Development Of Novel Water Equivalent Liquid Scintillator For 3D Dose Measurement Of Radiotherapy

The complexity of radiotherapies requires sophisticated dose-measuring equipment.An ideal dose-measuring equipment should provide high spatial resolution and real-time accurate measurement.The detector system of a scintillator provides real-time measurement of small fields with high gradients,and has good repeatability,linear response,radiation damage resistance and excellent spatial resolution.The scintillator’s composition is similar to that of water,so no calibration is required when measuring the dose at the point,but scintillators are not water after all,and when using large scint,l!ators,the difference becomes noticeable due to the cumulative effect.The scintillator used for radiotherapy dose measurement should have good water equivalence and have the same effective atomic number,electron density and mass density as water.In this thesis,scintillators were used as the research system,and substances with high atomic numbers were added to the matrix to adjust the water equivalence of the scintillators.A water-equivalent plastic scintillator doped with high atomic number nanoparticles was developed,and its performance and water equivalence were studied.The plastic scintillator is easy to carry and has good stability,but its shape cannot be changed once it is prepared.Moreover,the preparation of large volume plastic scintillator requires a higher process,so we also designed a water-equivalent liquid scintillator.A new water-equivalent liquid scintillator was prepared by using PPO as the first luminescent substance,CdSe quantum dots(QD)as the second luminescent substance and dodecylbenzene as the matrix.Compared with plastic scintillator,liquid scintillator preparation process is more operable,which can not only prepare water equivalent,but also adjust the content of doping QD at any time to meet the requirements of different human tissue density.Meanwhile,different shapes can be designed by designing containers with different human body structures.Water-equivalent liquid scintillator can choose the appropriate emission wavelength QD to meet the needs of different photomultiplier tubes.Moreover,the intensity of the light signal collected by the large volume scintillator can be greatly improved because of the good penetration of the long wavelength fluorescence emitted by QD.A plastic scintillator made of polystyrene(PS),PPO,POPOP,and oil-soluble ZrO2 nanoparticles(less than 10 nm)was developed.ZrO2 nanoparticles were prepared via the solvothermal synthesis method and its surface was modified by y-methacryloxy propyl trimethoxyl silane(MPS).The prepared oil-soluble ZrO2 nanoparticles were characterized.Styrene monomer,PPO,POPOP and specific amount of oil-soluble ZrO2 nanoparticles were blended,and azobisisobutyronitrile(AIBN)was used to initiate thermal polymerization of the particles to create a transparent plastic scintillator.The newly prepared plastic scintillator showed good transparency and stable irradiation performance.To prepare water-equivalent plastic scintillators,inductively coupled plasma emission spectrometer(ICP)was used to determine the content of Zr and Si in ZrO2 nanoparticles.A water-equivalent plastic scintillator doped with oil-soluble ZrO2 nanoparticles was developed,and its performance and water equivalence were examined.An empirical formula was used to calculate the approximate proportion of doped nanoparticles needed for the tissue equivalent plastic scintillator.Thereafter,the doping proportion of nanoparticles was simulated in the Monte Carlo simulation software Geant4,and water-equivalent plastic scintillators were prepared according to the obtained proportion.The linearity,multi-measurement stability and long-term stability of the newly prepared water-equivalent plastic scintillator were then tested.We measured the physical density of the water-equivalent ZrO2 plastic scintillator;used the CT density value and the electron density corresponding relationship to estimate the electron density of the ZrO2 plastic scintillator;tested the water equivalence of ZrO2 plastic scintillator under low-energy X-ray and high-energy X-ray conditions.The results showed that the density and electron density of the prepared water-equivalent plastic scintillator doped with ZrO2 nanoparticles were very close to that of water,and the ability of the plastic scintillator to attenuate ionizing radiation is very close to that of water.A new low-toxic water-equivalent liquid scintillator with PPO as the first luminescent substance,oil-soluble CdSe quantum dots as the second luminescent substance,and dodecylbenzene as the solvent was developed.Performance tests confirmed the feasibility of quantum dots as the second luminescent material,and the optimal concentration of QD as the second luminescent material.An empirical formula was used to calculate the approximate proportion of the quantum dots doped by the tissue equivalent liquid scintillation,after which the doping proportion of the quantum dots was accurately simulated in the Monte Carlo simulation software Geant4,and the water equivalent liquid scintillation was prepared according to the ratio simulated by Monte Carlo.The properties and water equivalence of water equivalent liquid scintillation were tested with a series of tests,The results show that CdSe quantum dots can be used as the second luminescent substance of the scintillation liquid.The fluorescence intensity of the liquid scintillation reached the maximum values when the doping concentration of QD is 0.005wt%.The liquid scintillation has stable properties and good water equivalence when the doping concentration of QD is 0.34wt%.To sum up,this paper presents a method of designing water equivalent scintillation material.Plastic and liquid scintillators with water equivalent were successfully developed to solve the problem of water equivalence of a detector.It can be potentially applied in environmental measurement,radiation protection and radiation point dose measurement,2D and 3D dose measurement.