Air shower simulations with CORSIKA#

simtools uses CORSIKA 7, a detailed Monte Carlo simulation framework for extensive air showers initiated by high-energy cosmic rays. This includes the generation and propagation of Cherenkov light.

References:

Building CORSIKA for simtools#

CORSIKA 7 is built using the CORSIKA coconut build system:

./coconut --expert --without-root \
    --without-COASTUSERLIB --without-conex < /dev/null; \

Build configurations are provided by the CTAO corsika7-config repository. Each supported CORSIKA release has dedicated configuration files, including variants for flat and curved atmospheric geometries.

Installed interaction models#

The interaction models available to simtools depend on the CORSIKA installation. Production containers record every compiled model and atmosphere-geometry combination in $SIMTOOLS_CORSIKA_PATH/build_opts.yml. List the variants in the active environment with:

simtools-simulate-prod --list_available_corsika_models

For example, an installation may provide EPOS/UrQMD and QGSJet-III/UrQMD binaries for both flat and curved atmospheres. simtools validates the requested high-energy model, low-energy model, and required atmosphere geometry against the manifest before starting a simulation. An unsupported combination is rejected with the list of available variants.

The manifest is required. Missing manifests, missing executables, and unsupported model/geometry combinations are configuration errors.

CPU vectorization#

Patches to CORSIKA to make use of CPU vectorization for Cherenkov light generation are available from the CTAO corsika-opt-patches repository. Patches are version-specific and currently available only for flat-atmosphere configurations.

References:

CORSIKA interaction tables#

CORSIKA requires precomputed interaction tables for the supported physics models, including:

  • EGS4 (electromagnetic)

  • URQMD (low-energy hadronic)

  • EPOS (low-/high-energy hadronic)

  • QGSJET-II/III (high-energy hadronic)

  • further models will be added as needed in future releases

For efficiency, interaction tables are distributed separately from the executables and must be installed manually.

Tables are provided by the CTAO repository: CTAO corsika7-interaction-tables repository.

The environment variable $SIMTOOLS_CORSIKA_INTERACTION_TABLE_PATH must point to the interaction-tables subdirectory. Follow the repository instructions for download and installation.

Hadronic transition energy#

The CORSIKA HILOW data card defines the laboratory-frame energy at which the simulation changes from the low-energy to the high-energy hadronic interaction model. Configure it with --corsika_hadronic_transition_energy; values without a unit are interpreted as GeV.

When this option is set, simtools writes HILOW explicitly. When it is omitted, no HILOW card is written and the selected CORSIKA executable determines the transition. CORSIKA container builds record this value for every executable variant in build_opts.yml; inspect it with:

simtools-simulate-prod --show_options corsika_hadronic_transition_energy

Older build manifests do not contain this metadata. In that case, the command reports the affected variants without failing.