Preparation And Storage Properties Of C12A7: Pr³⁺ Phosphors For X-ray Imaging

Computed radiography?CR?has potential applications in medical diagnosis and disease therapy due to its many advantages,such as low-cost,easy operation and good compatibility.Especially,the quality of X-ray imaging depends on the storage properties of X-ray phosphors,which are coated on the surface of imaging plate of the CR system.However,commercial halide-based phosphors are not environmental-friendly and show poor chemical and physical stabilities under high energy X-ray bombardment.Thus,researchers concentrate on developing new oxide-based phosphors which can avoid these problems.Among of them,Tb³⁺doped 12CaO·7Al₂O₃?C12A7?phosphors with a unique nanocage structure have been used for X-ray imaging based on the green emission of Tb³⁺.However,for the CR technology,the photo-stimulated luminescence signals are dectected by photomultiplier?PMT?,which has the highest sensitivity in blue region.Therefore,to obtain high-quality X-ray imaging,it is essential to develop novel rare earth ion doped C12A7 based on blue-emitting phosphors.In this dissertation,Pr³⁺doped C12A7 based blue-emitting phosphors for X-ray imaging were prepared by a combustion method in combination with a subsequent annealing under different conditions.We studied the effects of the heattreatment conditions on the X-ray storage properties of the samples and discussed the X-ray storage mechanism.For as-prepared C12A7:x%Pr³⁺phosphors with different doping concentrations?x=0.1,0.3,0.5,1.0?upon 232nm excitation,it can be found that several of visible emission peaks,which originate from the³P₀-³H_J?J=4,5,6?transitions of Pr³⁺,appear in the 470-660 nm region.In paticular,the blue emission peak at 486 nm has a dominant contribution to the emission spectra and the strongest blue emission intensity can be obtained as the Pr³⁺doping concentration is 0.3%.On the basis of the afterglow decay and the photo-stimulated luminescence?PSL?analysis,it can be found that there exist shallow and deep traps in the as-prepared phosphors and the strongest PSL intensity under 808 nm stimulation can be obtained for the as-prepared phosphor with x=0.3%.For the C12A7:0.3%Pr³⁺phosphor annealed in N₂ ambient,the narrower distribution of the grain size and the stronger PSL intensity can be obtained as the annealing temperature and time are 1200?and 2 hours,respectively.The thermoluminescence and the PSL analysis suggest that the trap depths in the C12A7:0.3%Pr³⁺sample were⁰.71 and⁰.83 eV,respectively,and the trap depths and the numbers of the traps vary with the annealing temperature and time.For the C12A7:0.3%Pr³⁺phosphor annealed in vacuum,it can be found that the photoluminescence intensity and the PSL intensity are enhanced compared to the as-prepared sample.This phenomenon can be explained by the decreased number of encaged OH^-groups and the increased number of empty cages in C12A7 after the thermal annealing in vacuum.The thermoluminescence analysis shows that the existed traps might originate from empty cages?acting as F⁺-like color centers?in the C12A7 host.As the X-ray absorbed dose is in the range of 0.9-7.9 Gy,the PSL intensity of the C12A7:0.3%Pr³⁺phosphor annealed in vacuum has a linear relation with the X-ray absorbed dose.To apply the C12A7:0.3%Pr³⁺phosphor annealed in vacuum?coated on the surface of the imaging plate?for X-ray imaging,a part of fishtail was used as an object.Upon X-ray irradiation?applied dose:5.2 Gy?,a clear blue X-ray image on the surface of the imaging plate was obtained by 808 nm stimulation?the diameter of the fishbone is in the range of 80-100?m?.