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The IRIS service modules (addIRISServiceModules and
addIRISServiceModulesSegmented) created their medium by hand as
new TGeoMedium("ALUMINUM", 4, ...): a hard-coded medium ID 4 and all
tracking parameters 0, i.e. ifield = 0. ID 4 is already TRK_AIR$, the
medium of the TRKLayer* mother volumes and the VD petal cases.
Geant4 VMC keys media by ID (TG4GeometryManager::FillMediumMapFromRoot):
the second medium with ID 4 reuses the first one's entry and overwrites
its ifield with 0. With /mcDet/setIsZeroMagField true, every volume of
TRK_AIR$ then gets a zero field, so tracks cross each ML/OT layer
volume (1.2-1.5 cm of air around the staves) without bending. The
geometry file still shows TRK_AIR$ with ifield = 2.
A muon gun (p 2-5 GeV, |eta| < 0.8, A3IP + TRK, ALICE 3 field 2 T)
measures the field each pair of consecutive layers turned the momentum
in: before 0.84 T (7 -> 9 cm) rising to 1.95 T (60 -> 80 cm), the VD
2.00 T; after, every pair 1.99-2.01 T.
The aluminium is now registered in TRKServices::createMaterials with
the TRK's field parameters (ALICE3_TRKSERVICES_ALUMINIUM) and taken
from MaterialManager::getTGeoMedium, which gives it a unique ID. The
material is unchanged (A, Z, density, radiation and interaction
lengths).
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Signed-off-by: Felix Schlepper <felix.schlepper@cern.ch>
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The IRIS service modules (addIRISServiceModules and addIRISServiceModulesSegmented) created their medium by hand as new TGeoMedium("ALUMINUM", 4, ...): a hard-coded medium ID 4 and all tracking parameters 0, i.e. ifield = 0. ID 4 is already TRK_AIR$, the medium of the TRKLayer* mother volumes and the VD petal cases.
Geant4 VMC keys media by ID (TG4GeometryManager::FillMediumMapFromRoot): the second medium with ID 4 reuses the first one's entry and overwrites its ifield with 0. With /mcDet/setIsZeroMagField true, every volume of TRK_AIR$ then gets a zero field, so tracks cross each ML/OT layer volume (1.2-1.5 cm of air around the staves) without bending. The geometry file still shows TRK_AIR$ with ifield = 2.
A muon gun (p 2-5 GeV, |eta| < 0.8, A3IP + TRK, ALICE 3 field 2 T) measures the field each pair of consecutive layers turned the momentum in: before 0.84 T (7 -> 9 cm) rising to 1.95 T (60 -> 80 cm), the VD 2.00 T; after, every pair 1.99-2.01 T.
The aluminium is now registered in TRKServices::createMaterials with the TRK's field parameters (ALICE3_TRKSERVICES_ALUMINIUM) and taken from MaterialManager::getTGeoMedium, which gives it a unique ID. The material is unchanged (A, Z, density, radiation and interaction lengths).
Mediums should be registered with the manager to avoid this.