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I was reading my notes (the topic is called 'nuclear chemistry') today when I noticed a line

"alpha (He2+) and beta (e-) can't be emitted simultaneously. After emission of alpha or beta, gamma photons must be emitted to decrease the energy of Excited Nucleus"

Can someone please explain how emission of alpha and beta particles might excite the nucleus

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    $\begingroup$ If you know about excited atoms, read about the nuclear shell model. $\endgroup$
    – rob ♦
    Commented Apr 12, 2023 at 13:38

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After the emission of an alpha or beta particle, your parent nucleus has decayed into its daughter nucleus. That daughter nucleus may or may not be in its ground state. If it is in its ground state, you do not get a gamma at all. But more often than not, the daughter nucleus is left in an excited state, perhaps because some spins of the nuclei aren't aligned properly etc. That surplus energy is then most often shed by the emission of a gamma.

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"Excited nucleus" isn't really a proper name (though it is not wrong).

Here's a sample decay chart: enter image description here

The excited state is called a nuclear isomer (so it has the same $A$ and $Z$, just not in a ground state configuration. They have a little $m$ in the superscript to indicated they are metastable (which is also an acceptable name).

They are metastable because they are stable against the strong interaction (they would have to have enough energy to emit a radiative pion, which would take $10^{-20}$s, give or take), and decay electromagnetically, by rearranging the charge distribution.

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