Energy resolved imaging with a stratified phosphor detector

Polycrystalline solid state solutions of Ba_ySr_1 − yCl₂ have been produced; these adopt the cubic fluorite phase for y ≤ 0.30. The cubic structure minimizes optical scattering from grain boundaries, and so for y ≤ 0.30 the materials are transparent, a key advantage for ceramic scintillators and phosphors. The substitution of Sr²⁺ ions with Ba²⁺ ions has the advantage that it substantially increases the x-ray absorption coefficient with respect to pure SrCl₂. Additional doping with rare earth ions such as Sm²⁺ and Eu²⁺ gives bright x-ray phosphor materials. The Sm or Eu-doped materials show a broad 4f⁵5 d¹ → 4f⁶ emission peaked at 685 nm or a broad 4f⁶5 d¹ → 4f⁷ emission peaked at 406 nm respectively. These materials have been tested as x-ray phosphors and the spatial resolution was determined to be at least 6 LP/mm, whilst the x-ray radioluminescence intensity is around 40% that of the commercial x-ray phosphor Gd₂O₂S:Tb. A stratified phoswich structure, comprising Eu and Sm-doped layers of Ba_ySr_1 − yCl₂ was produced in which the relative intensities of the two emissions varies with x-ray beam energy; this can be used for energy discrimination in imaging by way of emission spectra as opposed to the more commonly used pulse shape discrimination. A dual energy imaging technique based on these bi-layered structures and utilizing a semi-professional grade digital SLR camera is described and composition-sensitive imaging has been demonstrated.