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Originally Posted by Sylas
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Originally Posted by papageno
This is the typical situation for Mössbauer effect.
Take a crystal lattice containing radioactive (gamma emitting) Iron isotopes.
The excited Iron nuclei emit gamma photons. If the energy of that photon is lower than the energy necessary to excite a phonon (quantum of lattice vibrations), the emission of the gamma photon is effectively recoil-less. The recoil is transferred to the whole crystal (10^23 atoms), rather than to the emitting atom (which would make the atom vibrate in the lattice).
The spectral line corresponding to this recoil-less emission, is very narrow, and makes some nice experiments possible.
For example, it has been used to measure gravitational red-shift on Earth, between the basement and the top of a tower.
In a teaching lab, I used it to measure the hyperfine field in Iron.
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This much I think I understand. |
When you jump, the Earth recoils.
However, the Earth's mass is so much larger than yours, that the recoil is negligibly small.
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Originally Posted by Sylas
As a test of my understanding... it seems to me that the recoil-less emission should mean that photons can be absorbed by an atom, and then re-emitted with exactly the same frequency to all intents and purposes, with no energy loss to the lattice. The re-emitted photon can continue to pass through the lattice, possibly being absorbed and re-emitted a number of times, all with no loss of energy. But so far I see nothing to constrain it to remain in a straight line.
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The gamma photons are not constrained on a line.
Whether the emission (absorption does not seem very likely, since the gamma photon needs to hit a nucleus) is recoil-less, depends on the lattice.
The phonon spectrum is not necessarily isotropic, so you might have absence of recoil in one direction, but recoil in another direction even though the energy is the same.
The emission by the nucleus is isotropic.*
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Originally Posted by Sylas
I've seen references compare this with "resonance fluorescence of the yellow D lines of sodium in sodium vapour". See, for example Recoilless nuclear resonance absorption of gamma radiation, which is Rudolf Mössbauer's Nobel lecture. In this case, the fluorescence means that the whole gas glows as photons bounce around within the gas. I guess the same thing happens for gamma photons in a crystal lattice at the excitation frequency of the Mössbauer effect. |
There is definitely similarity between transitions between atomic levels, and transitions between nuclear levels: the difference is the energy scale.
However, I do not expect absence of recoil in gas atoms, because the atom are not constrained. So I would expect broadening of the spectral line in the fluorescent emission.
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Originally Posted by Sylas
If a beam of gamma ray photons of exactly the right frequency are directed into the lattice, I would expect to see photons emerging from the lattice at the same frequency and in every possible direction.
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You have to consider the probability for a nucleus to absorb a gamma photon.
Then you have to consider the spectrum of the lattice vibrations, to see when recoil is absent.
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Originally Posted by Sylas
Have I got this right? When I first read Lyndon's paper, I got the impression that Mössbauer effect meant that the photons had no scatter angle, but now I think this is just another error on Lyndon's part. Or perhaps it is my error and I have misunderstood Mössbauer's lecture.
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Don't worry. Ashmore does not understand the Mössbauer, hence his use of the name is misguided.
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Originally Posted by Sylas
Unless I am badly mistaken, Lyndon's effect is completely the opposite of the Mössbauer effect in all the essential respects: - The Mössbauer effect works only for very very tightly constrained frequencies. The Lyndon effect allegedly works right across the spectrum.
- The Mössbauer effect is distinguished by no recoil and no redshift. The Lyndon effect allegedly involves a redshift to the photon and two recoils to an electron.
- The Mössbauer effect involves something comparable to fluorescence, in which a lattice "glows" with gamma ray photons emerging in all directions. The Lyndon effect allegedly involves no scattering at all, with the photons emerging with direction unchanged.
It is the last point on which I am least certain. |
The main point is that the recoil is absent because there is not enough energy to excite lattice vibrations.
Normal emission: jumping from a spring-board.
Mössbauer emission: jumping from a hard concrete wall.
* Well, it depends on magnetic field and temperature.
For anisotropic gamma emission, search for
low temperature nuclear orientation.