2.2.2 Monte Carlo Model
The MCNP5 Monte Carlo model geometry was defined to be equivalent to that used in the
PENTRAN model. The 60C0 source was distributed isotropically over multiple equi-probable
sources cells in one energy group. Source spectra were defined in a consistent manner with the
various multi-group libraries considered. Volumetric cell flux F4 mesh-tallies and energy
deposited F6 tallies were used in the Monte Carlo geometry description that was equivalent to
the discretized SN volumes. The MCNP5 F6 tallies were used along the central axis of the model
across the lead box, air region, and water phantom. All simulations were performed using a PC
cluster composed of 16 Dual Intel Xeon processors at 2.4 GHz with 2 Gb/processor, for a total
memory of 32 Gb. MCNP5 tallies along the central axis were obtained using variance reduction
techniques. The cell fluxes converged, on average, to within 3 % for the reference case.
2.3 Comparison of Deterministic and Monte Carlo Results
Calculations were performed to compare the SN and Monte Carlo results for the scalar
fluxes along the model central axis. Figure 2-2 depicts a 3-D scalar flux distribution computed by
PENTRAN for the reference case for different energy groups. The highest energy group (group
1) is forward peaked with some ray-effects along the central axis. However, as the radiation
down scattered to lower energy groups, it demonstrates more isotropic behavior evident in
energy groups 2 and 3. This simulation was based on the use of an S42 angular Legendre-
Chebychev quadrature (1848 directions/mesh) with P3 Scattering anisotropy using the CEPXS
library. This highlights the fidelity of the information available in a converged deterministic SN
computation.
2.3.1 Investigation of Angular Quadrature
Medical physics applications represented in SN models often require small mesh sizes,
relatively large models, and low density materials. These problems can lead to severe ray-effects,