Energy loss from stationary nuclei

The Mossbauer effect is the resonant absorption of low-energy y-rays by nuclei bound in solids in such a way that there is no energy loss due to nuclear recoil. It depends upon the monoenergetic nature of the y-ray emitted from an excited nucleus. When this ray falls on an unexcited nucleus of the same isotope, it will be absorbed if the nuclei are stationary relative to each other. However, if there is relative movement, there will be a Doppler shift in the frequency of the emitted y-ray so... 

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