Non-proportional scintillation response of NaI:Tl to low energy X-ray photons and electrons

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.