Resonant phonon scattering by interstitial O at 878GHz was investigated by means of phonon spectroscopy, using superconducting tunnelling junctions. It was found that the acoustic scattering cross-section was anisotropic. By studying the phonon scattering of 18O, it was shown that O concentrations of 7 x 1013/cm3 could be accurately determined.

C.Wurster, E.Dittrich, W.Scheitler, K.Lassmann, W.Eisenmenger, W.Zulehner: Physica B, 1996, 219-220, 763-5