Opticks: GPU ray traced optical photon simulation

Institute of High Energy Physics, CAS, Beijing, China.

^* Corresponding author e-mail: simon.c.blyth@gmail.com.

Published online: 7 October 2025

Abstract

Opticks is an open source project that accelerates optical photon simulation by integrating NVIDIA GPU ray tracing, accessed via the NVIDIA OptiX API, with Geant4 toolkit based simulations. Optical photon simulation times of 10 seconds per 100 million photons have been measured with the full JUNO geometry and optical model with a 3rd RTX generation GPU. The JUNO optical model incorporates optical processes inside PMTs including thin layer interference effects. The GPU geometry is auto-translated from the Geant4 geometry. Optical physics processes of scattering, absorption, scintillator reemission and boundary processes are implemented in CUDA based on Geant4. Wavelength-dependent material and surface properties as well as inverse cumulative distribution functions for reemission are interleaved into GPU textures providing fast interpolated property lookup or wavelength generation. In this work we describe performance measurements and new features developed to facilitate and optimize production usage of Opticks within the JUNO simulation framework. The features include geometry generalization enabling use of triangulated solids together with analytic geometry, out-of-core processing enabling simulation of events with more photons than can fit within available VRAM, interactive ray traced visualization and also the adoption of the Philox counterbased random number generator optimizing random number generation.