Deuterium as opposed to di-proton

Jorge Kluney

Hello,

I was having a difficult time understanding some instances of nucleosynthesis. Two hydrogens (each with just 1 proton) come together to form deuterium (1 proton and 1 neutron).... but why does this happen as opposed to forming a di-proton (2 protons). I've been told that it has something to do with the strength of strong nuclear force. That if the force were stronger than it would be a di-proton as opposed to deuterium.... but then the rate of stars burning would be too rapid.

Does one of the protons convert to a neutron while forming deuterium as opposed to a di-proton?
Thanks for any help.

Meir Achuz

The reason there is no stable di-proton has to do with the details of the nuclear force. The force between a neutron and a proton is slightly stronger than either the p-p force or the n-n force. (The technical reason has to do with "isotopic spin".) The n-p force is just barely strong enough to bind them into the deuteron, but the n-n and p-p forces are not strong enough to bind. The answer to your last question is that it is very rare for two protons to form a deuteron. The process would have to be something like p+p--> d+pion^+, which doesn't usually happen.
The deuterons in stars come from low energy, n and p coming together and binding, usually with the emission of a photon. The process is called
"radiate capture". It is similar to how electrons get captured by protons to form the hydrogen atom.

Jorge Kluney

Thanks for that reply. It was very helpful.

In regards to the deuterium being composed of n-p. How does one of those protons (hydrogen) become a neutron? Does it need to gain energy (sorry if this is the wrong descriptive term)? Does one of the existing protons have one of its 'u quarks' convert to a 'd quark'?
Thanks again.

Meir Achuz

If you are going back to the early universe, neutrons were produced as easily as protons, so both were around to create a deuteron by the process n+p--> d+ photon. If you start with p-p, forming a deuteron is vey unlikely since p+p--> d+pion can only happen at relatively high energy where forming a deuteron is unlikely.

Jorge Kluney

Thanks again!!
So, there was already free amounts of neutrons to be used in forming deuteron. Were those neutrons (neutrons free being joined with a proton in deuteron) unstable? Did they quickly breakdown to a proton-electron-antineutrino if they weren't held in a nucleus with a proton?

Meir Achuz

The neutron lifetime is about 15 minutes, which is almost infinite on the time scale of early nucleosynthesis.