3. Collision Track File Format

When collision tracking is enabled with mcpl=false (the default), OpenMC writes binary data to an HDF5 file named collision_track.h5. The same data may also be written after each batch when multiple files are requested (collision_track.N.h5) or when the run is performed in parallel. The file contains the information needed to reconstruct each recorded collision.

The current revision of the collision track file format is 1.0.

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Attributes:

• filetype (char[]) – String indicating the type of file. For collision-track files the value is "collision_track".

Datasets:

• collision_track_bank (Compound type) – Collision information for each stored event. Each entry in the dataset corresponds to one collision and contains the following fields:

  • r (double[3]) – Position of the collision in [cm].

  • u (double[3]) – Direction unit vector immediately after the collision.

  • E (double) – Incident particle energy before the collision in [eV].

  • dE (double) – Energy loss over the collision (\(E_\text{before} - E_\text{after}\)) in [eV].

  • time (double) – Time of the collision in [s].

  • wgt (double) – Particle weight at the collision.

  • event_mt (int) – ENDF MT number identifying the reaction.

  • delayed_group (int) – Delayed neutron group index (non-zero for delayed events).

  • cell_id (int) – ID of the cell in which the collision occurred.

  • nuclide_id (int) – ZA identifier of the nuclide (ZZZAAAM format).

  • material_id (int) – ID of the material containing the collision site.

  • universe_id (int) – ID of the universe containing the collision site.

  • n_collision (int) – Collision counter for the particle history.

  • particle (int) – Particle type (0=neutron, 1=photon, 2=electron, 3=positron).

  • parent_id (int64) – Unique ID of the parent particle.

  • progeny_id (int64) – Progeny ID of the particle.

In an MPI run, OpenMC writes the combined dataset by gathering collision-track entries from all ranks before flushing them to disk, so the final file appears as though it were produced serially.