How to define a limitset applicable to the whole world?

[wdconinc]

There are examples that demonstrate how to define limit sets, but they all are applicable to detectors only. Is there a way to define a limit set that applies to the entire world? Use case: suppressing low energy neutrons with min kinE.

[MarkusFrankATcernch]

First of all, "Limitset" objects are applicable to regions.
Regions in G4 are defined by at least one volume (and AFAIK implicitly everything which is inside).

In the compact xml description limitsets are an attribute of the DetElement.
However, in the geometry constructor the limit-set is propagated to volumes
and in the Geant4Converter all volumes with the same limit-set are grouped to the same region.

Hence a single subdetector could also use multiple limit-sets.

Before there is activity coming up: If the volume is the "world_volume" will G4 be happy with this ?

Otherwise this could probably be relatively easily be implemented. "World" is nothing else than a DetElement with a placement (aka Volume). Wether this is done by some plugin or an extension to compact will have to be seen.

[wdconinc]

If the volume is the "world_volume" will G4 be happy with this ?

In another simulation environment (without DD4hep), we have been successfully setting geant4 user limits on the world volume (https://github.com/JeffersonLab/remoll/blob/develop/src/remollDetectorConstruction.cc#L238 and call stack therein).

"World" is nothing else than a DetElement with a placement (aka Volume).

So, it may be possible to add the limit set to an explicit world definition at e.g. https://github.com/AIDASoft/DD4hep/blob/master/examples/ClientTests/compact/WorldVolume.xml#L41-L52 (with maybe some additional code in https://github.com/AIDASoft/DD4hep/blob/master/DDCore/src/DetectorImp.cpp#L786 to set the limits)?

[MarkusFrankATcernch]

Will have to see how it will be technically done.
If G4 accepts it, dd4hep should not be the barrier.

[MarkusFrankATcernch]

Please see #1071
Should be merged soon.

[MarkusFrankATcernch]

I assume this to be fixed.

[wdconinc]

Not had a chance to test yet, but will reopen if not fixed.

[MarkusFrankATcernch]

Reopening is just fine. The aim is to maintain a "proper" worklist without too many already processed items.