I have a question. From this government reference: http://www.ncnr.nist.gov/resources/n-lengths/ the thermal neutron cross section for stable isotope Be-9 = 0.0076 barns. This means Be-9 is not expected to absorb a thermal neutron, the probability of this is very, very low. The Be-9 isotope has 5 neutrons and 4 protons. What I cannot understand is why the next possible stable isotope, B-10, which also has 5 neutrons, has an extremely high neutron cross section = 3,835 barns, compared to Be-9 ? Of course B-10 has 5 neutrons and 5 protons. Both isotopes would have an unpaired n in the last n energy shell...correct ? The n packing situation for both Be-9 and stable B-10 would be, starting at ground state: n nn nn So, what aspect of the nuclear shell model explains such a drastic difference in neutron cross section for these two stable light isotopes that both have 5 neutrons ? Why does B-10 accept the thermal neutron to form completed neutron shells: nn nn nn but, Be-9 isotope does not accept a neutron ? I know the shell model is incomplete for heavy isotopes, but it appears that predictive problems begin at Be to B, which are not very heavy. Thanks for any help with this question.