MCNP can tally reactions in materials that aren't present in the geometry as long as they are on a material card. These are often used for activation foil tallies of reaction rates since the foil is so small that it doesn't make sense to put it in the model explicitly.
MCNP can tally reactions in materials that aren't present in the geometry as long as they are on a material card. These are often used for activation foil tallies of reaction rates since the foil is so small that it doesn't make sense to put it in the model explicitly.