My watch list  

Proton-proton chain reaction





The pp chain reaction

  The first step involves the fusion of two hydrogen nuclei 1H (protons) into deuterium 2H, releasing a positron and a neutrino as one proton changes into a neutron.

1H + 1H → 2H + e+ + νe

with the neutrinos released in this step carrying energies up to 0.42 MeV.

This first step is extremely slow, because it depends on an endoergic beta positive decay, which requires energy to be absorbed, to convert one proton into a neutron. In fact this is the limiting step, with a proton waiting an average of 109 years before fusing into deuterium[citation needed].


e+ + e → 2γ (whose energy is: 1.02 MeV)


2H + 1H → 3He + γ (whose energy is: 5.49 MeV)

From here there are three possible paths to generate helium isotope 4He. In pp1 helium-4 comes from fusing two of the helium-3 nuclei produced; the pp2 and pp3 branches fuse 3He with a pre-existing 4He to make Beryllium. In the Sun, branch pp1 takes place with a frequency of 86%, pp2 with 14% and pp3 with 0.11%. There is also an extremely rare pp4 branch.

The pp I branch

3He +3He → 4He + 1H + 1H + 12.86 MeV


The pp II branch

       3He + 4He 7Be + γ
       7Be + e 7Li + νe
       7Li + 1H 4He + 4He


90% of the neutrinos produced in the reaction 7Be(ee)7Li* carry an energy of 0.861 MeV, while the remaining 10% carry 0.383 MeV (depending on whether lithium-7 is in the ground state or an excited state, respectively).

The pp III branch

       3He + 4He 7Be + γ
       7Be + 1H 8B + γ
       8B 8Be + e+ + νe
       8Be 4He + 4He



The pp IV or Hep


3He + 1H → 4He + νe + e+

Energy release




The pep reaction



1H + e + 1H → 2H + νe




  1. ^ Claus E. Rolfs, William S. Rodney: Cauldrons in the Cosmos, The University of Chicago Press, 1988, 354. pp
  2. ^ Int'l Conference on Neutrino and Dark Matter, Thursday 07 Sept 2006, Session 14

See also

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Proton-proton_chain_reaction". A list of authors is available in Wikipedia.