Why is there a sudden drop-off in half-life of isotopes at around 130 neutrons? Is there a name for this?

Question

Pertaining to the chart of nuclides, there is a region above Bismuth, in which the relatively continuous trend of stability is interrupted by a batch of isotopes all with extremely short half-lives. I know that "magic numbers" contribute to the stability of nuclei via shell-filing, is there such a thing as an "anti-magic" number which leads to this "bay of instability" style feature? I assume this is a well studied phenomenon, I just know not what keywords to look for in order to look into it further.

I find it particularly fascinating as it is so close to the last few stable elements, the drop-off in half-life is precipitous and it affects a lot of isotopes in this area. Is it something to do with how alpha decay operates? Every isotope in this loosely-defined zone seems to undergo alpha decay. I also note that generally, above Bismuth the 128-neutron line is extremely unstable, with half-lives much more gradually increasing as $N$ increases.

Answer 1

The instability is actually centred on N=128 neutrons, not N=130, and is caused by N=126 being a neutron magic number. Isotopes with magic numbers of neutrons have greater binding energies than those that don't, so it is energetically favourable for isotopes with N=128 to decay to an N=126 isotope by alpha decay. This causes a peak in alpha decay energy ($Q_\alpha$) at 128 neutrons and corresponding dip in half-life ($t_{1/2}$), as shown in Figure 3 from this paper. According to the Geiger-Nuttall Law, the half-life for alpha decays decreases exponentially with increasing $\sqrt{Q_\alpha}$.

I don't know if the dip has a name.

Answer 2

It happens to be $2$ past the magic proton number $82$ and $2$ past the magic neutron number $126$ where the major instability begins. I think this makes the isotopes in this block of instability doubly "unmagic". It also happens to be one alpha particle more than a stable lead $208$ nucleus where this begins. What is also striking, is that the isotones with $125$ neutrons are on average more stable than those with $126$.