  Using highly monochromatic X-ray synchrotron irradiation ranging from 9 keV
to 100 keV, accurate Lu2SiO5:Ce3+,Ca (LSO), Lu3Al5O12:Pr3+ (LuAG),
Lu2Si2O7:Ce3+ (LPS) and Gd2SiO5:Ce3+ (GSO) non-proportional response curves
were determined. By utilizing information from escape peaks in pulse height
spectra the response curve can be extended down to several keV. A detailed
study of the non-proportionality just above the K-edge is presented and from
that a method, which we named K-dip spectroscopy, is obtained to reconstruct
the electron response curve down to energies as low as 100 eV.
