Development of water-equivalent plastic scintillator doped with zirconium oxide nanoparticles

Scintillator-based detectors can provide real-time measurement of small fields with high dose gradients and have the advantages of good repeatability, linear response, and excellent spatial resolution. For radiotherapy dose measurement, water-equivalency of the scintillator can be beneficial based on current clinical standards. It would ideally match water in effective atomic number, electron density, and mass density.

Plastic scintillators are primarily composed of hydrocarbon molecules. While their interaction with photons exhibits properties similar to water, they are incompletely equivalent. This study aimed to develop a water-equivalent plastic scintillator by doping the scintillators with a specific proportion of oxide nanoparticles. The nanoparticles must be less than 10 nm to maintain transparency.

Zirconium oxide (ZrO₂) nanoparticles smaller than 10 nm were synthesized, and their surface was modified using methacryloxy propyl trimethoxyl silane (MPS) to ensure good dispersibility. The precise elemental composition of the modified ZrO₂ nanoparticles was determined using inductively coupled plasma (ICP) analysis to develop water-equivalent plastic scintillators. The initial doping ratio of MPS-ZrO₂ in a water-equivalent plastic scintillator was estimated using an empirical formula. Meanwhile, the precise doping ratio of MPS-ZrO₂ in water equivalent plastic scintillator was determined through a simulation performed with the Monte Carlo (MC) program GEANT4. Finally, the water-equivalent plastic scintillator was synthesized by in situ polymerization, and its water equivalence and dosimetric performance were validated using experimental tests.

Transmission electron microscope imaging indicated that the newly prepared MPS-ZrO₂ nanoparticles exhibited a size of approximately 4-5 nm, with uniform distribution and no aggregation. The ICP analysis determined ZrO₂ and MPS contents in the nanoparticles to be 67.47% and 31.82%, respectively. Based on the empirical formula and MC simulation, the optimal doping concentration of MPS-ZrO₂ nanoparticles in the water-equivalent plastic scintillator was 0.53 wt%. The physical density of the synthesized plastic scintillator was measured at 1.049 ± 0.127 g/cm³, with an electron density of 3.536 × 10²³ E/cm³, closely matching that of water. The maximum deviation in x-ray attenuation between water and the plastic scintillator was 1.27% for kV x-rays and 0.37% for megavoltage x-rays. Additionally, the plastic scintillator demonstrated excellent dose linearity, reproducibility across multiple measurements, and long-term stability.

The effective atomic number of plastic scintillators can be adjusted to match that of water by doping them with high particle number density nanoparticles of less than 10 nm. The precise doping ratio can be determined using empirical formulas and MC simulation. This method enables the development of plastic scintillators equivalent to various human tissues and addresses diverse clinical needs.