Celeritas is a new Monte Carlo transport code designed to accelerate scientific discovery in high energy physics by improving detector simulation throughput and energy efficiency using GPUs.
We've encountered an issue where the exterior boundary of a unit is a union so we cannot eliminate the surfaces from the boundary, resulting in a coincident (or nearly coincident) boundary in multiple levels. This results in a tracking error when crossing from the parent to the daughter because we end up in the "exterior" of the daughter universe. (It can also cause assertion errors when the daughter universe is complex because we start by validating that the point is "inside".) This shows up in the ATLAS tile calorimeter which has a union that contains another physical volume.
The proposed short-term solution is to keep logically eliminating boundary surfaces on the daughters when we can, but we will also support coincident surfaces (rather, the same surface appearing in multiple universe, potentially with transformations and negations applied).
Change proto construction from top-down to bottom-up so that we build the child CSG trees before the potential enclosing units and therefore know whether there are any boundary surfaces that cannot be eliminated.
Retain an ordered list of the boundary's surfaces so that a congruent shape but with a transformation applied will have the correct surface mapping. Because of surface regularization (flipping normal directions and senses) we will have to retain knowledge of whether the surface sense needs to be flipped as well when going from parent to child.
Add data structure in input and runtime for storing this mapping on volumes. We should be able to just have a single new Id index for each volume: null if no mapping is needed, offset into an array of daughter local surfaces and bools for sense flipping. (We won't need to store as a map or store the length because each volume already has a "face" map that will be the same length if the mapping is set.) Faces that can be deleted from the daughters can be left as null IDs.
Probably want to add a new accessor to the VolumeView for these mappings?
Inside find_next_step and cross_boundary, when descending from the "current surface level", we have to interrogate the current volume's map to find whether the next lower level has a corresponding surface, then updating the on-surface (ID and sense) that level in the LocalState object for the daughter. (Both of these calls depend on the local surface state.)
Long term solution for the Geant4 representation might be to have only a stack of universe IDs, a local tracker, and a detailed mapping of parent/child universes: because for this geometry specification we do not need to worry about parents truncating the children via overlaps. This is something I'd thought about for being more efficient but the tricky case of not being able to eliminate daughter surfaces may make it a requirement.
Parent universe where the daughter is a union of two spheres:
Daughter universe after the CSG tree has been simplified to remove the "boundary" (see #1256):
We've encountered an issue where the exterior boundary of a unit is a union so we cannot eliminate the surfaces from the boundary, resulting in a coincident (or nearly coincident) boundary in multiple levels. This results in a tracking error when crossing from the parent to the daughter because we end up in the "exterior" of the daughter universe. (It can also cause assertion errors when the daughter universe is complex because we start by validating that the point is "inside".) This shows up in the ATLAS tile calorimeter which has a union that contains another physical volume.
The proposed short-term solution is to keep logically eliminating boundary surfaces on the daughters when we can, but we will also support coincident surfaces (rather, the same surface appearing in multiple universe, potentially with transformations and negations applied).
Idindex for each volume: null if no mapping is needed, offset into an array of daughter local surfaces and bools for sense flipping. (We won't need to store as a map or store the length because each volume already has a "face" map that will be the same length if the mapping is set.) Faces that can be deleted from the daughters can be left as null IDs.VolumeViewfor these mappings?find_next_stepandcross_boundary, when descending from the "current surface level", we have to interrogate the current volume's map to find whether the next lower level has a corresponding surface, then updating the on-surface (ID and sense) that level in theLocalStateobject for the daughter. (Both of these calls depend on the local surface state.)Long term solution for the Geant4 representation might be to have only a stack of universe IDs, a local tracker, and a detailed mapping of parent/child universes: because for this geometry specification we do not need to worry about parents truncating the children via overlaps. This is something I'd thought about for being more efficient but the tricky case of not being able to eliminate daughter surfaces may make it a requirement.
Parent universe where the daughter is a union of two spheres:
Daughter universe after the CSG tree has been simplified to remove the "boundary" (see #1256):