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Originally Posted by Sylas
PS. I had not heard of the Mössbauer Effect before reading Lyndon's stuff. Initially I accepted his description of straight line transmission with no scatter, and repeated it myself in some posts. I now suspect this is wrong, and that photons can be scatted in all directions. The distinguishing feature is not a lack of scatter, but negligible recoil in the atom, and hence negligible change in wavelength. I'll be checking this further to confirm or retract; whether I'm right or wrong I'll have learned something.
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Ashmore called his "effect"
double Mössbauer, because he does not actually understand what characterizes the real Mössbauer effect.
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.