Comparison of photon flux per unit lethargy for a sphere containing element U, corresponding to an isotropic source at 5 MeV (the thick-target bremsstrahlung model is activated). Slight discrepancies occur between TRIPOLI-5 and OpenMC or PENELOPE, due to stopping power data: OpenMC and PENELOPE compute these data internally, accounting for the electron density of the material, whereas TRIPOLI-5 reads values from the NIST ESTAR database. PENELOPE was modified to ignore delta-ray and triplet production electrons, since these are not included in OpenMC and TRIPOLI-5. Figure taken from reference [36].