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Apr13-08, 10:49 PM
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Quote Quote by malawi_glenn View Post
Boron-10 have 5 neutrons, one of them is unpaired and can easily fill the second subshell (1p_3/2). In boron-11 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 boron-11 would lower the neutron cross section from 3835 barns in boron-10 to 0.0055 barns in boron-11

My next question is: "why are both boron-10 and boron-11 "stable ?". Let me attempt to answer my question and you can correct my errors. For boron-11, it appears the stability comes from the fact that the 1p_3/2 shell is complete with 4 neutrons. For boron-10 it appears the stability comes from the fact that it is an "odd-odd" (Z-N) isotope with equal number of protons and neutrons in both 1s and 1p_3/2 shells--an example of "pairing-energy". There are only three other known examples of odd-odd (Z-N) isotopes that are stable against beta-decay: deuterium, lithium-6, nitrogen-14.

But this leads to another question. Why does boron-10 convert to lithium-7 and alpha particle when it absorbs a low energy neutron ?--why does it not just convert to stable boron-11 ? Is there more energy in the added neutron than is needed to form stable boron-11 ?

Quote Quote by malawi_glenn View Post
B-13 will have even smaller neutron cross section, since it have 8 neutrons - a magic number.
This statement I do not understand. B-13 does not have a smaller cross section than boron-11. Boron-13 has a neutron cross section = 767 barns, boron-11 has cross section of 0.0055 barns:
Boron-13 is also beta unstable, with half life of 0.0174 sec. Boron-11 is beta stable.

So, the concept that magic neutron number = 8, thus isotope is more stable, does not work for element boron. Boron-13 isotope (which has a magic # = 8 neutrons) is neither more stable, nor has smaller cross section, than non-magic boron-11 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 boron-13.