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Creating excited states

  1. Oct 13, 2011 #1
    Can one go about creating excited states of a nucleus without using neutrons?

    For example, Aluminium 26 exists in a ground state that has a half-life of about 70000 years, and decays by beta particle emission. It can also exist in an excited state that decays with a half-life of about 6.3 seconds (Wikipedia). Unfortunately this emits a positron with the decay, or captures an electron. Is there a beta emitter with these characterisitics?
     
  2. jcsd
  3. Oct 13, 2011 #2
    If you can get a gamma of the exact right energy to excite the nucleus to the state you want you should be able to hit it with that.
     
  4. Oct 16, 2011 #3
    Actually I was wondering if there was a mechanism to excite it without using something like that. I know that microwave radiation can pump energy into a nucleus under some circumstances (e.g. when the energy levels are broadened into bands under the influence of a magnetic field)

    I guess the real question is how strong a magnetic field do you need to broaden an energy level to the point where the ground state levels are connected by a ladder of much smaller steps.

    Gamma rays are too uncontrollable to be efficient in small systems. So are neutrons. Protons and electrons are tough to use but preferable because you can constrain uncertainty along some dimensions by trading off against other dimensions.
     
  5. Oct 16, 2011 #4

    Bill_K

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    NO, you cannot excite a nucleus with microwaves! The energy of microwaves is many magnitudes too small. Are you seriously wanting to create a nuclear isomer in your kitchen??

    Isomers are not produced by exciting the ground state, they're produced in the same reaction that originally made the nuclide, ground state and excited state both being produced at the same time.
     
  6. Oct 16, 2011 #5
    You can create an excited state of a nucleus with microwaves and magnetic fields - that is what NMR technology is about. Of course the excitation is tiny and does not fundamentally change the nucleus, and is just reemitted a little later.

    The Zeeman effect splits electronic states of an atom. If you put the atoms in a lattice (even without a magnetic field!), the electronic levels broaden and form a band instead of a sharp line. I was wondering if there was a similar mechanism with nucleii.

    If you look at http://atom.kaeri.re.kr/ton/nuc12.html and check out U236 you see a metastable state at "0.000MeV" which has a half-life of 121ns compared to the ground state with a half-life of 23420000 years, and both disintegrate via spontaneous fission. Since they use three decimal places I assume the separation between those states is under 500eV. It is an even-even nucleus so magnetism is not going to work to bridge the gap. But a 500eV X-ray might. How small is the gap actually?

    Np237 is similar, but the first excited state is at 2.8MeV. I found some others with differences as little as 75keV. Al26 has a gap of 228keV. Interestingly, there are some where the excited state has a longer half-life than the ground state!

    The ones that decay by IT back to the ground state are uninteresting (to me at least).
     
  7. Oct 17, 2011 #6
    NMR is totally different, that is just about the energy difference between nuclei whose magnetic dipole moment is aligned with the magnetic field and those which aren't. I.e. the nucleus is just rotated in space. It isn't an "excited state" of the nucleus.

    There IS a nuclear Zeeman effect, it is associated with the Mossbauer effect, but the shifts in the energy levels of the nucleus are very small. Check out this Mossbauer spectrum for Fe-57, the splitting is of the order of 10^-7 eV: http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/ironze.html.

    Hmm, ok so it seems there are some low energy nuclear transitions based on this effect. Check out http://en.wikipedia.org/wiki/Isotopes_of_thorium, it talks of a nuclear isomer with an excitation energy of ~7.6 eV. Looking at the reference for this measurement they say it is due to the energy splitting of the ground state doublet. Pretty cool.
     
  8. Oct 17, 2011 #7

    Bill_K

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    I wasn't aware of this unique state, Th-229m, which apparently has a remarkably low excitation energy of 7.6 eV. But even this is still in the ultraviolet range, shorter than visible light. The Wikipedia article suggests it might prove possible to populate the state by means of a laser.

    You're thinking of doing this with a microwave? An energy of 7.6 eV corresponds to a wavelength of about 160 nm, whereas commercial microwave ovens operate at 2.45 GHz, with a wavelength of 122 mm, six orders of magnitude lower.
     
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