I am trying to configure a moderated Cf source using the source_newgen.f file. I have prepared a text file containing energy bin values and emission intensities according to ISO8529-1:2021. However, when plotting the spectrum obtained with a USRBDX scoring neutron fluence through a sphere surrounding the source point, I notice that the low-energy part of the spectrum is not correctly reproduced.
I suspect that one issue is related to the difference in binning between ISO8529-1:2021 and FLUKA, as the former uses non-uniform bin widths, while FLUKA assumes uniform binning. How can I resolve this discrepancy to achieve a correct spectrum reproduction?
I have also two more question regarding the source file itself:
To obtain an isotropic source, is it sufficient to uncomment the line call sample_isotropic_direction(direction_cosx, direction_cosy, direction_cosz) in the code?
While, If I want to define a non-isotropic source instead, how should I modify the beam dimensions? Should this be done within the source routine? Since the BEAM card seems not working.
Looking at the pdf you attached, the table specifies a histogram (note the extra energy value without an emission rate). However, the emission rate is the total number of neutrons, so you need to divide the rate with the energy with of the bin, to be able to use it in the source routine.
Then comes the other problem, the source routine samples linearly in a bin, thus creating slopes in each bin. You need to sample linearly along the logarithic value of the energy to get the same result as it is shown in the pdf file.
To make it easy, please find my modified source routine, and spectrum file attached.
Dear David,
thank you very much for your quick reply!
I run the simulation with your source file and I got the correct spectrum.
However, I wanted to ask you a clarification: you calculated the dN/dE as Bi/(Emax-Emin), but since we have logarithmic bins, shouldn’t we calculate it as Bi/log(Emax/Emin). The latter is also suggested in the ISO itself but if I apply it I don’t get the expected spectrum.
And one last question, when using an external source, the beam size has to be defined inside it? If I modify it in the BEAM card I get always the same result as in the picture below:
The source routine expects the number of particles in a bin divided by (Emax—Emin). This is because FLUKA energy spectrums are always normalized to the bin’s energy width.
For extended sources, you need to set the sampling in the source routine as well, the extended source options in the input file are ignored.
