http://en.wikipedia.org/wiki/Mössbauer_effect I was looking at this article, trying to prepare myself for experimenting this effect. What I understood is that Moessbauer effect explains the reason why you can have gamma absorbtion/emission in solids while you can't for gases. However at the description it says: From this I don't understand what's the problem of the recoiling... You send some gamma on a nucleus, it gets excited and then it emits the extra energy in another gamma and recoiling. Why should we care about the last gamma's energy? Secondly, what is the reverse transition? If it means that they can be reabsorbed by another nucleus of the same matterial, then it should excite it in the same energy level and not less. Suppose you have a 3MeV ray emitted from the 1st nucleus, it should excite the 2nd to 3 MeV again...and so on... Finally I don't understand why nuclei have fixed positions in a crystal and the recoil is taking place for the crystal as a whole. Couldn't they be recoiled but nothing happen to the crystal? like for example cause some oscillations within it. As far as I know the states [gas to solid] is an atomic thing, so why would it concern the nuclei?