Gamma-Ray Attenuation and Spectral-Angular Transport in Transparent Graded-Z Multilayer Shields

Transparent radiation shields that combine efficient photon attenuation with controlled secondary emission are of interest for medical imaging, nuclearmedicine, scientific instrumentation, and radiation-monitoring environments. In this study, a transparent graded-Z multilayer shield composed of bismuth-rich borosilicate (BBS), lutetium aluminium garnet (LuAG), and silica (SiO2) is investigated using Geant4 Monte Carlo simulations and compared with commercial RS-253 glass at matched areal density. The graded-Z architecture exhibits higher mass attenuation coefficients, shorter mean free paths, and smaller half-value layers over the photon energy range 0.06-1.33~MeV, indicating enhanced attenuation efficiency without increased thickness. Secondary-photon spectra reveal energy-dependent fluorescence governed by the staggered K-edges of Lu and Bi, while fluorescence yield decreases at higherenergies as Compton scattering becomes dominant. Angular analysis shows forward-peaked transmission for both materials, with the graded-Z multilayerdirecting a larger fraction of transmitted energy into the forward direction. These results demonstrate that graded-Z shielding redistributes photontransport spectrally and directionally, offering tunable performance while not always minimizing forward energy leakage. To the best of our knowledge, this work provides the first quantitative implementation of the graded-Z shielding concept in fully transparent glass-based materials while jointly resolving secondary gamma-ray energy spectra and angular photon transport. The results demonstrate that transparent graded-Z shields actively redistribute photon propagation spectrally and directionally, and establish spectral-angular scoring as an essential metric for evaluating next-generation transparent radiation-shielding windows.