What is the reason why some reactions give a continuum for the energy of the released neutrinos and others give only a discrete value ($\ ^7B$ and $pep$ )?
Thanks for your help. Please feel free to use the figures below.
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Discrete spectra like that are characteristic of final-states of exactly two particles. For two fixed masses and a fixed total energy there is only one way to divide the energy that also conserves momentum.
With three or more particle in the final state there is a whole family of solutions, which leads to the broad spectra seen for $pp$, $^8\mathrm{B}$ and $hep$.
Compare to the original expectation for the energy spectrum of beta decay electrons, and the proposal of (what became known as) the neutrino in the first place.
