Research On The Designing And Fabricating Of High- Reflectivity And High-transmittance Films Directly Coated On Scintillator

Positron emission tomography?PET?is the most sensitive medical imaging device in all imaging modes and represents the highest level of nuclear medical imaging technology.The scintillator-based high-energy ray detector module is the key component of PET equipment,and its role is to absorb the incident high-energy rays?such as gamma rays,X rays,etc.?,and producing subsequent electrical signal processing circuit,which could be used for analysis of gamma photon energy,function location and other information.With the expansion of PET application scope,it's important to promote the performance of PET system to achieve higher energy resolution and time resolution of the detector.In the PET system,lutetium-yttrium oxyorthosilicate?LYSO?and bismuth germinate?BGO?are the common used scintillators.Rflectance matching the emission spectrum of scintillators can improve the energy resolution of detector.The light transmittance of the crystal end affects the detector's initial photoelectron rate,which thereby affects the light output and time resolution of detector.In order to improve the energy resolution and time resolution of PET system,this paper proposes research on three aspects:?1?Design and fabricating of high reflective film based on BGO scintillation characteristics.?2?Design and fabricating of high transmittance film based on LYSO scintillation characteristics.?3?Thin layer insertion method to improve the reflectance at large incident angle.This paper designed a high reflectivity?HR?film stackwith a total layer number of 80 layers at 20°angle based on Cherenkov radiation inside BGO scintillators.The film stack consists of four?/4 wavelengths basic reflective film stacks,which were inducted from a?/4 wavelengths film stacks with 20 layers at wavelength of 340nm,whose dielectric material was Ta₂O₅/SiO₂/MgF₂.In this study,the HR film was fabricated by electron beam evaporation,and the samples were tested,indicating a good performance.Based on the principle of multilayer dielectric film stack,we designed an anti-reflectivity?AR?film stack with 11 layers,which matches the emission spectrum of LYSO scintillator and consists of Ta₂O₅/SiO₂/MgF₂.The HR film was fabricated by electron beam evaporation too according to the designing result.Samples were tested,showing a good perfoemance.In this study,we proposed a thin layer insertion method to improve the reflectance at large incident angle.A MgF₂ layer was inserted between LYSO scintillator and HR film stack,and two conditions,including air and glue as the coupling medium between the scintillator end surface and photoelectric sensor,were took into consideration.The influence of the thickness of MgF₂ thin layer on the reflectance of film stack was investigated,and the optimal thickness of MgF₂ thin layer was determined.