Velocity modulation of gamma-rays

The recoil-free y-ray energy of a typical Mossbauer transition is so precisely defined that its Heisenberg width corresponds to the energy change produced by an applied Doppler velocity of the order of 1 mms. It is therefore possible to imagine a particular relative velocity between source and absorber at which the y-ray energy from the source will precisely match the nuclear 

A Mossbauer spectrum comprises a series of measurements at different velocities (that is energies) across the resonant region. The convention universally adopted is that a closing velocity between source and absorber (i.e. a higher energy) is defined as positive. 

It has already been shown in Chapter 1 that the resonant absorption curve for an ideally thin source and absorber has a width at half-height F, which is twice the Heisenberg width of the emitted y-photon. The Doppler velocity v corresponding to this energy F is given by 

A full list of such values in Appendix I shows that the velocities range from 

3-1 X 10 for up to 202 mms for Re. They are all very small when compared with the tremendous velocities ( 7 x 10 mm s ) used by Moon in 1950 to detect nuclear resonance fluorescence without recoilless emission, and show dramatically that the Mossbauer technique eUminates both recoil and thermal broadening. The Heisenberg relation means that an excited state with a shorter half-life has a greater uncertainty in the y-transition energy and hence a broader resonance line. 

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