  Non-proportional response of the scintillation yield of NaI:Tl was measured
using highly monochromatic synchrotron irradiation ranging from 9 to 100 keV.
Special attention is paid to the X-ray escape peaks. They provide us additional
information about non-proportional response in the range 0.9 to 12 keV. A rapid
variation of the non-proportional response curve is observed near the Iodine
K-electron binding energy. A dense sampling of data is performed around this
energy and that data are used to apply a method, which we call K-dip
spectroscopy. This method allows us to derive the electron response curve of
NaI:Tl down to energies as low as 30 eV. A comparison of our data with data of
others employing different methods is made. Advantages, limitations and
peculiarities of presented techniques and methods are discussed.
