B Input Energy of Radioactive Decays

[A M]

TL;DR Summary

We know that nuclear fusion doesn't occur under normal conditions (e.g. in a helium balloon).
Because for the fusion of even the lightest elements (like hydrogen isotopes), fairly high energy is needed to 'break' the electrostatic repulsion barrier; although the released energy might be much higher.

But how about some radioactive decays? (α, SF, CD, ...)
Don't we need energy to to separate some nucleons of a radioactive nucleus?

 

[Orodruin]

Yes, decays also need to overcome an energy barrier. This is done through quantum mechanical tunneling through the energy barrier. The difference in relation to fusion is that you only have a single radioactive particle and even a small probability per time of decaying will eventually lead to a decay. For fusion, the particles also need to meet up and when they do they have a single chance of tunneling through the energy barrier with very low proability.

 

[A M]

Yes, decays also need to overcome an energy barrier.

Would you please explain a little more?

 

[snorkack]

For example of tunnelling where it is electrons that tunnel: we can observe both field emission and photoelectric effect. There is a definite energy barrier (as demonstrated by photoelectric effect) and yet electrons do have a small chance of getting through without any additional energy (as shown by field emission).

 

[A M]

Yes, decays also need to overcome an energy barrier. This is done through quantum mechanical tunneling through the energy barrier. The difference in relation to fusion is that you only have a single radioactive particle and even a small probability per time of decaying will eventually lead to a decay.

What does this probability come from? The higher energy they need to overcome that barrier or sth more complicated?

 

[mfb]

Yes. We had this discussion and even a graph for alpha decays in the previous thread. The energy released is not directly the same as the energy barrier but the two are closely linked.

 

[A M]

Thank you for the reply, but I haven't quite understood.
What if the nuclei were in a situation without that amount of energy available? The decay wouldn't occur?

 

[Orodruin]

Thank you for the reply, but I haven't quite understood.
What if the nuclei were in a situation without that amount of energy available? The decay wouldn't occur?

Yes it would. It is quantum tunnelling. In quantum mechanics a particle can pass an energy barrier that it classically would not be able to.