  Rf spectroscopy is one of the most powerful probing techniques in the field
of ultracold gases. We report on a novel rf spectroscopy scheme with which we
can detect very weak signals of only a few atoms. Using this method, we
extended the experimentally accessible photon-energies range by an order of
magnitude compared to previous studies. We verify directly a universal property
of fermions with short-range interactions which is a power-law scaling of the
rf spectrum tail all the way up to the interaction scale. We also employ our
technique to precisely measure the binding energy of Feshbach molecules in an
extended range of magnetic fields. This data is used to extract a new
calibration of the Feshbach resonance between the two lowest energy levels of
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