Hi there,
I will assume then that you want to score your neutrons at the moment they are created but please note that this is not what is explained in the paper you were referring to in your initial question.
To score the neutrons at the moment they are created I would use the mgdraw.f routine, in particular the USDRAW entry. There you can implement your logic to score the neutrons just after the reaction that generates them, maybe this post could give you some hints.
But, if you plan to use these neutrons as source for a second simulation you should be careful or you could end up counting some of them more than once. For instance, imaging you score the properties of one neutron, but you let it continue running in your initial simulation, it may give rise to the creation of more neutrons, and you would also count them, which would be wrong if then you use all of them in your second simulation (you would simulate twice the “life” and products of the first neutron). To avoid this, you should discard the neutrons after you “score” them, for instance by zeroing their statistical weight.
Note that if you do that then you shouldn’t score anything else during your simulation, at least not blindly because you would be discarding an important part of it, the neutrons.
Kind regards,
Francisco