Helium nucleus emitted by unstable isotopes (5,8)

I believe the answer is:

alpha particle

I'm a little stuck... Click here to teach me more about this clue!

'helium nucleus' is the definition.
'alpha particle' can be an answer for 'nucleus' (thesaurus). I am not certain of the 'helium' bit.

'emitted by unstable isotopes' is the wordplay.
I cannot quite see how this works, but
'by' could be 'apart' (synonyms) and 'apart' is found within the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Another definition for alpha particle that I've seen is " Helium nucleus emitted by radioactivity".)

I've seen this clue in The Times.

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