It was noted that the Eu2+-doped material could be made sensitive to stimulation further into the infra-red by doping with Ca2+ or Sr2+. Electron paramagnetic resonance, electron nuclear double resonance (and their optically detected versions) showed that, upon X-irradiation, FA(Br-,Ca2+) centers were formed. These acted as photo-stimulated electron traps in Ca2+-doped material; where one of the 2 nearest Ba2+ neighbors along the c axis was replaced by Ca2+. The FA(Br-,Ca2+) center had tetragonal symmetry; as did the F(Br-) center. The optical absorption band of the FA(Br-,Ca2+) centers was red-shifted. No such FA(Br-,Ca2+) centers were detected after additive coloration alone. The red-shift of the photo-stimulation spectrum was attributed to the formation of FA(Br-,Ca2+ or Ba2+) centers as electron traps.

S.Schweizer, P.Willems, P.J.R.Leblans, L.Struye, J.M.Spaeth: Journal of Applied Physics, 1996, 79[8], 4157-65