Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Nucleus of atom

  1. May 31, 2007 #1


    User Avatar

    What has mainstream decided is the arrangement of the protons and neutrons in the nucleus of the atom?
    Lattice or cluster structures or ???
  2. jcsd
  3. May 31, 2007 #2
    Nothing, the nuclear structure is explained in terms of the interactions between neutrons and protons (coloumb force between protons and residual strong force between protons and neutrons, which holds the nucleus together). Any absolute statement on the position violates the uncertainty principle.

    (residual) strong force : http://hyperphysics.phy-astr.gsu.edu/hbase/forces/funfor.html#c2

    HINT : make sure you understand the difference between strong force and residual strong force :wink:
    Last edited: May 31, 2007
  4. May 31, 2007 #3


    User Avatar

    Hi marlon!
    Here is what I got from that link.
    Therefore, the uncertainty principle and the Pauli exclusion principle would have to be constrained in order to be able to obtain a structured model.
    The Strong Force
    A force which can hold a nucleus together against the enormous forces of repulsion of the protons is strong indeed.
    The strong force between nucleons may be considered to be a residual color force.

    The liquid drop model is given at
    Liquid Drop Model of Nucleus

    The Weak Force
    One of the four fundamental forces, the weak interaction involves the exchange of the intermediate vector bosons, the W and the Z. Since the mass of these particles is on the order of 80 GeV, the uncertainty principle dictates a range of about 10-18 meters which is about 0.1% of the diameter of a proton.
    To try and find a model for protons and neutrons it now becomes necessary to understand Quarks and how that 0.1% of the diameter of a proton could be structured to be considered to be a residual color force
    What are the competing models? What are they using as a constraint (I assume, changes to the uncertainty principle and the Pauli exclusion principle).
  5. May 31, 2007 #4
    We are not really done with studying the nuclear structure, even in terms of effective forces between nucleons. It is however true that we already reached an excellent level of understanding.

    What remains to be done is really to understand to role of quarks and gluons in the nuclear medium. For instance, we have a good deal of hints pointing towards the fact that hadrons in the nuclear medium are not the same as free hadrons. The propagate differently. There is color transparency, nuclear shadowing, and many technical details (some of them very minute), but the mere fact that the neutron is stable inside the nucleus is already an excellent indication.

    So overall, the picture is still the following (IMHO) : proton and neutrons in the nucleus are vey much alike orange and apples stacked together. Their "quantumness" is tiny enough so that we can apply semiclassical methods and get excellent results. The inherent quantum and extreme-relativistic nature of quarks and gluons inside hadrons however makes it very difficult to go beyond this simple picture, and in fact very little progress has been made since about Heisenberg was working on this problem (ok I am a bit provocative here :biggrin:)
  6. May 31, 2007 #5


    User Avatar

    Hi humanino!
    Maybe you or someone can steer me in another direction.
    (You will probably need to help me understand the papers.)

    From my understanding, and also from what you have said, what we have figured out is from high energy.
    Is there a low energy approach (ground state) low temp.?
    Is there any "mapping" coming out of this low energy approach?
  7. Jun 1, 2007 #6
    This is disturbing me.
    Can you elaborate on the feeling that you have that what we have figured comes from high energy ?
    What do you call high energy ?

    See, there is a natural scale in QCD, the real theory of the strong interactions. It is approximately the scale corresponding to the size of hadrons, and also the scale at which the QCD coupling constant is 1 (and perturbation calculations must fail). So usually, high energy refers to an energy scale much larger than this, say several tenth of GeV at least. There, the coupling constant of QCD is smaller than 1 and perturbation calculations allow you to use QCD approximately enough. At a much smaller energy scale, say much less than 0.1 GeV, we have chiral perturbation theory making wonders.

    So the issue with strongly interacting systems is really in between those two scales, where QCD perturbation calculations fail and hadron structure is manifest, aroung 1 GeV or so. This is the intermediate energy scale.
  8. Jun 1, 2007 #7


    User Avatar

    You are right. I was too vague and general by saying high energy.
    I was thinking of less than 0.1 GeV for low energy.
    On reflecting, I get the feeling that we cannot probe into the nucleus or get any information of what is happening inside the nucleus at energy levels of less than 0.1 GeV. We do not have any tools that can probe with any accuracy. (uncertainty principle)
    What I was wondering was having a “trapped” nucleus that would be probed/pocked with ??? and from that it would be possible to figure out where the neutrons and protons are located or what they are doing.
  9. Jun 1, 2007 #8


    User Avatar

    Good Day!
    A search gave me more than what I'll ever be able to learn.
    Do you have any suggestions of where to start to figure out where the neutrons and protons are located or what they are doing?

    I'll keep the following for "light" reading. :rofl:
    Nuclear Science: Impact, Applications, Interactions
    Collisions of Cooled and Polarized Sodium Atoms

    Study Group on Radioactive Nuclear Beams

    Mass Measurements of Exotic Nuclei at GANIL using the CSS2 and CIME Cyclotrons
  10. Jun 1, 2007 #9


    User Avatar

    Staff: Mentor

  11. Jun 1, 2007 #10
    Just to be clear, constructing more accurate probes will not eliminate the uncertainty coming from the HUP. It seems that you are suggesting that the accuracy of measurement devices and the inaccuracy due to the HUP are somewhat related. THEY ARE NOT. The HUP has NOTHING to do with devices what so ever and is due to the QM nature of atomic scaled phenomena. Theoretically, we could make devices that determine the exact position of an electron with some momentum p. The point is that if we measure the same electron with same momentum p, the acquired value for the position with be again an exact number. But that number will differ from the previous position value. NOT because of the device but because of the HUP ! You see ?

  12. Jun 2, 2007 #11


    User Avatar

    I’ve have gone through the light reading and I would like to add http://www.jlab.org/highlights/phys.html
    Jefferson Lab

    Before going into the reference papers (which cost money) presented by Jefferson Lab. I would like to get some explanations of some points.
    Keep in mind that you will not see me in a lab. or behind a calculator. I’m only trying to understand what is going on. (That’s Life!!)
    An understanding seems to require knowing what is happening between the proton drip-line and the neutron drip-line. (Nobody knows. It’s ongoing research)
    I understand that the description that are used are representing a math formula for a symmetric action.
    (I’ve expanded my the vocabulary but it’s not memorized yet so I’ll make mistakes)

    Study Group on Radioactive Nuclear Beams
    Nuclei with an excess or deficiency of neutrons relative to the valley of stability are unstable and they decay [e.g. via the emission of  particles (e- or e+)]. The existence of nuclei is limited by frontiers in three directions of the nuclear chart: i) the neutron rich limit for fixed Z; ii) the proton rich limit for fixed N; iii) the large mass limit. We have not fully reached any of these limits except for the very light nuclei. (proton drip-line, neutron drip-line)
    My first question ….
    What is suppose to be happening with the following description? What are they extrapolating?
    Colour Transparency
    Letting the mini-state evolve during its travel through different nuclei of various sizes allows an indirect but unique way to test how the squeezed mini-state goes back to its full size and complexity, i.e. how quarks inside the proton re-arrange themselves spatially to ``reconstruct'' a normal size hadron.
    “travel through different nuclei of various sizes” Too general for me to understand.
    Can I express “squeezed” as setting borders/contained to a “waves”. Like saying it’s not spread all over the place. Maybe …. A plaquette?
    ”goes back to its full size and complexity” What kind of formula does this express?

    I thought that the “complexity” was cut off by inserting a number by hand. (renormalizing)
    In your words …. What is he saying?
    Remember that I’m thinking along the lines of structure that is separate from movement/dynamics/actions. I’m looking for the “model” that can do all that action.
    I’ll be keeping/referring to http://hyperphysics.phy-astr.gsu.edu/hbase/particles/quark.html#c1 to try to remember everything.
    Quarks and Leptons are the building blocks which build up matter, i.e., they are seen as the "elementary particles". In the present standard model, there are six "flavors" of quarks. They can successfully account for all known mesons and baryons (over 200)
  13. Jun 4, 2007 #12


    User Avatar

    After finding the right words for a search I think I did very well.

    Your description/explanation,
    Now, I think I understand what you said.
    I wrote down dozens of questions as I was doing my search.
    They were all the wrong questions to ask.

    Inside the nucleous is the standard model.(a concept) It's a relic of pre-computer days.
    How don’t know how anyone can answer any question from someone who does not know the Parton Model, which seems to be the working tool.
    Handbook of perturbative QCD
    I dont know the Parton Model. So I realize that I cannot even formulate a proper question.
    It is no wonder that so many people are being led down the garden path and that you all have to keep trying to get rid of misconceptions.

    The generalized parton distributions, introduced nearly a decade ago, have emerged as a universal tool to describe hadrons in terms of quark and gluonic degrees of freedom
    The ultimate goal of the GPD approach is to provide a three-dimensional spatial picture of the nucleon, direct measurement of the quark orbital angular momentum, and various inter- and multi-parton correlations.

    HAS generalized parton distributions (GPD) PUBLISHED THEIR “three-dimensional spatial picture”?

    I did find a nice conceptual picture of the proton
    How the Proton Got its Spin
    Maybe one question can be answered
    What is a sea of gluons?
  14. Jun 4, 2007 #13
    1) The gluon (or quark) "sea" is the "cloud" of virtual gluons (quarks) and their antiparticles that "exist" inside the nucleon. You can think of them as a sea of particles constantly popping in and out of existence, if you want. Often when you scatter a probe particle off of a nucleon, it doesn't hit one of the physical quarks "making up" the nucleon -- these are called the valence quarks -- but instead one of these temporary quarks or gluons.

    2) Keep in mind there's a big difference here between nucleon structure and the nuclear structure. The nucleon -- a proton or neutron -- is a strongly-coupled, very "quantum" object. This is what people use PDFs, sea quarks/gluons, etc. to talk about. The nucleus, on the other hand, can be rather well-described as humanino said semiclassically: as a bunch of protons and neutrons coupled together. You still have to talk about the nuclear wavefunction, but you can do decently well without introducing seas of virtual particles, etc.


    Typically, parton distribution functions are given in terms of the fractional momentum carried at a given collision energy (actually momentum transferred). This is the quantity most relevant to experiment.

    You would have to be careful in transforming this into a spatial picture. Keep in mind that the quantum world is odd, and in a very real way it doesn't make sense to ask "how are the partons distributed inside the nucleon" without specifying how you intend to measure those partons. Different ways of measuring give different results.

    To put it a different way, it's simply *not the case* that you can imagine the nucleon as a particular physical arrangement of quarks & gluons, not at all. It's a strongly-coupled QFT bound state, and so you would have to specify the state of all the quantum fields that make it up.

    It's not even the case that you can imagine even the *nucleus* as a physical arrangement of protons an neutrons. As I said above, you do get a decent picture by thinking of the nucleus as made up of a fixed # of protons and neutrons. But the nucleus is still a quantum object, and so we have to specify it in terms of its wavefunction. This is actually relatively straightforward, and gives a good broad-based picture of nuclear structure. But again it doesn't correspond to a simple physical arrangement of protons and neutrons; instead you get something a little like the orbitals that describe the atom's "shape."
  15. Jun 4, 2007 #14


    User Avatar

    Hi damgo!
    You say "inside the nucleon". Is that because that is the only place that we have reactions that we can input into the formulas?
    Would it be speculation or experimental evidence to say that the sea is everywhere?

    Are there ways of producing "a spacial picture" within those constraits?
    What about the picture that I found of the proton? Is that one of the quantum possibilities?
    How the Proton Got its Spin
  16. Jun 4, 2007 #15
    Hi jal!
    Just terminology. The gluon/quark "sea" that is everywhere is typically called the QCD vacuum. When you talk about sea quarks people assume you mean inside some strongly-coupled composite particle.

    Sure... there are lots of them, that's the problem. :)

    Looking at that paper, it's not clear to me exactly what they're doing, but the general idea is clear. Roughly the thing to do is take the momentum-parameterized information on the structure -- the pdfs or the form factors -- and Fourier transform it to get position information.

    Keep in mind that the picture shown in that article is just one of an infinite series of pictures: you get a different picture for every value of parton momentum. And, of course, that's just one way of coming up with spatial visualization information; I've seen several. It's better to think of these things as more of "visualization tools" to help find pretty representations of the complex underlying reality, rather than as any one of them being a true picture of how things "look."
  17. Jun 4, 2007 #16


    User Avatar

    I may not have all the fact but it appear to me that "waves" and "particles" are the tools being used to find the true picture of how things "look".
    Your way of expressing it would be equivalent to saying, "Forget it. There isn't a structure/picture."
    I do not support "a complex underlying reality" that we cannot figure out.
    I go along with quantization/discrete to a minimum scale. That scale is now being investigated.
  18. Jun 4, 2007 #17
    I guess I may not have been clear. I'm not saying that there isn't an underlying reality. I'm saying that the picture of the nucleus of being made up of particular components existing in well-defined positions -- that picture is simplistic and wrong. Quantum mechanics gives us an underlying picture, but it's a wavefunction, not an ordinary picture. One can (at least in principle) specify the superposition of field configurations that we call a nucleon. As far as our current understanding goes this is the underlying reality of that nucleon. This is, however, not just a classical picture of component objects clumped together somehow. To go from the underlying quantum reality to a simple picture involves throwing out lots of information -- serious (over)simplification -- so naturally there are many ways to do it, depending on what kind of information you want to show in the simple picture.
  19. Jun 4, 2007 #18


    User Avatar

    I do not like to argue about faith and opinions.
    Do you have some other facts to present that we can discuss.
    You said only, "typically called the QCD vacuum".
    Have you got an experiment that can be discussed?
  20. Jun 5, 2007 #19
    Or in short, you just described the dynamical quarks : http://cerncourier.com/main/article/44/5/13/1/cernlatt2_6-04

    Their presence also explains why the mass of a proton exceeds the sum of the three valence quarks which are bounded with each other.

  21. Jun 5, 2007 #20
    Google for the Casimir Effect and read the link i posted above on dynamical quarks.

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?