2.2. Building the Elements
At the same time that neutrons and protons are interconverting, they are also colliding among themselves to create deuterons: n + p D + . However, at early times, when the density and average energy of the CBR photons are very high, the newly formed deuterons find themselves bathed in a background of high-energy gamma rays capable of photodissociating them. Since there are more than a billion photons for every nucleon in the Universe, before the deuteron can capture a neutron or a proton to begin building the heavier nuclides, the deuteron is photodissociated. This bottleneck to BBN persists until the temperature drops sufficiently so that there are too few photons energetic enough to photodissociate the deuterons before they can capture nucleons to launch BBN. This occurs after e± annihilation, when the Universe is a few minutes old and the temperature has dropped below 80 keV (0.08 MeV).
Once BBN begins in earnest, neutrons and protons quickly combine to form D, 3H, 3He, and 4He. Here, at 4He, there is a different kind of bottleneck. There are no stable mass-5 nuclides. To jump this gap requires 4He reactions with D or 3H or 3He, all of which are positively charged. The Coulomb repulsion among these colliding nuclei suppresses the reaction rates, ensuring that virtually all of the neutrons available for BBN are incorporated in 4He (the most tightly bound of the light nuclides), and also that the abundances of the heavier nuclides are severely depressed below that of 4He (and even of D and 3He). Recall that 3H is unstable, decaying to 3He. The few reactions that manage to bridge the mass-5 gap lead mainly to mass-7 (7Li or 7Be, which, later, when the Universe has cooled further, will capture an electron and decay to 7Li); the abundance of 6Li is 1 to 2 orders of magnitude below that of the more tightly bound 7Li. Finally, there is another gap at mass-8. This absence of any stable mass-8 nuclei ensures there will be no astrophysically interesting production of any heavier nuclides.
The primordial nuclear reactor is short-lived, quickly encountering an energy crisis. Because of the falling temperature and the Coulomb barriers, nuclear reactions cease rather abruptly as the temperature drops below ~ 30 keV, when the Universe is ~ 20 minutes old. This results in "nuclear freeze-out", since no already existing nuclides are destroyed (except for those that are unstable and decay) and no new nuclides are created. In ~ 1000 seconds BBN has run its course.