  Caesium iodide is one of the more extensively studied scintillators. Here we
present X-ray luminescence spectra, scintillation light output and decay curves
as function of temperature, from room temperature down to below 10 K. Features
of the observed intrinsic luminescence are explained in terms of radiative
recombination of on- and off-centre STE. A model permitting interpretation of
the dynamics of luminescence changes in CsI with temperature is suggested. This
model includes adiabatic potential energy surfaces (APES) associated with
singlet and triplet states of self-trapped excitons (STE) and explains the
variation of the luminescence spectra with temperature as a result of
re-distribution in the population between on- and off-centre STE. The
temperature dependence of the scintillation light yield is discussed in the
framework of the Onsager mechanism.
