MCNP: Is it possible to make a "hole" in an fmesh tally?

[ssapp]

Hello,

I am running a MCNP calculation where there is a source in the center of a building. I have put a fmesh tally parallel to the ground so that I can see how the particles disperse throughout the building and outside the building. However, I only really care about the results from the fmesh tally voxels outside the building's walls. Is there any way I can make a "hole" in my fmesh tally so the particles aren't tracked by the fmesh inside the building?

Thank you.

 

[DEvens]

If you are doing a rectangular mesh then you can limit it to whatever rectangular region you are interested in. Then you can cover various regions with multiple tallies. From your description I would guess that to be the way to go.

If you are doing a cylindrical tally then you can't skip tallying from r=0 to some radius. The best you can do is one radial division for the part you want to ignore. It will have the same angular division at every radius, so if you have N angular divisions you will need to ignore all of them for the radius you want to ignore.

Tallying only slows the code a small amount. So having a small number of tallied voxels in places you don't care about is not a huge deal.