
#1
Apr408, 03:55 PM

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?




#2
Apr508, 12:04 AM

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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?




#3
Apr508, 05:49 AM

P: 2

The keyword "shell model" was exactly what I needed, thanks.




#4
Apr808, 01:08 PM

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).



#5
Apr908, 09:14 PM

P: n/a

But, I have a question.
The element Boron has two stable isotopes, B10 and B11. They differ by one neutron. The neutron capture cross section of B10 is 3835 barns (this means it has high probability of capturing another neutron). The neutron capture cross section of B11 is 0.0055 barns (low probability). So, my questionwhy is there such large difference in neutron capture cross section for two stable isotopes of the same element that differ by only one neutronhow does the shell model explain this ? 



#6
Apr1008, 02:48 AM

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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.phyastr.gsu.edu...ear/shell.html B13 will have even smaller neutron cross section, since it have 8 neutrons  a magic number. 



#7
Apr1008, 12:04 PM

P: 909

And why some radio nucleus are likewise to receive fast neutrons, and some radio nucleus slow neutrons?




#8
Apr1008, 12:12 PM

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This is treated in Krane's textbook: Introductory Nuclear Physics Every nuclear physicist should own a copy of it :) 



#9
Apr1008, 12:22 PM

P: 909

Can you please give me some analogy, or with 2 sentences tell me what is the deal about...



#10
Apr1308, 10:49 PM

P: n/a

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 ? 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. 



#11
Apr1408, 01:40 AM

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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 B13 was stable;) }
I dont have the time really to look up why B10 absorbing a thermal neutron decays into Li7 + alpha but it should the explanation you mention. Alos you do not say what "low energy" means here. 



#12
May408, 11:58 AM

P: 909

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.




#13
May408, 12:05 PM

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In what situation? In a nuclear power plant reactor or in a nuclear bomb?
Have you tried google? 



#14
May408, 12:11 PM

P: 909

Thay said it in general, the important is to keep the coefficient constant. Is their statement correct?




#15
May508, 04:57 AM

P: 909

In nuclear power plant.




#16
May508, 11:15 AM

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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 reemit it again  over the whole solid sphere, so they can act as 'neutronmirrors'. This you use in an atombomb, where one does not want to loose neutrons. 



#17
May508, 12:17 PM

P: 909

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"?



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