Inducing and detecting collective population inversions of M\"ossbauer   nuclei
  Up to now, experiments involving M\"ossbauer nuclei driven by x-rays have been restricted to the low-excitation regime. Here, a setup is proposed which promises significant excitation, ideally exceeding full inversion of the nuclear ensemble, at x-ray light sources under construction. We further introduce a method to experimentally verify such inversions, in which population inversions manifest themselves in symmetry flips of suitably recorded spectra. It neither requires per-shot spectra of the incoming x-ray pulses, nor absolute measurements of the scattered light intensity. 
K. P. Heeg, C. H. Keitel, J. Evers
quant-ph physics.atom-ph physics.optics