Samarium doped alkaline earth halides as red-emitting scintillators and phosphors

Abstract

This thesis is concerned with the manufacture, spectroscopic characterisation, and radiation detection performance of three rare earth doped alkaline earth halides; these were designed for scintillation or phosphor detection of x-rays and γ-rays. The materials are transparent polycrystals of lanthanum or praseodymium stabilised cubic barium chloride ((La,Pr)₀.₁₂₅Ba₀.₈₇₅Cl₂.₁₂₅), BaCl₂ - SrCl₂ solid solutions, or single crystals of CaF₂. The primary dopant investigated was Sm²⁺ since this has a red emission in all the materials which is well matched to the spectral sensitivity of silicon photodiodes. The cubic structure of the polycrystalline materials is essential for optical transparency, and so the structural stability of the materials has been investigated using x ray diffraction and thermal analysis. For CaF₂ large single crystals were unintentionally produced without following the usual Bridgman-Stockbarger or Czochralski methods. All of the materials showed predominantly Sm²⁺ ions, and only in CaF₂ could evidence of Sm³⁺ ions also be seen.

The spectroscopy of the 4f⁵5d¹ → 4f⁶ red emission, including lifetimes, and absorption of Sm²⁺ ions in all these materials is reported; a strong thermal cross over to 4f⁶ → 4f⁶ emission is observed and successfully modelled. A time correlated single photon counted system has been built to measure the scintillation decay time of these materials. The system yields decay times in excellent agreement with the literature values. The performance of the materials as scintillators is limited to varying degrees by the formation of colour centres which slow the electron-hole recombination process after x-irradiation. Ba₀.₃Sr₀.₇Cl₂:Sm was found to be a bright and fast x-ray phosphor. The integrated intensity (per x-ray half thickness of material) of the radioluminescence is ~ 30 % that of the commercial material, the scintillation lifetime is ~ 30 μs (c.f. milliseconds for Gd₂O₂S:Tb³⁺) and the imaging resolution is 6 LP/mm (c.f. 4.2 LP/mm for Gd₂O₂S:Tb³⁺). CaF₂:Sm²⁺ was shown to be a red-emitting scintillator with a decay time of ≤ 1 μs and a light output of 15,000 photons/MeV when cooled by dry ice. The x-ray imaging resolution was high at 8.5 LP/mm. Several of the materials have been tested for performance as neutron detecting phosphors by adding neutron capture elements such as gadolinium or lithium, the strongest emission observed was 6 % the integrated intensity of the standard material ⁶LiI(Eu²⁺).