NSR is a means to distinguish the neighbouring energylevels due the hyperfine splitting. It is thus a method in material study. To attain the energy needed to excite nuclear energy levels, hard X-rays are required. Thanks to synchrotron radiation, one can gain a steady stream of X-rays with high brilliance.
The probed material, of which the energy levels are studied, gets exposed to the X-rays. If the synchrotron radiation first passes through an undulator, the spectrum is broadened and multiple nuclear excitation are possible.
The light is absorbed and re-emitted, thus creating a scattering phenomenon. The emitted electromagnetic waves have different wavelenghts due to the different excited energy levels. These wavelengths interfere with eachother, the superposition leads to quantum beats in the temporal evolution of the decay.
In NSR one thus studies this temporal evolution to reverse engineer with fourier transformation to the wavelengths and the energy levels of the probed atoms.