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For the pp chain, the second process is listed as the d(p,γ) 3He reaction: D + p --> 3He + gamma ray.

I cannot find an explanation why a gamma ray is created in this process. Can anyone explain why this process must result in production of a gamma ray? I understand that accelerating charges produce EM emission, but the other processes in the pp chain do not specifically produce gamma rays directly, only indirectly. For example, in the first step, two protons fuse to generate deuterium, a positron, and a neutrino. The positron annihilates with an electron to produce a gamma ray - but not one originally from the fusion reaction itself! What is different about this second process?

I also understand that a blackbody generates light due to temperature, but I'm interpreting this reaction (D + p --> 3He + gamma ray) as that it would produce a gamma ray if it were isolated regardless of environment.

Why does this reaction produce a gamma ray of an exact energy? There must be an exact mechanism at play, can anyone explain it to me?

Thank you!

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    $\begingroup$ If the nucleus is created in an excited state, you will get a gamma out. $\endgroup$
    – Jon Custer
    Commented Mar 4, 2023 at 1:23
  • $\begingroup$ The annihilation makes two photons. $\endgroup$
    – J.G.
    Commented Mar 4, 2023 at 8:24
  • $\begingroup$ Conservation of energy at the center of mass?For some bizarre reason I can find only in Daltons the masses involved.3He is 3.16, whereas proton is 1.01 and Deuteron 2.01. There are 0.14 Daltons left over from a fusion at rest, which are taken up by the gamma . hand waving. $\endgroup$
    – anna v
    Commented Mar 4, 2023 at 8:49

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A suggestion with no numerical evidence.

For example, in the first step, two protons fuse to generate deuterium, a positron, and a neutrino.
Thus you have three particles which between them are sufficient to ensure that energy and momentum are conserved. Any "extra" energy as the result of the reaction can be taken off by the particles which are produced with the deuterium in a non-excited state.

Can anyone explain why this process $[\rm D + p \to He3 + \gamma]$ must result in production of a gamma ray?
Noting that only one particle is produced the $^3_2\rm$He nucleus will be in an excited stated as a result of the reaction.
A way that the $^3_2\rm$He nucleus can move down towards the ground state is via the emission of gammas.

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