Proton microbeam characterization using a single-particle scintillator

OBJECTIVE: To develop a simple and high-resolution method for measuring both the local dose and lateral beam width of a microproton beam using light emission from a single micron-sized scintillator particle. APPROACH: We used ZnS:Ag (silver-activated zinc sulfide) scintillator powder, known for its favorable dose response and peak emission at 450 nm. Proton microbeams (3.4 MeV) were delivered using the Single-Particle Irradiation system to Cell (SPICE) at the National Institute for Quantum and Radiological Science and Technology. The scintillator powder was dispersed on a thin film placed over a cell culture dish, and the emission from individual particles was recorded using a microscope-coupled camera. By scanning the proton beam in 400 nm steps across the particles, we measured variations in light intensity to evaluate both irradiation dose and beam profile. MAIN RESULTS: The luminescence intensity increased linearly with irradiation time and varied consistently with beam position relative to the particle center. Despite the particle diameter exceeding the beam width, emission modulation enabled precise localization of the beam center. Measured lateral beam widths using four particles with diameters of approximately 8 μm were 1.9 ± 0.3 μm in the X direction and 1.8 ± 0.4 μm in the Y direction, demonstrating sub-micron resolution beyond the imaging system's pixel size. SIGNIFICANCE: This method enables simultaneous estimation of dose and beam width using a single scintillator particle, without requiring complex microdosimetric equipment. It provides a practical and accurate tool for microbeam characterization, contributing to improved dosimetry and targeting precision in radiobiological research at the cellular and subcellular scale. By exploiting emission modulation within a single scintillator particle, beam profiles can be reconstructed with spatial precision beyond the optical pixel resolution.