Does the shell theory explain why it becomes unusually unstable once there’s two neutrons past the shell (and not when there’s two protons past the shell)?
For alpha decay, a bunch of two protons and two neutrons need to detach. Two protons will have a greater intrinsic chance of breaking away, because of charge repulsion from other protons. So it’s detaching the neutrons which is hardest.
So, if you are considering various nuclei with two nucleons outside the filled shells, and asking when alpha emission faces the lowest energy barrier, it might be the case in which the two protons come from a filled shell (and can use charge repulsion to escape), and the neutrons are the two loose nucleons.
And also, why does the decay mode suddenly change to alpha particles?
The proton shell after Z=82 seems to be the threshold at which electrostatic repulsion between protons, wins out over strong-force cohesion among nucleons. Although it can take a while… the half-life of bismuth-209 is 10^19 years!
For alpha decay, a bunch of two protons and two neutrons need to detach. Two protons will have a greater intrinsic chance of breaking away, because of charge repulsion from other protons. So it’s detaching the neutrons which is hardest.
So, if you are considering various nuclei with two nucleons outside the filled shells, and asking when alpha emission faces the lowest energy barrier, it might be the case in which the two protons come from a filled shell (and can use charge repulsion to escape), and the neutrons are the two loose nucleons.
The proton shell after Z=82 seems to be the threshold at which electrostatic repulsion between protons, wins out over strong-force cohesion among nucleons. Although it can take a while… the half-life of bismuth-209 is 10^19 years!