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Reaction of radiation with Nitrogen-14

Physics Asked on October 2, 2021

When solar wind brings high energy neutrons into contact with Nitrogen-14, a well known process occurs yielding Carbon-14 + a proton. N14 is assumed to exist in upper atmosphere as N2 gas, but the assumption is that in order for this reaction to occur, it must be atomic N14 – if this is correct, the question is what drives the molecular nitrogen gas (triple bonded and very stable) to an atomic state? Chemistry exchange referred this to Physics.

One Answer

The reaction releases around 4MeV of energy, which is vastly more than the chemical binding energy of nitrogen or the cyanide radial. This means the reaction splits the original $N_2$ molecule into two separate atoms of nitrogen and carbon 14.

The process that creates the carbon 14 nucleus is:

$$ {}^{14}N_7 + n to {}^{14}C_6 + p + mathrm{energy} $$

The reaction is with thermal neutrons so the energy of the neutron is negligible. That means the energy released is equal to the difference in the binding energies of the nuclei. The mass defect of ${}^14N_7$ is $0.108506u$ while the mass defect of $^{}14C_6$ is $0.113027u$ so the carbon 14 nucleus energy is lower by $0.004521$ amu or about $4.2$ MeV. The proton will carry away most of this energy because it's much lighter than the carbon 14 nucleus, but the KE of the carbon nucleus will still be around $0.28$ MeV.

Answered by John Rennie on October 2, 2021

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