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In nuclear fusion part of nucleons' rest mass goes into radiation energy. Does it mean that inertial mass of result nucleus decreased?

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In a word, yes. The nucleus has less mass (rest energy) than the sum of its protons and neutrons. The difference between these two quantities is called the mass defect, and it's equivalent to the binding energy of the nucleus. This binding energy is the energy that would be released if the nucleus were assembled from its constituents, and, conversely, it's also the energy you would need to expend to fully disassemble the nucleus.

If a nuclear reaction releases energy, then the resulting nuclei will have a higher binding energy than the initial nucleus. In the case of gamma decay, this difference is precisely the energy of the photon emitted. In other cases, things are slightly more complicated, due to the presence of multiple products, but the principle is still the same: the difference in binding energy is equal to the energy released in the reaction.

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