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Resting masses of isolated neutrons versus neutrons in atoms

  1. Jul 12, 2015 #1
    I'm a high school student reading through a book on the discovery of the Higgs boson, and, among several other things, there's one part that I don't understand completely.

    I understand that the Higgs field is what gives mass to lots of particles that would otherwise be the same without the Higgs breaking their symmetry, and I also understand that particles like protons and neutrons aren't massive just because of the Higgs.

    The explanation that the book offers is that the mass of a neutron is caused by the kinetic energy of the constituent quarks held together by the strong interaction. That much makes sense to me, but why wouldn't neutrons in a nucleus have higher mass? They're bound to protons by the strong interaction as well, and that interaction holds a lot of energy that we've used to make bombs and power plants and stuff. Are neutrons that are 'alone' lighter than neutrons next to protons?

    Sorry if this is a question that's been covered before. I tried a lot of different ways to search my question but didn't get the answer I was looking for.

  2. jcsd
  3. Jul 12, 2015 #2


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    Sure! But it's the other way around. Neutrons (and protons) in nuclei are lighter than their unbound partners. Or more precisely - nuclei are less massive than the sum of the masses of their individual constituents. This is something that is quantified by the "mass defect" of nuclei - the difference between the mass of the nucleus, and the sum of the individual masses of protons and neutrons. For 16O this is - 123.53 MeV/c^2. http://www.wolframalpha.com/input/?i=mass+16O+-(8*mass+neutron)+-+8*(mass+proton)+

  4. Jul 12, 2015 #3


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    No, if they were the nucleus would be unstable. There is some additional energy due to the kinetic energy of the nucleons, but this is more than compensated by the fact that the potential energy is smaller.
  5. Jul 12, 2015 #4
    So, I think I see how some of their mass becomes the energy binding the nucleus together, but why doesn't that energy stay as mass?

    Like, if the mass of a neutron comes from the binding energy of the quarks, then why doesn't more mass come from the binding energy of a nucleus? What's the difference?
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