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In a quiz contest, I came across the following question: What is the term used to describe the splitting of a heavy nucleus into two lighter nuclei? Two options provided were: a) Alpha Decay b) Fission. Apparently, the correct answer is b) Fission. But even in Alpha Decay, a Helium nucleus is formed (i.e., the alpha particle), along with the daughter nucleus; both of them are obviously lighter than the parent nucleus, and so, doesn't this also fit the required criteria?

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up vote 2 down vote accepted

Alpha decay is treated as a special case and is usually not included in the term "fission", although it arguably is a fission (especially in the case of light elements where the alpha can represent an appreciable fraction of the original mass).

So, for the purposes of a quiz you want "fission".

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Even Carbon-nucleus is emmited in some decays. Then it would be about the speed of the thing. – huseyin tugrul buyukisik Aug 31 '12 at 15:16

This is about the speed of the product. If both pproducts are nearly same speed and nearly same mass, indeed it is fission. But one of them is emmited very very fast, this is decaying.

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