I think I have found my mistake, I’ve been doing it the other way around. What I need to score the number of (n,alpha) interactions is a USRBIN card with WHAT(2) = NEUTRON followed by an AUXSCORE with WHAT(2) = 4-HELIUM. That way the USRBIN card scores the density of stars produced by neuitrons, and then the AUXSCORE card filters those stars to only count the alpha particles. Is that right?
Thanks!
Joaquin