Neutrons Interacting With Nuclei

[Cult of Reason]

On page 278, Example 4, of Ohanian Physics Second Edition Expanded, by Hans C. Ohanian, published 1989, ISBN 0-393-95750-0, it says:


EXAMPLE 4. Inside a nuclear reactor containing uranium fuel, the fission reactions produce an abundant flux of fast neutrons of a speed of about 2 x 10^7 m/s. These neutrons are used to trigger more fusion reactions. But before they can be so used, they must be slowed down to a much lower speed. The slowing down is accomplished by colisions: the uranium fuel in the reactor is surrounded by water or by graphite and the neutrons lose their kinetic energy in collisions with the nuclei of these materials.[Etc..]

Emphasis mine. I am wondering how a neutron can possibly bounce off a nucleus. It has no overall charge. Do the quarks inside move around like electrons in an atom? If that were the case, I would expect the neutrons to be attracted to the nucleus as the down quarks would be attactred to the nucleus and not the up quark, so the down quarks would be closer and so the net fore towards the nucleus would be positive. Therefore I conclude that the electric force is NOT the cause for neutrons bouncing off nuclei. In that case, what is? The strong/colour force? Weak(??? I thought weak only affected such things as beta decay) force? Could someone knowledgeable about this part of Physics (ComradeRed?) please explain this to me, and why nuclei bounce off some nuclei and cause others to split etc.. Just remember that I start university in October. ;)

Many thanks in advance.

[encephalon]

If I remember correctly, I think you're complicating it more than things need to be. Nuclear fission was accomplished really before quantum mechanics was in full swing, and as such the basic principles of how it happens doesn't even take quarks into consideration. You can probably dissect it in more classical physics and have a better grasp of the basics.

Graphite is very tightly bound, making them good "walls," so to speak. Remember, you're only trying to decrease the kinetic energy of the neutron, not anything else. So in this respect, it would be like throwing a tennis ball at series of concrete walls until the ball slows down. Using a substance other than graphite, though, might be more like a net of some sort, where the ball has a chance of passing through.

In the nucleus, there are just enough protons to keep the nucleus together (balanced with the neutrons already in the nucleus), so there's not much to attract a stray neutron to the nucleus. While I'm sure that there's a more complicated explanation using quantum physics, I think it's unnecessary to have a basic grasp of it. All you're really doing is bouncing a tennis ball against walls (therefore absorbing its kinetic energy) to slow it down.

It should probably be noted that there are different emergent levels of physics; that is, what applies to quarks and such doesn't necessarily apply to the structures composed of quarks. It seems like the more macroscopic you get, the more things resemble classical newtonian physics.. which seems to explain the slowing down of neutrons well enough.

That said, I've no idea if there's a quantum explanation of it yet, although there probably is. But there are a lot of things in classical physics, so far, that we've little explanation for on the quantum level. If there is, though, comradered would probably know more than anyone here.

[Cult of Reason]

It seems to me that you are basically saying that the strong force is doing this?

[ComradeRed]

Well, alpha decay can take the form of a single subatomic particle being ejected from the atom (though most often it is in the form of a helium nucleus).

The exact reason why is not too well known...although Heisenberg did concoct some scheme using "Isospin" which is somehow related to the angular momentum of the particle, and connected this to radioactive decay. I think this is merely mathematical gymnastics than a breakthrough.


Do the quarks inside move around like electrons in an atom? Yes, sort of...but not really :lol:

It's hard to say because we don't really see quarks "move" very well. They do, however, emit particles to interact via strong force!

For the most part, it is a rather rigid configuration for all practical purposes (whatever that may be!).


If that were the case, I would expect the neutrons to be attracted to the nucleus as the down quarks would be attactred to the nucleus and not the up quark, so the down quarks would be closer and so the net fore towards the nucleus would be positive. Therefore I conclude that the electric force is NOT the cause for neutrons bouncing off nuclei. In that case, what is? The strong/colour force? Weak(??? I thought weak only affected such things as beta decay) force? Well, first of all, I am not sure if quarks really "click" together like lego bricks into said fermions (neutrons and protons).

I would expect it to be more of a soup than a collection of blocks. That's my personal bias, however.

The case for Neutrons bouncing off would be alpha decay, though this may very well be debateable by some hard core particle physicists!

Others would say it is electro-weak force (there was a unification a while back of electromagnetism and weak force).

These are all rather interesting questions for a particle physicist to think about and, not being one, I wouldn't know what to say.

But you can't neglect good ol' kinetic energy ;) That's principally how it is done in practice in nuclear power plants.

You see, the material used in nuclear power plants are really unstable, so a nudge by a sub-atomic particle would be enough to cause decay. It can easily get out of hand in a chain reaction too.

[Cult of Reason]

The case for Neutrons bouncing off would be alpha decay,

They enter, and then leave? Would it necessarily be the same neutron that is ejected as what came in?


These are all rather interesting questions for a particle physicist to think about and, not being one, I wouldn't know what to say.

So this is an unknown thing then? Right.

Since I have already started this thread...:

Having been taught how light is absorbed by a material, I am now wondering why the unabsorbed light in most solids is reflected, rather than passing right through the material. I am currently assuming that it is as a result of interaction with the electron cloud. I am thinking that possibly the photon enters the electron, exerting a force on it as if it was sent by another electron, and that the electron then ejects another photon with exactly the same energy out in the opposite direction. That, however, I do not think would account for when the light comes in at an angle.

[ComradeRed]

They enter, and then leave? Would it necessarily be the same neutron that is ejected as what came in?
No, why does it have to be?

More likely than not, it decays into some two other particles (like radon into whatchmakalit and who-seywhatsit -- technical terms there :P).

No one really cares that much to investigate it.



So this is an unknown thing then? Right.
Sort of, most people work with it at the classical level since quantum mechanics doesn't really change it. It's past the classical limit, so classical mechanics is still applicable I believe.



Having been taught how light is absorbed by a material, I am now wondering why the unabsorbed light in most solids is reflected, rather than passing right through the material. I am currently assuming that it is as a result of interaction with the electron cloud. I am thinking that possibly the photon enters the electron, exerting a force on it as if it was sent by another electron, and that the electron then ejects another photon with exactly the same energy out in the opposite direction. That, however, I do not think would account for when the light comes in at an angle. It has to do with the wavelength of the light.

Remember that you can write the wave equation as:
P*e^i(S/hbar) = wave-equation
for the amplitude P, euler's e, the squareroot of -1 i, the shorter planck's constant hbar, and the Action (time integral of Lagrangian) S.

There are points when it is zero for all practical purposes (e.g. n*pi for all non-negative numbers n). I would speculate that the reflection is when the wavelength is not equal to n*pi.

It is thus "hit" by the physical thing and shown. Otherwise, it "doesn't exist" and "passes through" at those nodes.

You would have a better argument if you pointed out photons are used as the mediating bosons of electromagnetism, and it was EM repulsion. But that wouldn't work too well ;)

[mikelepore]

Originally posted by [email protected] 21 2006, 05:23 AM
how a neutron can possibly bounce off a nucleus
The exclusion principle. You can't have two or more particles of any one type which have the same set of quantum numbers together in a very small space. That maintains some separation between any bound group of particles. It regulates the radius of the nucleus. (It also regulates atomic spacing in molecules. It makes things vibrate when they get too close, so that their average separations will increase.) If an incoming neutron or proton has enough kinetic energy to fuse with the nucleus, and if the resulting configuation is stable, that means it's initial energy has put it into a new quantum number state that was formerly empty.

Mike Lepore - [email protected]