MCNP6.2 - Are results of FMESH tallies already divided by cell volume?

• 19matthew89
In summary, the FMESH tally results are already divided by the cell volume, as shown by the comparison with an F4 tally and the availability of the "out=cf" option. This is similar to TMESH where the average fluence is calculated by multiplying the particle weight by the track length and dividing by the volume. Although not explicitly stated in the MCNP manual, the FMESH calculation includes a division by the mesh cell volume, not the geometry cell. Therefore, the F4 and FMESH cell tallies should match if the geometry cell and mesh cell are coincident.
19matthew89
TL;DR Summary
It's not clear whether the results of mesh tally for FMESH are already divided by the cell volume
Hi everyone,

I am struggling to understand whether the results of FMESH tallies are already divided by the cell volume or not.

I'd actually expect so considering:
1. the comparison with an F4 tally in the same cell where results are comparable only if I assume that the mesh tallies results are already divided by the cell volume
2. the fact that the option "out=cf" is available. With this option "the volume and the tally results multiplied by the volume are also printed". And so it would make sense multiplying the standard tally results by the volume only if its standard output were already divided by the volume
3. for "similarity" with TMESH, where indeed "average fluence is particle weight times track length divided by volume in units of number/cm2"

But I could not find anywhere in MCNP manual (MCNP® Code Version 6.3.0 Theory & User Manual) a clear statement about it.

Could you confirm, please?

The FMESH calculation includes a division by the mesh cell volume, not the geometry cell.

So if you had a geometry cell and a mesh cell that were totally coincident you'd expect the F4 and the FMESH cell tally to match.

19matthew89
Alex A said:
The FMESH calculation includes a division by the mesh cell volume, not the geometry cell.

So if you had a geometry cell and a mesh cell that were totally coincident you'd expect the F4 and the FMESH cell tally to match.
Great! Thanks!