect parameters available, version 4 of PEGS wasmodified to allow the user to override the PEGS defaults and to specify directly the six parametersneeded for any material being considered. However, as the six parameters used in PEGS merelyparameterize δ, the stopping powers calculated with these values will be slightly different (≤ ±0.5%)from those calculated directly from Berger and Seltzer’s original δ values (e.g., see Figure 1 ofSeltzer and Berger[152]). Because some benchmarking application require stopping power valueswith extremely fine precision (see, e.g., [101] or [137]), a further override option was developed forPEGS by Duane et al.[52] in which the values of δ and Īadj used in [20] can be used in PEGS. Whenthis option is invoked (through the PEGS input flag EPSTFL), PEGS reproduces exactly values ofcollision stopping power found in ICRU Report [79]. Provided with the implementation of thisoption in PEGS are values of δ and Īadj for 100 elements and 345 compounds.For materials not included in Table 2.3 or for which the density effect parameters or values arenot input, the general prescription of Sternheimer and Peierls[165] is used as follows:1. The value of Īadj is determined as described above.2. m s is always taken as 3.3. C is given by Equations 2.273 and 2.274.4. For solids and liquids:(a) If Īadj < 100 eV, then x 1 = 2.0 andx 0 = 0.2 when −C < 3.681 or x 0 = −0.326C − 1 otherwise.(b) If Īadj > 100 eV, then x 1 = 3.0 andx 0 = 0.2 if −C < 5.215 or x 0 = −0.326C − 1.5 otherwise.5. For gases:(a) If −C < 10.0 then x 0 = 1.6and x 1 = 4.0.(b) If 10.0 ≤ −C < 10.5 then x 0 = 1.7and x 1 = 4.0.80