AUGER-FREE LUMINESCENCE OF THE BAF2SR, BAF2MGF2 AND CSBRLIBR CRYSTALS UNDER EXCITATION OF VUV PHOTONS AND HIGH-ENERGY ELECTRONS

The emission spectrum, the time dependence of the luminescence, the excitation spectrum and the reflectance spectrum have been measured for the pure BaF2 crystal, the BaF2:Sr crystals with Sr++ concentrations of 2 and 5 mol%, the BaF2:MgF2 crystal with a mole mixing ratio of 1:2, and the pure CsBr crystal and the CsBr:LiBr crystal with a mole mixing ratio of 1:1. The measurements have been made by using synchrotron orbital radiation and high-energy electrons produced by gamma-rays. It is shown that the BaF2, the BaF2:Sr and the BaF2:MgF2 crystals have the Auger-free luminescence having decay times around 0.90 ns. The slow and fast luminescence components of the BaF2 crystal are suppressed by introduction Sr++ and Mg++ ions. It is, however, clearly seen that the suppression of the slow component is more effectively occurring than that of the fast component, especially in the BaF2:MgF2 crystal. By analyzing the Auger-free luminescence spectrum, it is shown that the valence band widths are 2.7+/-0.3 eV, 2.7+/-0.3 eV and 4.2+/-0.3 eV and the band-gap energies are 11.1+/-0.3 eV, 11.1+/-0.4 eV and 11.9+/-0.4 eV for BaF2, BaF2:Sr and BaF2:MgF2 Crystals, respectively. The introduction of LiBr into CsBr has resulted in a significant enhancement in the luminescence intensity, attaining 40 times larger intensity than that of CsBr crystal at room temperature, due to the suppression of the non-radiative relaxation of the outer-most core holes due to the absorption induced by the Urbach effect. The observations of the increases in the band gap-energy, the exciton energy and the decay time of the CsBr:LiBr crystal compared with those of the CsBr crystal support the argument given in the present study regarding the suppression of the Urbach effect in the CsBr:LiBr crystal. The decay times observed are 0.20+/-0.05 ns and 0.90+/-0.06 ns for CsBr and CsBr:LiBr crystals, respectively.