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

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.
  • #1
19matthew89
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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?

Thanks in advance
 
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  • #2
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.
 
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  • #3
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!
 

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

Are the results of FMESH tallies in MCNP6.2 already normalized by the cell volume?

No, the results of FMESH tallies in MCNP6.2 are not automatically normalized by the cell volume. Users need to manually divide the tally results by the cell volume if normalization is required.

How can I normalize FMESH tally results by cell volume in MCNP6.2?

To normalize FMESH tally results by cell volume, you need to calculate the volume of each mesh cell and then divide the tally result by this volume. This can be done post-processing the output data.

What is the purpose of FMESH tallies in MCNP6.2?

FMESH tallies in MCNP6.2 are used to score various quantities (such as flux, dose, or energy deposition) over a defined mesh grid. This allows for detailed spatial distribution analysis of the scored quantity within the simulation geometry.

Can I directly obtain volume-normalized results from FMESH tallies using any built-in feature in MCNP6.2?

No, MCNP6.2 does not have a built-in feature to directly obtain volume-normalized results from FMESH tallies. Users must perform this normalization manually after obtaining the tally results.

What steps should I follow to ensure accurate volume normalization of FMESH tally results?

To ensure accurate volume normalization of FMESH tally results, follow these steps: 1) Define the mesh grid accurately in the FMESH card. 2) Calculate the volume of each mesh cell based on the grid dimensions. 3) Extract the tally results from the output file. 4) Divide each tally result by the corresponding cell volume. 5) Verify the normalization by checking consistency across the mesh grid.

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