Preparation And Luminescence Mechanism Of C12A7:Tb³⁺Phosphor For X-ray Imaging Storage

X-ray imaging storage materials(photostimulablematerials)have been applied in many fieldssuch as computed radiograghy(CR),erasable/rewritable optical memory media,radiation dosimetry and biomedicine.These phosphors can generateamounts of electron-hole pairs and store electron or hole in traps fordays or months upon X-ray,UV or visible light excitations andrelease it as luminescence by an optical stimulation with longerwavelengths.During the CRoperation,the latent image is stored in the imaging plate using photostimulable materials after X-ray penetrating human body.The image can be readoutby a stimulation with longer-wavelength light.At present,CR has a low cost,good compatibility and portability exhibits a salient feature in trauma orthopedics and neonatal imaging during operation still have wide application in medical diagnostics.Thereinto,the phosphors coating the substrate of the imaging plate is the core of CR system.Thus far,commercial photostimulable BaFBr:Eu2+ phosphors for clinical use have strong blue photostimulable emission with a high sensitivity,fast response,and high conversion efficiency(CE)satisfy the requirements of the CR system.However,theyshowed poor chemical and thermal stabilitiesdue tohigh energy X-ray bombardment and an inferiorspatial resolution which limits the development of imaging plate.Since toxic substancesare generatedafter halogenide decomposition,it will cause serious environment pollution.To solve the the aforementionedproblems,it is one of the hot fields to explore the preparation,design and the mechanisms of photostimulated luminescence based on rare earth doped oxide-based X-ray image storage phosphors.In particular,there is a prevailing effort in preparingnovel oxide-based photostimulable phosphors by cationicsubstitution orheat-treatment under differentconditions toimprove their storage properties.However,the absence of X-ray imaging has limited their development and applications.Therefore,it is still a challenge to find a newoxide-based photostimulable phosphor for X-ray imaging.The host,12CaO·7Al2O3(C12A7)with unique nanocage(inversed zeolitic-like)structure,might be a potential candidate due to its highly variable properties and good chemical and thermal stabilities.Its unit cell consists of 12 cages forming the positively charged framework and two free O2-anions([Ca24Al28O64]4+·2O2-)occupying randomly 2 of the 12 cages to maintain the charge neutrality.Under different treatment conditions,the free encaged O2-anions can be replaced by electrons or other anions,like OH-,H-,O-,e-,F-and Cl-anions shows variable optical and electron properties.Each cage has a mean effective charge of +1/3(+4 charges for 12 cages),functioning as an F+-like center(trap)to capture an electron.In addition,the studies on Eu2+-doped C12A7 phosphors show a goodlong afterglow and optical storage properties.Therefore,nanocage-structure C12A7 might be a kind of candidate host material.Therefore,rare-earth doped C12A7 phosphors might be a high efficient potential candidate material for X-ray imaging storage.12CaO·7Al2O3:Tb3+ phosphors were prepared by a self-propagating combustion methodin combination with heat treatment under different conditions.Then,the storage properties,the photostimulated luminescence mechanism and the X-ray imaging of C12A7:Tb3+ phosphors were investigated.Furthermore,the influence of different encaged anions including O2-,H-,OH-and electrons forphotostimulated luminescence and modulation of trap depth are discussed.12CaO·7Al2O3:Tb3+ phosphors doped by different alkaline earth cation(Mg2+,Sr2+)were prepared by the same method and the storage properties were explored.Based on the analysis upon the influence of doping Sr2+ for X-ray imaging properties,the enhancement of X-ray absorptioncoefficient and conversation efficiency were realized.The main research findings are listed as follows:Lanthanide-doped nanocaged oxide C12A7:0.5%Tb3+ phosphors with a cubic structure were ignited at temperatures ranging from 700 to 900 °C via a combustion method.At the ignition temperature of 700 °C,a sub-micrometer average grain size was obtained.Under 239-nm excitation,strong green emission at 541 nm originating from the 5D4–7F5 transition of Tb3+ was observed.Photostimulated luminescence and thermoluminescence analyses indicated that there were deep traps in the C12A7:Tb3+ whose depths were estimated to be 0.7–0.9 eV.Photostimulated luminescence and photoconductivity measurements showed that the storage time was longer than 48 h.The photostimulated luminescence mechanism upon 239 nm is demonstrated using the conduction band modelbased on photoconductivity and ESR measurements.The photostimulated luminescence intensity is proportional to the X-ray irradiation time when the irradiation time is shorter than 40 min and it approximate to be a saturating value when the irradiation time is longer than 60 min.The CE of the C12A7:Tb3+ phosphors was ~1.28 pJ/mm2/mR.The spatial-resolution capability of C12A7:Tb3+phosphors reached 30 ?m,and their spatial resolution reached ~15 lp/mm,which is superior to those of commercially applied BaFBr:Eu2+ phosphors(5 lp/mm).Our results suggest thatthe C12A7:0.5%Tb3+ phosphorswith the sub-micrometer grain size,cubic structure,and high CEare potential candidates for application in X-ray imaging systems for medical diagnostics.We have been successfully fabricated rare-earth doped photostimulable phosphors,C12A7:0.5%Tb3+,by a combustion method in combination with the treatment under different conditions and investigated how the different encaged anions including O2-,H-,OH-and electrons influence their X-ray storage properties.ESR and FTIR transmittancedata confirm the existences of encaged electrons,O2-(indicating the existence of encaged O2-anions)andOH-anions in C12A7:Tb3+.The encaged-anion-dependent photostimulated luminescence and thermoluminescenceanalyses suggest that the more encaged OH-anions appear in C12A7,the weaker PS luminescence intensity,the shallower the depths of the two deeper traps(Etrap1 = ~0.72 eV and Etrap2 = ~0.95 eV)and the smaller the glow-peak intensity ratio(Itrap2/Itrap1)become.Our results suggest that the strategy for improving X-ray storage time and the quality of X-ray imaging is put forward by reducing encaged OH-anions in C12A7:Tb3+ through heat-treatment in air.Through the replacement of Ca2+ by alkaline earth ions,C12A7:0.5%Tb3+,C12A7:0.5%Tb3+,1%Mg2+ and C12A7:0.5%Tb3+,1%Sr2+ phosphors were prepared by a self-propagatingcombustion method in combination with heat treatment under different conditions.The influence of cation doping on theX-ray storage properties of C12A7:0.5%Tb3+ phosphors was discussed.The XRD andEDS analyses show that Sr and Mg substitute for Ca site.In addition,photostimulated luminescence and thermoluminescence results suggest that C12A7:0.5%Tb3+,1%Mg2+ phosphors are more suitable for application in optical storage devices,while C12A7:0.5%Tb3+,1%Sr2+ phosphors are more appropriate for X-ray imaging storage.To understand the influence of doping concentration on X-ray absorption of the phosphors,C12A7:0.5%Tb3+,x%Sr2+(x=0,1,3,5)phosphors with different Sr doping concentrations were prepared by a self-propagating combustion method.From photostimulated luminescence,it can be observed that the luminescence intensity is almost in proportion to the Sr2+ doping concentration.Furthermore,C12A7:0.5%Tb3+,x%Sr2+ exhibits a larger dynamic response range,showing a linear growth,compared tothe undoped phosphors.At the doping concentration ofx=5,the strongest photostimulated luminescence can be achieved due to the increased X-ray absorption originating from the replacement of Ca by Sr.For the phosphorC12A7:0.5%Tb3+,x%Sr2+(x=5),the CE iscalculated to be ~1.42 pJ/mm2/mR,which is higher than that of C12A7:0.5%Tb3+ phosphors(~1.28 pJ/mm2/mR).