Quote by malawi_glenn
Boron10 have 5 neutrons, one of them is unpaired and can easily fill the second subshell (1p_3/2). In boron11 we already have 6 neutrons, and the (1p_3/2)sub shell is filled, hence the lower probability to add another neutron

Thank you, I can see where filling the 1p_3/2 shell with 4 neutrons in boron11 would lower the neutron cross section from 3835 barns in boron10 to 0.0055 barns in boron11
My next question is: "why are both boron10 and boron11 "stable ?". Let me attempt to answer my question and you can correct my errors. For boron11, it appears the stability comes from the fact that the 1p_3/2 shell is complete with 4 neutrons. For boron10 it appears the stability comes from the fact that it is an "oddodd" (ZN) isotope with equal number of protons and neutrons in both 1s and 1p_3/2 shellsan example of "pairingenergy". There are only three other known examples of oddodd (ZN) isotopes that are stable against betadecay: deuterium, lithium6, nitrogen14.
But this leads to another question. Why does boron10 convert to lithium7 and alpha particle when it absorbs a low energy neutron ?why does it not just convert to stable boron11 ? Is there more energy in the added neutron than is needed to form stable boron11 ?
Quote by malawi_glenn
B13 will have even smaller neutron cross section, since it have 8 neutrons  a magic number.

This statement I do not understand. B13 does not have a smaller cross section than boron11. Boron13 has a neutron cross section = 767 barns, boron11 has cross section of 0.0055 barns:
http://www.site.uottawa.ca:4321/astr...x.html#boron13
Boron13 is also beta unstable, with half life of 0.0174 sec. Boron11 is beta stable.
So, the concept that magic neutron number = 8, thus isotope is more stable, does not work for element boron. Boron13 isotope (which has a magic # = 8 neutrons) is neither more stable, nor has smaller cross section, than nonmagic boron11 isotope. Also, the concept that having completely filled first three shells with neutrons (1s, 1p_3/2, 1p_1/2) thus leads to great isotope stability is falsified with boron13.