The recoil phenomenon can spoil resonant scattering processes. However, if you’ve ever studied resonant scattering of light on atoms, you did probably not examine recoil. Here, you will see why. And you will appreciate why recoil is important when γ-rays scatter on nuclei.
Much of the initial activities that led to the development of nuclear resonant scattering originated from interest in similarities between “light optics” and “γ-ray optics”. Search the meaning of the three concepts from light optics listed in the form hereunder, and translate them to their counter-part in γ-ray optics. (Wikipedia or the “IUPAC gold book” are good places to start looking for the meaning of these concepts).