X-ray excited optical luminescence of metal oxide single crystals

The X-ray excited optical luminescence response of sapphire (α-Al2O3), aluminium lanthanate (LaAlO3), and magnesium oxide (MgO) were measured at room temperature as a function of incident photon energy in the range from 500 to 1000 eV. The luminescence yield varies weakly with energy above 600 eV and is dominated by the presence of the O K absorption edge in all cases and the La M2,3 edge for LaAlO3. The emission spectra in the visible range measured at a fixed incidence X-ray photon energy of 1000 eV shows for LaAlO3 a simple spectrum composed of two broad lines, a dominant sharp line for sapphire arising from Cr3+ impurities, and a complex spectrum for MgO, containing both broad and sharp spectral lines originating predominantly from impurities. SrTiO3 and Nb-doped SrTiO3 crystals were also measured and are found to have much smaller luminescence yields than the other materials investigated. Our results show that residual impurities that are inevitably present in high quality bulk crystals can be employed as colour centres for the detection of X-ray light in X-ray transmission spectroscopy.