  Fluorescence lifetimes of Eu$^{2+}$ doped in CaF$_2$ are measured at various
temperatures between 4K and 450K. The lifetime increases with between 100K and
300K, in contrast to the usual lifetime-temperature dependence. At higher or
lower temperatures the lifetime decreases with increasing temperature. The
phenomenon is explained by simulation of the energy levels and lifetimes of
low-lying $4f^65d$ states involved in the fluorescence.
