Nuclear Forward ScatteringIntroduction to the theory behindWhile conventional Mössbauer spectroscopy (MS) measures the resonant absorption process, nuclear forward scattering (NFS) is regarded as a scattering process. After the excitation of a nucleus by an incoming gquantum the subsequent nuclear decay takes place
The probability for internal conversion is for iron much larger than that for the radiative decay, only 8.2% of the excited nuclei decay by emitting a photon. Absorption and emission processes involved in the radiative decay can proceed without changing any quantum number of the involved nuclei and lattice. In this case the coherence of the radiation is preserved, the scattered quanta are scattered into forward direction (and Braggdirections in case of a single crystal) and different nuclear transitions interfere with each other. The theory describing the whole process consists of two parts.
The theory of nuclear resonant scattering was introduced and developed in the 60's and 70's by Trammell [], Kagan and Afanas'ev [,,,,], Trammell and Hannon [,,,,]. The following introduction to the theory of forward scattering is based on the review articles [,,]. 
