When comparing *F8 (p mode) transport and *F8 (p, e mode) transport, (Figure 4-5) shows
significant differences in the outcomes for the same simple model. This directly results from
accessing different calculation modes in MCNP5 with regard to electron physics. If electron
transport is turned on (mode p, e), then all photon collisions except coherent scatter lead to
electrons that are banked for later transport. Alternatively, if electron transport is turned off
(achieved by omitting the 'e' on the MCNP5 Mode card), then a "thick-target bremsstrahlung"
model (TTB) is used. This model generates electrons, but assumes that they are locally "slowed
to rest". The TTB production model contains many approximations compared to models used in
actual electron transport. The choice of which tally is to be used and the detailed physics
treatment by which the Monte Carlo code is executed is essential to the success of the EDK-SN
method. In conducting computations for generation of the EDKs, we used the full detailed
physics treatment, implementing a *F8 (p, e mode) tally, as opposed to the alternative methods
that lead to less robust electron physics, essentially employing a collisional kerma. A detail
discussion on photons interactions in matter is presented in Appendix A.
4.4 EDK -SN Code System Methodology
4.4.1 Net Current
For any SN calculation, the PENTRAN discrete ordinates code preserves angular information
explicitly in parallel data storage arrays. This allows one to calculate Jser and to apply the pre-
computed electron absorbed dose kernel along the net current direction, Figure 4-6, where:
J =J i^+J 7+J. k
net nx ny =(4-3)
J, = J J-,
(4-4.a)