Photochromism in Calcium and Strontium Fluoride Crystals Doped With Rare-Earths Ions

This paper reports the absorption spectra of photochromic centers in CaF ₂ and SrF ₂ crystals doped with Ce ³⁺ and Gd ³⁺ impurities and thermal decay of the centers in temperature range 80-500 K. The ionized photochromic color centers are generated in crystals under low-temperature X-rays irradiation. These centers are transformed into photochromic color centers upon heating of crystals. All color centers decay at temperature about 500 K.