Opticks : Gpu Optical Photon Simulation for Particle Physics with Nvidia OptiX
Simon C Blyth1*
1Department of Physics, National Taiwan University, Taipei, Taiwan
* presenting author:Simon Blyth, email:blyth@hep1.phys.ntu.edu.tw
Opticks is an open source project that integrates the NVIDIA OptiX
GPU ray tracing engine with Geant4 toolkit based simulations.
Massive parallelism brings drastic performance improvements with
optical photon simulation speedup expected to exceed 1000 times Geant4
when using workstation GPUs. Optical photon simulation time becomes
effectively zero compared to the rest of the simulation.

Optical photons from scintillation and Cherenkov processes
are allocated, generated and propagated entirely on the GPU, minimizing
transfer overheads and allowing CPU memory usage to be restricted to
optical photons that hit photomultiplier tubes or other photon detectors.
Collecting hits into standard Geant4 hit collections then allows the
rest of the simulation chain to proceed unmodified.

Optical physics processes of scattering, absorption, reemission and
boundary processes are implemented as CUDA OptiX programs based on the Geant4
implementations. 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.

Geometry is provided to OptiX in the form of CUDA programs that return bounding boxes
for each primitive and single ray geometry intersection results. Some critical parts
of the geometry such as photomultiplier tubes have been implemented analytically
with the remainder being tesselated.
OptiX handles the creation and application of a choice of acceleration structures
such as boundary volume heirarchies and the transparent use of multiple GPUs.

OptiX supports interoperation with OpenGL and CUDA Thrust that has enabled
unprecedented visualisations of photon propagations to be developed
using OpenGL geometry shaders to provide interactive time scrubbing and
CUDA Thrust photon indexing to provide interactive history selection.

Validation and performance results are shown for the photomultiplier based
Daya Bay and JUNO Neutrino detectors.


Keywords: Neutrino , Photon, Geant4 , Simulation, GPU