I Why no heavy delayed neutron emitters?

[snorkack]

TL;DR

Delayed decays don´t seem to occur in elements heavier than Tl

As a rule, starting with He, the heavy isotopes of all elements are delayed neutron emitters:

1. He-4 stable
2. He-5 unbound
3. He-6 β^- only 807 ms
4. He-7 unbound
5. He-8 β^-n 16% of 120 ms

then:

1. Li-7 stable
2. Li-8 β^- only 839 ms
3. Li-9 β^-n2α 50% of 178 ms

etc... till

1. Tl-205 stable
2. Tl-206 (RaE) β^- only 252 s
3. Tl-207 (AcC) β^- only 286 s
4. Tl-208 (ThC'') β^- only 183 s
5. Tl-209 β^- only 130 s
6. Tl-210 (RaC'') β^-n 0,01% of 78 s
7. Tl-211 β^-n 2,2% of 80 s

But then:

1. Pb-208 (ThD) stable
2. Pb-209 β^- only 3,24 h
3. Pb-210 (RaD) β^- only 22 y
4. Pb-211 (AcB) β^- only 2170 s
5. Pb-212 (ThB) β^- only 10,6 h
6. Pb-213 β^- only 610 s
7. Pb-214 (RaB) β^- only 1625 s
8. Pb-215 β^- only 142 s
9. Pb-216 β^- only 40 s
10. Pb-217 β^- only 20 s
11. Pb-218 β^- only 15 s

... and that´s it? No Pb isotopes are delayed neutron emitters.
This seems also to be the case for all heavier elements to about 100. Heavy isotopes are pure beta emitters, with no delayed neutrons. Why?
Also, is it possible for an isotope to undergo delayed fission? As in β^-f? Are there any such isotopes?

 

[Astronuc]

TL;DR Summary: Delayed decays don´t seem to occur in elements heavier than Tl

This seems also to be the case for all heavier elements to about 100. Heavy isotopes are pure beta emitters, with no delayed neutrons. Why?

It has to do with the nuclear energy levels, which are characteristic of the radionuclide. One could ask why branching fractions are the way they are, or why gamma transitions are the energy they are.

See Figure 2 in this document - https://ec.europa.eu/programmes/era..._part_2_Experiment_procedure_for_students.pdf

However, in some cases a daughter nucleus is formed after a β- decay, where the
excitation energy is higher than the neutron-separation energy. This nucleus will emit again a
neutron, almost promptly after its formation. As these neutrons are generated from several
milliseconds to several minutes after the fission event, they are called delayed neutrons.

I think one is referring to elements of Z up to 100. Some isotopes are alpha emitters, and some do both alpha or beta, otherwise, as the n/p ratio increases, the neutron rich nuclei decay by beta emission, if not by alpha.

Some nuclei decay by 'spontaneous emission', e.g., ²⁵²Cf, and important nuclide for startup neutron sources in nuclear reactors.

Looking at the Tl isotopes 209 through 214, one sees a decrease in half-life and high fraction of β-,n decay. There are heavier nuclides that may decay by β-,n, e.g., ²²¹Bi, ²²⁷At, ²³³Fr, and the heavier isotopes of those elements. I suspect they have very short half-lives and are difficult to produce given the relative high n; they are also odd Z elements, Bi (Z=83), At (Z-85), Fr (Z=87).

²³⁰Ac is perhaps the lightest nuclide that can undergo 'spontaneous fission' (SF), but is more likely to undergo beta decay with a half-life of 122 s.

The heavier nuclei are of less interest in terrestrial systems, since they are hard to produce, the half-lives are short, and they are not usually found in nuclear reactors where delayed neutrons are important. Instead research has focused on those delayed neutron nuclides that are important fission products, e.g, ⁸⁷Br or ⁹³Rb.