Correlation of XANES features with the scintillation efficiencies of Ce doped alkaline earth lithium silicate glasses

Cerium-activated, lithium-silicate glasses are widely used as thermal neutron detectors because of their versatility, robustness and low cost. The glasses convert the energy of the neutrons to visible light pulses that may be counted. This process, scintillation, is generally thought to be composed of three steps: ionization, energy transfer, and luminescence. If defects are present, they can trap the excitations, altering the scintillation output. These features have been discussed previously. The presence of magnesium in these glasses increases scintillation efficiency, but as previously observed the effect drops by a factor greater than 2.5 with substitution through the series of alkaline earths. Here, cerium activated glasses of composition 20Li2O·15MO·64.4SiO2·0.6Ce3O3 (where m is Mg, Ca, Sr, or Ba) exhibit scintillation efficiencies that vary by more than a factor of 2.5 with the alkaline earth. Previous work has suggested a correlation between the microstructure of these glasses and scintillation efficiency. Measurements of the Ce LIII x-ray absorption edge in the Mg, Ca and Sr glasses display a feature near the absorption edge that is suggestive of the presence of Ce4+. The area of this peak is, in fact, correlated with the scintillation efficiency of the glass. The amount of Ce4+ indicated by the intensity of this feature is, however, too high to be a permanent population. The authors suspect that the feature is a transient phenomenon related to creation of Ce4+ and trapped electrons due to photoionization by the x-ray beam