ITER has a lot of neutrons. The moon is supposed to have a neutron flux based on its surface constantly absorbing cosmic rays and getting transmuted to unstable isotopes as a result. Fission plants have neutrons.
What if you could start with hydrogen-1, absorb a neutron, absorb a neutron again, wait for beta decay, and absorb one more neutron? You would go from H-1 to He-4 while never having to overcome proton-proton repulsion. The second proton "sneaks in" as the second neutron of tritium, then it turns into a proton "for free" if you're patient. The best efficiency would be to start with He-3 and just add the last neutron, but I understand He-3 is not abundant. I did a spreadsheet on this and it wasn't promising, but maybe I was wrong.
Has anyone read about this being studied before? Anyone know where I can find cheap neutrons? Are the absorption cross sections high enough?