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]

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.

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.