Spectroscopic and radioluminescence properties of two bright X-ray phosphors: Strontium barium chloride doped with Eu2+ or Sm2+ ions

Abstract

The optical fluorescence, X-ray phosphor and scintillation properties of europium or samarium-doped solid solutions of strontium and barium chloride, together with their preparation and structure, are reported. Strontium chloride can accommodate up to 30% of barium ions on the strontium site whilst still retaining the cubic fluorite structure, and it is shown that that polycrystalline samples of volume ~1cm3 with high transparency can be produced. The increase in average atomic number provided by the barium content over pure strontium chloride means that strontium barium chloride with additional (divalent) europium or samarium doping shows good performance as an X-ray phosphor, emitting in the blue or red, respectively. The red-emitting samarium doped material is of particular significance as the emission is well matched to the spectral sensitivity of new silicon-based photodiodes. Europium-doped strontium barium chloride shows photoluminescence centred at 406nm with a lifetime of 1.0µs. As a scintillator, the conversion efficiency is 26,000 photons/MeV, and the decay time is 1.6µs, whilst as an X-ray phosphor the relative brightness is 40% that of the commercial X-ray phosphor gadolinium oxysulphide doped with terbium. Samarium-doped strontium barium chloride shows intense red 4f55d1→4f6 emission at room temperature which switches to 4f6→4f6 emission at low temperatures. The temperature–induced change in emission intensities and lifetimes is successfully modelled as a thermal crossover between 4f55d1 and 4f6 excited states which are in close proximity. At room temperature, the 4f55d1→4f6 emission is centred at 680nm and has a lifetime of 15µs; the scintillation efficiency is 22,000 photons /MeV with a lifetime of 30µs. As an X-ray phosphor, the efficiency compared to gadolinium oxysulphide doped with terbium is 30%. The role of native defects in the performance of the materials as scintillators and phosphors is briefly discussed.