THE EGS5 CODE SYSTEM

(c) If 10.5 ≤ −C < 11.0 then x 0 = 1.8
and x 1 = 4.0.
(d) If 11.0 ≤ −C < 11.5 then x 0 = 1.9
and x 1 = 4.0.
(e) If 11.5 ≤ −C < 12.25 then x 0 = 2.0
and x 1 = 4.0.
(f) If 12.25 ≤ −C < 13.804 then x 0 = 2.0
and x 1 = 5.0.
(g) If −C ≥ 13.804 then x 0 = −0.326C − 2.5
and x 1 = 5.0.
6. a = [−C − (2ln 10)x 0 ]/(x 1 − x 0 ) 3 .
The final stage in determining the density effect correction for gases is to scale the results in
terms of the gas pressure since the tabulated values are all given for NTP (0 ◦ C, 760mm Hg). If the
pressure correction factor is
then
and
GASP = actual pressure/pressure at NTP
C(GASP) = C(NTP) + ln(GASP)
x 0 (GASP) = x 0 (NTP) − 1 2 log 10(GASP)
x 1 (GASP) = x 1 (NTP) − 1 2 log 10(GASP) .
PEGS routine SPINIT initializes the stopping power routines for a particular medium. Routine
SPIONB, which is a function of Ĕ0, AE, and POSITR, evaluates Equation 2.257 for a positron if POSITR
is true, and for a electron if POSITR is false.
Note that limitations on the applicability of EGS5 for simulating electron transport at low
energies derive primarily from the breakdown of the Bethe-Bloch formula for the stopping power.
The presence in Equation 2.257 of a term containing the logarithm of the ratio of the electron
energy and the mean ionization energy implies that there must exist some energy (dependent on
Z) at which the Bethe-Bloch expression will yield physically unrealistic negative values. In addition,
Equation 2.257 was derived under approximations which are strictly valid for high energy particles,
large energy losses and large recoil energies, so the accuracy of the Bethe-Bloch formula begins to
wane at energies ranging from around 1 keV for low Z materials to several tens of keV for high Z
materials.
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