| X-ray imaging has a wide range of applications in many fields such as nuclear medicine imaging,non-destructive evaluation,crystallography and astrophysics.With the development of X-ray imaging,especially hard X-ray imaging,many highperformance requirements have been put forward for the scintillation screen for X-ray imaging,which not only requires higher light yield and better spatial resolution,but also requires shorter decay time to achieve high time resolution.The scintillation screen is the core of the X-ray imaging system,so its performance plays a vital role in the imaging quality of the X-ray imaging system.For high-performance scintillation screens,scintillator with good luminescence performance is required,which are mainly reflected in the following aspects:1)Sufficient X-ray absorption capacity,which requires scintillation materials with higher atomic numbers and larger density.2)High light yield.The enhancement of light yield can directly improve detection efficiency.3)Shorter decay time,in order to achieve good time sensitivity of X-ray imaging.CsI(Tl)scintillator has the characteristics of high light yield,short luminescence decay time,moderate density and effective atomic number,so CsI(Tl)scintillation material is an ideal choice for the requirements of hard X-ray scintillation screen.Traditional scintillation screens are usually powder or film scintillation screens.Improving X-ray detection efficiency usually requires increasing the thickness of the scintillation screen.However,increasing the thickness of the scintillation screen will directly reduce the imaging resolution of the scintillation screen,because traditional scintillation screens cannot control the lateral propagation of the scintillation light.In order to increase the thickness of the scintillation screen while maintaining the spatial resolution of the scintillation screen,the scintillation screen with columnar structure has been proposed and gradually developed.The columnar structure is conducive to guiding the scintillation light to propagate along the column,which reduces the lateral propagation of the scintillation light,so that the imaging resolution of the scintillation screen and the quantum detection efficiency of X-ray imaging can be taken into account.The scintillation screen based on the silicon hole array template is a new type of structured scintillation screen developed in recent years.It is made by injecting scintillator into the holes of the silicon template.By growing a SiO2 reflection layer on the surface of the silicon hole wall,more scintillation light can be guided to propagate along the scintillation column,and the silicon on the hole wall has a light-absorbing effect,which can limit the lateral propagation of the scintillation light,so as to improve the scintillation screen X-ray imaging spatial resolution.In order to pursue higher Xray imaging spatial resolution,the development of structured scintillation screens based on silicon hole array templates is developing in the direction of small period and large aspect ratios.The period of scintillation micro-columns array is close to the order of microns,the spatial resolution of X-ray imaging can exceed 100 lp/mm.Generally,the spatial resolution of the structured scintillation screen will increase with the decrease of the scintillation micro-columns array period.However,as the micro-columns array period is further reduced,the fill factor of the scintillator in the template will be decreased.This will significantly reduce the X-ray detection efficiency of the scintillation screen.How to improve the spatial resolution of the scintillation screen while taking into account the requirements of X-ray detection efficiency is a challenge of current structured scintillation screen based on the silicon hole array template in development.This paper uses Geant4 Monte Carlo simulation toolkit,FDTD(finitedifference time-domain)method and Monte Carlo ray-tracing method to carry out the theoretical simulation and optimization design of the structured CsI(Tl)scintillation screen based on the silicon hole array template.The main results are as follows:1.In the case of structured CsI(Tl)scintillation screen with a 4 μm period and a 1 μm Si wall thickness,the effects of different thicknesses of the SiO2 reflective layer on the scintillation screen’s light output,spatial resolution and quantum detection efficiency were studied.The simulation results show that the thicker the SiO2 reflective layer,the better the scintillation micro-columns can guide the scintillation light,which leads to the stronger the light output capacity of the scintillation screen.As the thickness of the silicon dioxide layer increases,the impact on the spatial resolution is limited,so the quantum detection efficiency increases as the thickness of the SiO2 reflective layer increases.However,when the template is completely composed of SiO2,the spatial resolution will be reduced,which will have a certain negative impact on the quantum detection efficiency in high spatial frequency.When the thickness of the SiO2 reflective layer varies from 0 to 300 nm,the light output and detection efficiency of the scintillation screen increase significantly.When the thickness exceeds 300 nm,the change gradually becomes slow and tends to be stable.It can be seen that the thickness of the SiO2 reflective layer should be more than 300 nm.Although the thickness of the Si layer in the hole wall seems to have a limited impact on the light performance of the CsI(Tl)scintillating microcolumns,it is better to keep some properly.2.The CsI(Tl)scintillation screen based on the dual-period structure silicon hole array template is proposed.The light output,resolution and quantum detection efficiency of the double-period scintillation screen with a silicon hole wall thickness of 1 μm and a SiO2 reflective layer of 300 nm are theoretically simulated.The results show that compared with the single-period structure scintillation screen,the dual-period structure scintillation screen has obvious advantages in the filling rate and light guide efficiency.When the average secondary period of the dual-period structure is 4 μm,3 μm and 2 μm,compared with the corresponding single-period scintillation screen,the light output of the scintillation screen is increased by about 60%,90%,and 200%,respectively.For the spatial resolution,although the dual-period scintillation screen is not as good as the single-period scintillation screen,but the difference is very limited.For a given spatial resolution,the light output of the dual-period structure is stronger,and for a given light output level,the spatial resolution of the dual-period structure is better.At the same time,the quantum detection efficiency has been considerably improved at both low and high frequencies.3.The structured CsI(Tl)micro-square-frustums scintillation screen with tilt angleθ=2.4° and structure period of 4 μm,the light output,spatial resolution and quantum detection efficiency of the scintillation screen with different top Si wall thicknesses were studied.When the top silicon wall thickness is greater than 1 μm,as the top silicon wall thickness decreases,the micro-square-frustum scintillation screen has different structural layers.At the same time,the effective part of the scintillator becomes more and more,so the light output of the scintillation screen is continuously enhanced,and at the same time,since the pixel period has not changed,the influence on the spatial resolution is relatively slight.However,when the top silicon wall thickness is reduced to less than 1 μm,a CsI(Tl)film layer appears on the surface of the scintillation screen.Although the light output still increases with the decrease in the top silicon wall thickness,the spatial resolution decreases significantly.The simulation result shows that the DQE increases at low-frequency and decreases at high-frequency with the decrease of top Si wall thicknesses.The simulated results can give the optimal tsi according to the comprehensive requirements of detection efficiency and spatial resolution in X-ray imaging. |
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