Afterglow Decay of CaF2

Calcium fluoride crystals, excited by low-energy ($\approx 10$ kV) electrons, are found to exhibit a weak but persistent afterglow in the temperature range 28-250°C. The portion of the decay curves between 2.5 and 30 sec for different temperatures of excitation has been analyzed assuming exponential decay laws for each of the several traps presumed to be present in the crystal. Traps of comparatively low depth (in the range 0.49-0.70 eV) which escaped detection by thermoluminescence experiments have been determined in this way. The results show that the value of the frequency factor $S$, which is about 2.2×${10}^8$ ${\sec }^{-1\mathrm{}}$ for excitation in the neighborhood of room temperature (28°C), is lowered by a factor of 10 for higher temperatures of excitation.