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Why does resonant absorption disappear at high Doppler velocities?

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Figure 1: Spectrometer utilizing Mössbauer effect concept Effect is usually observed by measuring transmission of gamma rays from radioactive source through absorber containing resonant isotope. — Mössbauer effect Figure 1: *Spectrometer utilizing Mössbauer effect concept* Effect is usually observed by measuring transmission of gamma rays from radioactive source through absorber containing resonant isotope.

Resonant absorption disappears at high Doppler velocities because the Doppler shift moves the energy of the emitted gamma rays away from the precise energy required for resonance absorption by the target nuclei.12

For resonance to occur, the energy of the incoming gamma ray must match the difference between two specific energy states of the absorbing nucleus. In a Mössbauer effect Doppler-velocity spectrometer, the source is moved at controlled velocities to shift the gamma-ray energy. When the source is stationary or moving at very low velocities, the energy matches the resonance condition, resulting in a drop in the counting rate due to absorption. However, as the velocity increases, the resulting shift in energy becomes large enough that the gamma rays no longer correspond to the required resonance energy, causing the resonant absorption effect to vanish.12