  The nonproportional scintillation response of LaBr3 doped with 5% Ce3+ and of
LaCl3 doped with 10% Ce3+ was measured using highly monochromatic synchrotron
irradiation. To estimate the photon response, pulse height spectra at many
finely spaced energy values between 9 keV and 100 keV were measured. The
experiment was carried out at the X-1 beamline at the Hamburger
Synhrotronstrahlungslabor (HASYLAB) synchrotron radiation facility in Hamburg,
Germany. Special attention was paid to the X-ray fluorescence escape peaks as
they provide us with additional information about photon response in the range
1.2 - 14.5 keV for LaBr3:Ce and 2.0 - 11.6 keV for LaCl3:Ce. A rapid variation
of the photon response curve is observed near the Lanthanum K- electron binding
energy for both scintillators. 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 curves of
LaBr3:Ce and LaCl3:Ce down to energies as low as 0.1 keV.
