# (Neutron capture) cross section

by MatiNuude
Tags: capture, cross, neutron
 P: 2 Cross section means the effective size of a nucleus for capturing a neutron, am I right? But what makes the cross sections for different nucleus different? I am a bit puzzled, why the cross section for Boron(for example) is so high?
 Sci Advisor HW Helper P: 4,738 in general, the more stable a nucleus can become if it captures a neutron, the higher cross section. Have you done the shell model in school yet?
 P: 2 The keyword "shell model" was exactly what I needed, thanks.
 P: 230 (Neutron capture) cross section basically, yes. Its a measure of the probability of the nucleus capturing the neutron. Yes, it varies element to element because of the shell model (ie. its harder to capture a neutron if there the element has a "magic" number of neutrons).
 P: n/a But, I have a question. The element Boron has two stable isotopes, B-10 and B-11. They differ by one neutron. The neutron capture cross section of B-10 is 3835 barns (this means it has high probability of capturing another neutron). The neutron capture cross section of B-11 is 0.0055 barns (low probability). So, my question--why is there such large difference in neutron capture cross section for two stable isotopes of the same element that differ by only one neutron--how does the shell model explain this ?
HW Helper
P: 4,738
 Quote by Rade But, I have a question. The element Boron has two stable isotopes, B-10 and B-11. They differ by one neutron. The neutron capture cross section of B-10 is 3835 barns (this means it has high probability of capturing another neutron). The neutron capture cross section of B-11 is 0.0055 barns (low probability). So, my question--why is there such large difference in neutron capture cross section for two stable isotopes of the same element that differ by only one neutron--how does the shell model explain this ?
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 probabilty to add another neutron.

And if you want to know in more detail you should study the physics of shell model more closely or wait til someone have time to give a longer explanation, I dont have time at the moment.

http://hyperphysics.phy-astr.gsu.edu...ear/shell.html

B-13 will have even smaller neutron cross section, since it have 8 neutrons - a magic number.
 P: 908 And why some radio nucleus are likewise to receive fast neutrons, and some radio nucleus slow neutrons?
HW Helper
P: 4,738
 Quote by Physicsissuef And why some radio nucleus are likewise to receive fast neutrons, and some radio nucleus slow neutrons?
That has to do with the pairing term (among several other things), it would take a quite long time to explain this altough;)

This is treated in Krane's textbook: Introductory Nuclear Physics

Every nuclear physicist should own a copy of it :)
 P: 908 Can you please give me some analogy, or with 2 sentences tell me what is the deal about...
P: n/a
 Quote by malawi_glenn 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 by malawi_glenn 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:
http://www.site.uottawa.ca:4321/astr...x.html#boron13
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.
 Sci Advisor HW Helper P: 4,738 In reallity there are more paramters than just the magic numbers and so on, they no not work 100%. This is due to the fact that the nuclear shell model is just a model, and works best for medium sized nuclei wich are stable. {I didnt look up if B-13 was stable;-) } I dont have the time really to look up why B-10 absorbing a thermal neutron decays into Li-7 + alpha but it should the explanation you mention. Alos you do not say what "low energy" means here.
 P: 908 And can I ask, how can we keep the neutrons, which want to escape, in the chain reaction? In my book says that it can be done by neutron caputre or neutron diffusion. I don't understand what they mean. PLease help! Thanks.
 Sci Advisor HW Helper P: 4,738 In what situation? In a nuclear power plant reactor or in a nuclear bomb? Have you tried google?
 P: 908 Thay said it in general, the important is to keep the coefficient constant. Is their statement correct?
 P: 908 In nuclear power plant.
 Sci Advisor HW Helper P: 4,738 what 'constant'? Have you confused this thread with another one of yours? Neutron capture is when you 'stop' a neutron from moving away. So you have great neutron absorbing material in the between the fuel rods. Then you want to have slow neutrons, so you have a moderating material, which works as diffuser, it does not stop the neutrons completley.' That is the two main parts in a reactor. Then there are nuclei that absorb a neutron, then re-emit it again - over the whole solid sphere, so they can act as 'neutron-mirrors'. This you use in an atombomb, where one does not want to loose neutrons.
 P: 908 And also there is thing called reflectior, which reflects the neutrons, off, right? Is reflection and scattering similar proccesses? Because when there is scattering the neutron is bouncing off the nucleus, and in reflection also the neutron is bouncing off the reflector, right? Which is that "great neutron absorbing material in the between the fuel rods"?
 Sci Advisor HW Helper P: 4,738 one often uses boron (If i remember correct) as asbsorbing material between fuel rods. Well neutron can be back-scattered, so that is a possibility yes.

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