An OpenCL-based photon-tracking simulation using a (source-based) ray tracing algorithm modeling scattering and absorption of light in the deep glacial ice at the South Pole or Mediterranean sea water.

This project is specific to the IceCube neutrino telescope (and Antares/KM3NeT) simulation and the following description contains lots of internal jargon. It requires the IceTray framework. It still has some IceCube-internal dependencies, but those might be removed in the future.

The conventional method of sampling the arrival time distribution of photons on photo-detectors in large-volume neutrino detectors is based on large, interpolated look-up tables. Generating such tables and finding robust interpolation methods has proven to be highly non-trivial. Existing tools such as ptd, photonics (recently combined with improved interpolation methods in photospline) and km3 all perform reasonable well. They all do, however, have drawbacks, such as binning artifacts, incomplete descriptions of the detector medium, large memory requirements and so on. All of those tools have to rely on interpolation, requiring careful fine-tuning of parameters during the generation of their respective photon tables. Finally, generating photon tables usually takes a comparably long time, making such tools inconvenient for systematic studies, where properties such as the absorption or scattering lengths need to be changed frequently. Usually, changing any parameter requires a full table re-generation.

The clsim photon tracker aims to work around these problems by tracking each single photon generated by a given source in the detector. This very time-consuming process can be sped up by factors of over 100 by using GPU hardware instead of CPUs. This has first been proven by tools like i3mcml and ppc. Instead of the vendor-specific CUDA framework used by those tools, clsim uses OpenCL with the aim of being portable to other architectures, such as AMD/ATI or even multi-core CPUs.

The first step in simulating the light yield from a given I3Particle at a DOM is to convert the particle into a series of light-emitting "steps". Each step is assumed to have a constant speed beta=v/c, determining the Cerenkov angle and a constant number of photons that should be emitted over a given length. By default, steps are created using a full Geant4 simulation, but alternative parameterizations can be used to speed up the process. (Geant4 tends to get rather slow at higher energies (E>10TeV).) The current version of clsim comes with a parameterization that is compatible to ppc. Parameterizations can be used for only a sub-set of particles and energies.

Once a set of steps is generated, they are uploaded to the compute device (i.e. the GPU). The GPU runs "kernels" in parallel that are responsible for creating photons from the steps, propagating them through ice layers and checking for collisions with DOMs. All photons that collided are saved with their full information (including direction, position on the DOM, wavelength, number of scatters, properties at its point of creation, ...). Those photons are then sent back to the host, converted to I3FrameObjects (I3Photon) and saved in the frame. In order to keep the GPU busy, multiple frames (==events) are simulated in parallel. The clsim module takes care of correctly re-assembling the events afterwards.

The output of the GPU simulation step is thus a list of photons at the DOM surface. These photons still need to be converted into hits, which is done using a dedicated module. This module finally writes a I3MCHitSeriesMap, compatible to all existing photon simulation output.

For a simple simulation run, you should process your I3MCTree using PROPOSAL. Then, just use the supplied "tray segment" to add all relevant services and modules to the I3Tray:

tray.AddSegment(clsim.I3CLSimMakeHits, "makeCLSimHits")

This reads the 'I3MCTree' and 'MMCTrackList' objects from each frame and adds hits named 'MCHitSeriesMap'. All photons are generated in a way compatible to ppc.

By default, clsim uses only the GPU for photon propagation (i.e. CPU devices are skipped during OpenCL device enumeration). This behavior can be changed using the UseGPUs and UseCPUs boolean options. The following example would only use the CPU for simulation and skip all GPU devices:

tray.AddSegment(clsim.I3CLSimMakeHits, "makeCLSimHits",
                UseGPUs=False, UseCPUs=True)

By default, clsim does apply photon yield parameterizations compatible to ppc. This can, however, be turned off to use a full Geant4 simulation run. In this case, full GPU simulation might not be necessary because the photon generation will become the bottleneck.

To use Geant4, just enable the UseGeant4 switch:

tray.AddSegment(clsim.I3CLSimMakeHits, "makeCLSimHits",
                UseGeant4=True)

Even in this mode, Geant4 will not be used for muons that have a length assigned. These are assumed to have been generated by PROPOSAL. Both, neutrino-generator and Corsika generate muons without lengths, so the module should generally do the right thing. To make absolutely sure that no parameterizations are used, also set the MMCTrackListName option to None. This will apply Geant4 to all particles in the I3MCTree (which are InIce and not Dark), even if they already have a length assigned to them. (You should make sure that this is what you really want, as PROPOSAL might already have added cascades to the muon track that would be added a second time by Geant4.)

In order to simulate low energies in a more correct way, you might want to consider disabling the DOM oversizing optimization. It is set to an oversize factor of 5 (in radius), which gives you a 25-fold increase in simulation speed at the expense of accuracy in timing and for tracks very close to DOMs.

To disable DOM oversizing (which might be a good idea especially when using Geant4) use the DOMOversizeFactor switch:

tray.AddSegment(clsim.I3CLSimMakeHits, "makeCLSimHits",
                DOMOversizeFactor=1., # disables oversizing (default is 5.)
                UseGeant4=True)       # enable or disable Geant4 as needed

By default, the 'SPICE-Mie' ice model is used. This can, however, easily be changed by supplying either a photonics-compatible ice description file or a ppc-compatible ice description directory using the 'IceModelLocation' parameter:

tray.AddSegment(clsim.I3CLSimMakeHits, "makeCLSimHits",
                IceModelLocation=expandvars("$I3_BUILD/clsim/resources/ice/ppc_aha_0.80"))

Another example (using a photonics file) would be:

tray.AddSegment(clsim.I3CLSimMakeHits, "makeCLSimHits",
                IceModelLocation=expandvars("$I3_BUILD/clsim/resources/ice/photonics_wham/Ice_table.wham.i3coords.cos090.11jul2011.txt"))

Except the files listed below, all of the code is under the ISC (OpenBSD/BSD 2 Clause equivalent) license.

The files clsim/private/geant4/TrkCerenkov.cxx and clsim/private/geant4/TrkCerenkov.hh are based on code from the Geant4 package released under the Geant4 license ( http://cern.ch/geant4/license ).

This product includes software developed by Members of the Geant4 Collaboration ( http://cern.ch/geant4 ).