The Wikipedia page for candidate nuclear reactions for terrestrial fusion power notes that the D + D and He-3 + T reactions can proceed through either of two alternative routes; for example:
(2i) D + D → T ( 1.01 MeV ) + p ( 3.02 MeV ) [50%]
(2ii) D + D → He-3 ( 0.82 MeV ) + n ( 3.27 MeV ) [50%]
I'm having trouble understand precisely what this means: is the cross section for D + D to fuse by either process equal to the sum of the cross sections for these processes or half this sum? From the definition of branching ratio, I would have said the latter but in this book by the IAEA (Fig 1.11, p.22) the authors simply plot the sum of these cross sections.
So, how is the cross section defined? For each process independently (in which case, take half the sum, since when two deuterons meet, there is a 50% probability of (2i) and a 50% probability of (2ii) occuring), or with the branching probability "built-in", in which case, just take the sum?
