PROPERTIES OF COLOR-CENTERS IN CAF2 SINGLE-CRYSTALS SUBJECTED TO HIGH ELECTRIC-FIELDS AND X-RAY-IRRADIATION

Optical absorption in the region 200 to 680 nm, and thermoluminescence, have been studied for CaF2 single crystals subjected to high d.c. fields (up to 70 kV cm-1) and later irradiated with X-rays for 2 h. Similar data were also taken when the crystals were X-ray-irradiated under high d.c. or a.c. fields. Under both conditions, absorption bands with peaks at 375 and 580 nm were obtained. Partial thermal bleaching measurements carried out on the optical absorption of CaF2 crystals subjected to a 50 kV cm-1 d.c. field and later irradiated with X-rays indicate no correlation between the destruction of the colour centres (responsible for the absorption bands) and the thermoluminescence exhibited by these samples. It is noticed that the influence of high d.c. or a.c. field on the colour centre concentration is different depending on whether the CaF2 crystals are first subjected to the field and later X-ray irradiated, or are X-ray irradiated under the fields.