Optical Characterization Measurements of Commercial Polyvinyltoluene Scintillators for Accurate Geant4 Detector Modeling

Accurate Monte Carlo simulation of plastic scintillator detectors requires precise wavelength-dependent optical properties. We present the first comprehensive measurements of wavelength-dependent refractive index $n(λ)$ and bulk absorption length $L_{\mathrm{abs}}(λ)$ for six commercial polyvinyltoluene (PVT)-based scintillators (Saint Gobain BC-400, BC-404, BC-408, BC-418, BC-422, and BC-430) across the 370-660 nm range. Using a precision refraction apparatus and variable-thickness transmission measurements, we characterize material-specific dispersion that deviates substantially from both the manufacturer specification and literature values for pure PVT. Validation simulations of a BC-422 detector demonstrate a 7.2% reduction in detected photon counts when using our measured $n(λ)$ compared to the constant manufacturer value, highlighting the practical importance of accurate optical property data for detector design and calibration. We successfully measure absorption lengths for BC-430 across most of the visible spectrum (2.2-12.4 mm) and for all materials at 372 nm (21-38 mm), while establishing lower bounds exceeding 750 mm for weakly absorbing formulations at longer wavelengths.