Point wise and group wise neutron cross section

Dear expert,

In a previous simulation, I have generated a thermal flux like the attached figure and used that fluence for estimating Mo production from U-235.

The spectra was read with an energy interval of 6.25E-06 GeV. So in this case, although the neutron energy was sampled at different eV values, has it still used group wise cross section?

Also, now since point wise cross sections are available, do we need to only add a blank LOW_PWXS card like this ?



Dear Riya,

Indeed the definition of the LOW_PWXS card as you did is correct if you want to activate the point-wise treatment to all materials in your simulation. By enabling the card this way there is no call for group-wise cross section treatment.

Couple of remarks on your input:

  • The definition of Am is not needed in the MATERIAL cards, it is enough to provide Z and A values.

  • As you are enabling the LOW_PWXS card for all materials, the LOW-MAT cards are no longer needed for 238U and 235U.

Little note, the maximum neutron energy in your input spectra is less than 1 eV while you want to sample at intervals of 6.25e-6 GeV which stands for 6.25e3 eV.

Hope this helps you.
Kind regards

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Thank you @msabateg for the response. And I am sorry for my mistake that I wrote the energy interval wrong (it should be eV, not GeV).

In my source file, I am reading the spectrum in the following way:

momentum_energy = sample_spectrum( “neutron_spectrum.txt”, “eV” )

Few of the spectrum is something like this: (unit: ev… #/cm^2)
6.250000e-06 3.73707e+11
1.250000e-05 7.47330e+11
1.875000e-05 1.12087e+12
2.500000e-05 1.49432e+12
3.125000e-05 1.86769e+12
3.750000e-05 2.24097e+12
4.375000e-05 2.61417e+12
5.000000e-05 2.98728e+12
5.625000e-05 3.36030e+12
6.250000e-05 3.73325e+12
6.875000e-05 4.10610e+12

6.249875e-01 4.23782e+11
6.249937e-01 4.23738e+11

Does FLUKA point wise treatment have this much low neutron energy cross section or is there any lower limit for this? Because then accordingly I can use binning in my spectra.



Dear Riya,

The lower energy limit for the point wise cross section treatment is the same as the lower limit for neutron transport in FLUKA, i.e., 1e-14 GeV that corresponds to 1e-5 eV.

As for the binning, you can use the one you think it is better for your simulations. The energy of the spectra does not need to match the energies of the point wise cross sections data base since a linear interpolation is performed between consecutive points with an uncertainty less than 1/1000.

Kind regards.

Thank you @msabateg for the guidance. The topic is now clear to me.