As point wise cross section for low energy neutron is used,
why fluka still use group wise cross section energy interval below 20 MeV by force?
Read the manual for the LOW-PWXS card, Note #1:
“The default (for backward compatibility) low-energy neutron treatment of FLUKA is the multigroup approach, with the possible exception of a few isotopes and channels as described in the LOW-NEUT card.”
So the LOW-PWXS card is required to supersede it with the new point wise treatment.
We may consider to take the latter as new default in the future.
I know the [LOW-PWXS] card,
but when the energy bin is set as this:
the result is 860 energy bin:
at energy below 20 MeV, the multigroup energy bin is used.
when the energy bin is set as this:
the result is 260 energy bin:
the multigroup energy bin is used.
it is weird.
It seems to me that this scoring is what one should expect if using the group-wise cross section treatment.
In the second case (0 < Energy < 1.5e-3), the maximum energy is below 20 MeV, therefore the 260 groups binning is used.
In the first case (0 < Energy < 0.05), the maximum energy is above 20 MeV, therefore the 260 groups binning is used below 20 MeV, while above, your binning is used.
One could confirm this hypothesis looking at your inputfile.
In fact, as recalled by Note 4 of LOW-PWXS, the group-wise binning is still used unless the point-wise treatment was requested for all isotopes.
As @amario implied, one should look at the LOW-PWXS card in your input to confirm that the latter was your case (if so, your surprise would be actually justified).