Why are only some particles fissionable/fissile?

[Unredeemed]

I understand how fission happens. But what I do not understand is why only a few certain nuclei are actually fissionable/fissile.

Can anyone help?

Thanks,
Jamie

 

[mgb_phys]

The stability of the nucleas depends on the ratio of protons to neutrons.
http://en.wikipedia.org/wiki/Isotope#Nuclear_properties_and_stability.

 

[Unredeemed]

The stability of the nucleas depends on the ratio of protons to neutrons.
http://en.wikipedia.org/wiki/Isotope#Nuclear_properties_and_stability.

If more neutrons means a more stable nucleus, then why in nuclear fission does the absorbtion of an extra neutron lead to the nucleus splitting in half?

 

[mgb_phys]

More neutrons only means more stable upto a certian level.
There is a most stable ratio of neutrons/protons which gets slightly larger with larger atoms.
So a small atom with an equal number of protons/neutrons would become very unstable if you added a single neutron. As you get larger the most stable configuration is a few more neutrons than protons - but if you add more neutrons than this you go off the line in the other direction and make the nucleus less stable.

The exact number to have best stability can be calcuated but the theory (Quantum Chromo Dynamics) is very complicated.

 

[Unredeemed]

More neutrons only means more stable upto a certian level.
There is a most stable ratio of neutrons/protons which gets slightly larger with larger atoms.
So a small atom with an equal numbe rof protons/neutrons would become very unstable if you added a single neutron. As you get lareger the most stable configuration is a few more neutrons than protons - but if you add more neutrons than this you go off the line in the other direction and make the nucleus less stable.

The exact number to have best stability can be calcuated but the theory (Quantum Chromo Dynamics) is very complicated.

Okay, thanks very much.

 

[granpa]

adding more neutrons results in a stronger force of attractin holding the neucleus together but it takes energy to produce neutrons which are themselves unstable. allowing a sungle neutron in such a nucleus to decay would cause the nucleus to expand but it also releases the energy stored in than nutron.

uranium decays by a different process than most. it splits in half. the protons are in 2 shells which simply separate.

 

[Vanadium 50]

adding more neutrons results in a stronger force of attractin holding the neucleus together

No, it only does this if the nucleus is proton rich. Adding a neutron to a nucleus that is already neutron rich makes it worse.

but it takes energy to produce neutrons which are themselves unstable.

Yes, if I want a beam of neutrons it takes energy and they are unstable. But what does this have to do with the question.

allowing a sungle neutron in such a nucleus to decay would cause the nucleus to expand

I don't think so.

uranium decays by a different process than most. it splits in half. the protons are in 2 shells which simply separate.

Alpha decay is common, and it's simply one example of spontaneous fission.

 

[granpa]

No, it only does this if the nucleus is proton rich. Adding a neutron to a nucleus that is already neutron rich makes it worse.
.

the strong force increases when you add neotrons. but if you hove too many neutrons then the energy released by allowing a neutron to decay is greater than the decrease in energy due to the strong force.

 

[Vanadium 50]

If your theory were true, Helium 5 would decay by beta emission, like a free neutron. In fact, it decays by simply falling apart to a neutron and an alpha.

 

[granpa]

yes and a nucleus of 2 alpha particles will fall apart too. there's something inherently unltable about those configurations. don't know what that has to do with what I am saying though.

 

[Vanadium 50]

. don't know what that has to do with what I am saying though.

It shows your theory is wrong. You said "the strong force increases when you add neotrons [sic]" until the neutron decays (via beta decay). Here's a situation where the nucleus becomes less stable and the neutron does not beta decay inside the nucleus.

 

[granpa]

a single exception doesn't necessarily prove a rule wrong. its a big complex world with lots of exceptions.

 

[Vanadium 50]

This is science. It matters when your theory gets something wrong. Especially when mainstream theory gets it right.

 

[malawi_glenn]

granpa, the strong force is a saturating force. You are mixing wild between concept of force and binding energy.

Uranium exihibit same fission spectra as the other heavy nuclei...


Unredeemed: All nuclei are fissionable, but not everyone you can gain energy by splitting them.

 

[granpa]

my 'theary' as you call it IS the mainstream view.

 

[malawi_glenn]

my 'theary' as you call it IS the mainstream view.

"uranium decays by a different process than most. it splits in half. the protons are in 2 shells which simply separate. "

Now first define WHAT uranium isotope you are referring to and state your source (peer reviewed article or textbook)

 

[granpa]

ok I'll bite. what other heavy nuclei and what fission spectrum and what does any of that have to do with what I said?

 

[granpa]

I was talking to malawi about the stability of lighter nuclei. please read the whole forum before you jump on people.

 

[malawi_glenn]

I can't see where you state that: "For light nuclei..."

Then you brought up the discussion about Uranium, therefor I asked what is so special about it and what isotope you are referring to. Don't you know what a fission spectra is?

 

[granpa]

well I would assume you mean the different masses of the resulting nuclei but that doesn't seem to follow from what you said since I can't see any relevance to anything being discussed.

 

[malawi_glenn]

well I would assume you mean the different masses of the resulting nuclei but that doesn't seem to follow from what you said since I can't see any relevance to anything being discussed.

eh?

"uranium decays by a different process than most. it splits in half. the protons are in 2 shells which simply separate. "

 

[granpa]

and?

 

[malawi_glenn]

and?

can't you see who ridiculous that post is?

First of all, what uranium isotope are you referring to. Second, what is so special about uranium? Third, it seems to me that you think they are split into equal halves, but infact the fission spectrum for uranium isotopes are smooth. Forth, what is "protons are in two shells which simply separate"?? Shells as in shells in nuclear structure theory or what?

 

[granpa]

split into equal halves

I never said that nor do I have any idea how you would have gotten it from any thing I said. the 2 proton shells are not equal. one is filled and the other isnt. in addition to the proton shells there are also neutron shells. each proton shell carries away a somewhat random number of neutrons. some of those neutrons then change to protons and some ore emitted

its really quite neat if you think about it. can you imagine how strongly they must repel one another? no wonder the atom bomb is so powerful.edit: yes this part of what I am saying is my own thinking. but I'm not aware of any conflict with 'mainstream' science.

 

[granpa]

oh wait. its been so very long since i gave any thought to any of this that I've forgotten many things.

the mainstream view is that there are shells but not 2 shells. my thinking is this. each shell consists of 5 subshells of 1, 3, 5, 7, and 9 pairs of protons. totalling 50 protons. one shell is full. the other not quite full. this agrees with most of the magic numbers of the nucleus. I've also observed that the number of neutrons not part of alpha particles (and presumably therefore in a shell of their own) in the nucleus is approximately (Z/12)^2. for uranium this is approximately 50 neutrons. one complete shell.

 

[Vanadium 50]

my 'theary' as you call it IS the mainstream view.

Please don't pretend to quote me and then misspell "theory" in the quote. It's a shabby attempt to make me look stupid, and it's not fooling anyone.

Second, malawi_glenn is right - you're mixing up different concepts.

Third, your description is most assuredly not mainstream view: I gave a specific example (5He) where your theory makes one prediction, the mainstream view another. You can't argue they are the same if the predictions are different. Furthermore, since you get it wrong and mainstream science gets it right, it's hard to argue that your theory is better.

Finally, you're digging yourself in a deeper and deeper hole. You're talking about uranium...no, light nuclei... no uranium. You're talking about mainstream science...well, actually, it's your ideas, not mainstream science. It splits in half...and by "half", you don't mean equal halves. We don't have to contradict you - you're doing it for us.

 

[granpa]

ok I think all pretense of intelligent discourse has evaporated. none of what you said makes any sense. I mis-type things constantly. you must be paranoid to think that I did it on purpose. I'm going to bed. have a good night or day or whatever it is where you are.

o and a are next to each other on dvorak keyboards.

 

[Vanadium 50]

the mainstream view is that there are shells but not 2 shells. my thinking is this. each shell consists of 5 subshells of 1, 3, 5, 7, and 9 pairs of protons. totalling 50 protons. one shell is full. the other not quite full. this agrees with most of the magic numbers of the nucleus.

No, it doesn't. You would predict magic numbers of 2, 8, 18, 32, 50 [and if you continue on 98, 128... ]. The magic numbers are 2, 8, 20, 28, 50, 82 and 126. Most means - at a bare minimum - "more than half". Out of 7, you get 3. That's not "most".

I think it's not fair to the OP for you to be posting your own ideas (particularly when they conflict with mainstream science) instead of mainstream science.

 

[granpa]

you got 2 8 18 32 50 right but it stops there and starts over. the next would be 52 then 58 68 82 100

anything over 100 is likely to be different anyway since so many shells become filled.

the main point I was making was that the nucleus is in balance between various forces. that's mainstream. it wasnt my intention to get into a big discussion about all this. but when my ideas are being so totally misrepresented what can I do?

 

[granpa]

the point being that the nucleus is in balance between the strong force, the electrostatic force, and a third force involved in neutron decay. I guess that's why they call the weak force the weak 'force'.

just realized that. it makes sense now.

 

[Vanadium 50]

you got 2 8 18 32 50 right but it stops there and starts over. the next would be 52 then 58 68 82 100

So you've got 4 right with your numerology, 4 wrong, and miss 3. I don't find that particularly impressive.

but when my ideas are being so totally misrepresented what can I do?

You could restrict your answers to mainstream physics and not bring your pet theories into it.

 

[QuantumPion]

Uranium does not split in half when it fissions. The fission products have a spectra which correspond to two peaks, e.g. Ce-133 and Tc-99.

 

[malawi_glenn]

http://en.wikipedia.org/wiki/Spontaneous_fission

But spontanous fission rate of Uranium is still very low compared to its other decay modes. So one has to specify I guess, so OP understands the entire picture.

It is also ambigous what the OP means by "Why are only some particles fissionable/fissile" but I suggest he is referring to why only some nuclei can be used in fission power plants - not why some nuclei undergo spontaneous fission.

Now if you bring up your own pet theories again, here or in superconductivity, you will run into trouble. Why can't you accept and follow the rules of this forum? If not, then why are you here?

Would you, when presenting your results in an article, say "nature occasionally has exceptions"? Totally non-serious...

 

[Vanadium 50]

I still believe it is more illuminating to consider alpha decay as simply a form of spontaneous fission. The underlying physics is the same - you just have a very asymmetric split. Many nuclei only have this mode, some, like U-235, have others.

 

[granpa]

here is a surprising result I just found. I used inverse 10th power but as you can see the reasoning would work just as well with any other law including logarithmic.

https://www.physicsforums.com/showthread.php?p=2055877#post2055877