I recently used the RESNUCL card to score the radionuclide activities produced over an irradiation period. I had a card for scoring all pathways, one for spallation and one for low energy, however the low energy entries were all zero. We confirmed that the all scoring included low energy neutron interactions but we were unsure why the low energy output was not reflective of this. It would be good to understand why this was the case and what could be done so that the low energy contribution could be scored separately.
Could you provide also the executable you used, and output files?
As an initial thought, you should include a PART-THR card for neutron transport down to thermal energies (1e-14 GeV).
I don’t know if its intentional but you are only scoring RESNUCLEi in regions SP_TARG and U0000, whereas for your USRTRACK you score more broadly. This isn’t a problem if its intended!
To score the low energy contribution separately I would suggest to use MGDRAW and define a filter there to score the productions from low energy.
I tried to reproduce your problem out of curiosity because I thought the low-energy option of RESNUCLEi could be quite useful.
However, it seems to be restricted to multigroup-transport for low-energy neutrons, i.e. it does not work with point-wise transport. This may also be stated in the manual page for RESNUCLEi, depending on how you interpret the documentation of WHAT(1) = 2.0. If it is truly limited to multigroup transport, I think that the documentation could be clearer. At the moment, you could interpret the text as “as long as the neutron energy is below the threshold for multigroup treatment, reaction products will be scored, even if point-wise treatment is activated.”.
I have attached the simple simulation I used to investigate the problem. A 1-MeV neutron pencil beam is moderated in water and used to activate a cadmium sample. One expects a large amount of residual nuclei from Cd-113 (n,gamma) Cd-114 neutron-capture reactions which are clearly low-energy products. However, the DEFAULTS setting PRECISIOn, which uses point-wise cross sections, prevents low-energy neutron products from being scored. Therefore, the easiest way to get nonzero scores is to change DEFAULTS to, e.g., SHIELDINg which uses multigroup transport (see documentation of DEFAULTS).
Personally, I found that the point-wise treatment activated by PRECISIOn was difficult to deactivate in a way that RESNUCLEi will score something. That’s why I resorted to the brute-force way of using different DEFAULTS (which changes a lot of other things as well, of course). For example, just adding a LOW-NEUT card with WHAT(6)=-1.0 is not enough. The best way to diagnose the problem is probably what Note 1 for the WHAT(1) = 2.0 setting of RESNUCLEi suggests: Enable verbose printing for LOW-NEUT and look for “RESIDUAL NUCLEI INFORMATIONS AVAILABLE” in the output.
Summary: Low-energy-neutron-product scoring most probably does not work with point-wise cross sections (which you are using in your simulation). FLUKA manual could be more explicit on this.
I hope this helps. @kltaylor 's answer is certainly the more general and flexible way to score anything you like.
While acknowledging the possibility of providing in the documentation a more clear explanation (although the mention for multigroup treatment explicitly announces what you experienced, i.e., that the residue separation does not apply to the recommended/default point-wise treatment), let me stress that there is no good reason to deactivate the point-wise treatment. When using it, the residual nuclei generated by low-energy neutrons are scored by RESNUCLEi together with the spallation products. (To separate them, Kate’s suggestion applies.)