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Radiation emission by matter

  1. Nov 2, 2015 #1
    as described by einstein in his paper published a few days ago in 1917, photon emission can be spontaneous or stimulated. In stimulated emission, atoms can be in an 'excited' state and the passage of a photon through such a population can can cause the ejaculation of energy in the form of an additional photon with the same energy and phase as the incident photon. This chain reaction leads to coherent radiation (masers, lasers, etc).

    But what happens if instead of whole atoms in the lasing medium, one just puts nuclei. Can a nucleus be in an excited state? (like bombarding it with neutrons first) If not, can one isolate nuclei from unstable isotopes and use those? Will it emit photons in the same way or particles like neutrons?

    A corollary question is what happens when electrons instead of photons are used to bombard an excited population of whole atoms? Do we get a cascade of electrons? An electron laser?

    We know what happens when we throw neutrons at excited (unstable) atoms like Pu239.....
     
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  3. Nov 2, 2015 #2

    mfb

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    Yes.
    Depends on the nucleus. Gamma radiation is just photon emission.

    Thorium-229 has an excited state just a few eV above the ground state. It might be possible to use this state for various laser-related applications.
    I don't think there is induced emission of electrons in a coherent way.
     
  4. Nov 2, 2015 #3
    Thank you. The emission of photons by stimulated nuclei (like the thorium you mentioned) should tell us about the energy levels in a nucleus, the same way we probe the quantum nature of the energy levels of electrons. Is this too hard to do? Why do we need to throw energetic particles at nuclei to dissect them? Can we not learn about nuclei by exciting them (or using their unstable isotope cousins) and then probing them with photons?

    .In thinking about the corollary question, I remembered cathode ray tubes and television. In those an electron beam is shot at whole atoms. Granted the target atoms are not in an excited state. But the result is emission of photons. Likewise, an X-ray tube uses higher energy electrons shot at a metal target. Again, X-rays are produced, not particles. We know we can make an electron gun and isn't that by definition the spontaneous emission of electrons?

    why hasn't stimulated emission of electrons been observed?

    Sorry for asking such simple questions but Dr. Google simply obfuscates my question with irrelevant minutiae. And physics textbooks don't even consider these questions.
     
  5. Nov 2, 2015 #4

    mfb

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    Gamma spectroscopy is done.
    The reverse is hard - you have to hit the nuclear transition energy very precisely, and usually there are no suitable monochromatic sources in that energy available. With one exception: other nuclei of the same isotope. See Mössbauer spectroscopy. The high frequency-sensitivity of nuclei has been used as first test of gravitational redshift on Earth.

    This is purely an effect of electrons outside nuclei. Electrons are accelerated or change energy levels and emit radiation incoherently.
    Sure, but you cannot make a laser out of that.

    I don't see which system you want to use to get coherent electron emission.
    Atom lasers exist, but they start with cooling atoms to get a coherent state.
     
  6. Nov 2, 2015 #5

    phyzguy

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    You get stimulated emission of photons because photons are bosons. With bosons, you can have arbitrarily many particles in the same quantum state. Electrons are fermions, and you can have at most one fermion in a quantum state. So stimulated emission of electrons is impossible.
     
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