Question

What decides the decay mode of a radioisotope? Take for example Plutonium-238. Why does it alpha decay instead of beta decaying, and why is alpha decay a helium nucleus instead of a helium nucleus?

edit: I know that alpha decay is a helium nucleus but why is it always a helium nucleus? Why can't isotopes only release 1 proton?

Answer 1

It depends on N/P ratio.

If N/P ratio is large then

One neutron (N) disintegrates into proton + electron + energy  

Electron cannot exist in nucleus hence it emits as beta particle and proton remains in the nucleus hence the N/P ratio decreases.

If the ratio is small enough and nucleus is unstable it releases a helium nucleus ( 2 protons + 2 neutrons) to attain stability for the nucleus. Stability depends on binding energy per nucleon. If 4 nucleons left it can attain stable state easily. Hence it releases an alpha particle.