This is a brief explanation of the fluxes file and its structure when using it in FISPACT-II. This follows from several questions regarding how it is structured and its usage.
The Fluxes File
When interested in studying activation and transmutation due to particle irradiation (not just decay), FISPACT-II requires an incident particle spectrum to be defined. This spectrum, which must be defined in the "fluxes" file (it can be called something else but this must be indicated in your files file), must match the same energy group structure as your nuclear data library. Because of this you do not need to define the energy bounds in the fluxes file, the entries must match your nuclear data group structure.
For example, TENDL 2017 that is provided with FISPACT-II version 4.0 is given in 709 group (https://fispact.ukaea.uk/wiki/CCFE-709_group_structure) therefore your spectrum should be binned in this structure (709 group has 710 energy values and thus 709 values - it is a histogram). These values are dimensionless and are normalised to 1. The flux value (from FLUX in input file) is specified in n /cm2 /s and scales these values when doing the collapse.
These values can be specified in any format (one per line, or many per line).
One important note is that values in the fluxes file are specified in descending energy (legacy reasons), so the highest energy bin (for TENDL 2017 this is 1 GeV) must be specified first.
The wall loading value (MW / m2) is needed after specifying these values and then a string representing the flux name (not really used anywhere but needed to run). These should be on newlines after the spectrum values are defined.
Let's look at an example:
For the sake of simplicity let's assume my nuclear data is 2 group, with bounds [1.0e-5, 1.0e2, 1.0e9] eV.
My spectrum is then 0.4 between 1e-5 and 1e2 eV and 0.6 between 1e2 and 1e9 eV, and my wall loading is 1.0 (not needed), and I call this "my flux".
My fluxes file would read:
The same logic applies to other groups (175, 709, 1102, etc).
Now it is typical that your input spectrum was generated via a transport code and is likely not binned in the same energy structure as the nuclear data (say your flux is in 175 and you're using TENDL2017 which is in 709). In this case you would need to use the arb_flux file with GRPCONVERT (https://fispact.ukaea.uk/wiki/Keyword:GRPCONVERT). Using this, you would need to define the energy structure with the values. If this applies to your case, please review the link for the GRPCONVERT keyword and add comments here if it is still unclear.
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