ALICE3/FT3: fix magnetic field silently disabled when FT3 is active

FT3Module::initialize_materials() registered its six support materials
via raw `new TGeoMedium(name, id, mat), bypassing o2::base::MaterialManager.
That 3-argument TGeoMedium constructor leaves all tracking parameters,
including ifield, at 0, and since these ids were never coordinated with
MaterialManager's auto-assigned global ids, they collided with other
detectors' media -notably the CAVE air medium that fills the gaps between
TRK's barrel layers.

Route these six materials/media through MaterialManager instead.

Author: Berkin Ulukutlu

Detectors/Upgrades/ALICE3/TRKFT3/FT3/simulation/src/FT3Module.cxx

@@ -14,6 +14,8 @@
 
 #include "FT3Simulation/FT3Module.h"
 #include "FT3Base/FT3BaseParam.h"
+#include "DetectorsBase/MaterialManager.h"
+#include "DetectorsBase/Detector.h"
 #include <TGeoManager.h>
 #include <TGeoMaterial.h>
 #include <TGeoMedium.h>
@@ -54,42 +56,77 @@ void FT3Module::initialize_materials()
     return;
   }
 
-  TGeoManager* geoManager = gGeoManager;
-
-  auto* itsH = new TGeoElement("FT3_H", "Hydrogen", 1, 1.00794);
-  auto* itsC = new TGeoElement("FT3_C", "Carbon", 6, 12.0107);
-  auto* itsO = new TGeoElement("FT3_O", "Oxygen", 8, 15.994);
-
-  siliconMat = new TGeoMaterial("FT3_Silicon", 28.0855, 14, 2.33);
-  siliconMed = new TGeoMedium("FT3_Silicon", 1, siliconMat);
-
-  copperMat = new TGeoMaterial("FT3_Copper", 63.546, 29, 8.96);
-  copperMed = new TGeoMedium("FT3_Copper", 2, copperMat);
-
-  TGeoMixture* kaptonMat = new TGeoMixture("FT3_Kapton", 4, 1.346); // C22 H10 N2 O5
-
-  kaptonMat->DefineElement(0, 12.0107, 6, 0.5641); // Carbon
-  kaptonMat->DefineElement(1, 1.00794, 1, 0.2564); // Hydrogen
-  kaptonMat->DefineElement(2, 14.0067, 7, 0.0513); // Nitrogen
-  kaptonMat->DefineElement(3, 15.999, 8, 0.1282);  // Oxygen
-  kaptonMed = new TGeoMedium("FT3_Kapton", 3, kaptonMat);
+  // NOTE: these materials/media used to be registered directly via
+  // `new TGeoMaterial(...)` / `new TGeoMedium(name, rawId, ...)` with small
+  // hand-picked ids (1-6). That bypasses o2::base::MaterialManager, whose job
+  // is to hand out globally-unique medium ids across all detectors. Since the
+  // 3-argument TGeoMedium constructor also leaves all tracking parameters
+  // (including ifield) at 0, and Geant4VMC's TG4GeometryManager::
+  // FillMediumMapFromRoot() maps ALL TGeoMedium objects into one global table
+  // keyed purely by that raw numeric id (last one processed wins), these raw
+  // ids collided with other detectors' properly-assigned medium ids (e.g. the
+  // CAVE air medium that fills the gaps between TRK layers) and silently
+  // switched the magnetic field off for them. Route through MaterialManager
+  // instead, like every other detector (incl. FT3's own Detector.cxx) does,
+  // so ids are safely auto-assigned and never collide.
+  auto& matmgr = o2::base::MaterialManager::Instance();
+
+  int ifield = 2;
+  float fieldm = 10.0;
+  o2::base::Detector::initFieldTrackingParams(ifield, fieldm);
+
+  float tmaxfdSi = 0.1;
+  float stemaxSi = 0.0075;
+  float deemaxSi = 0.1;
+  float epsilSi = 1.0E-4;
+  float stminSi = 0.0;
+
+  float tmaxfdPas = 0.1;
+  float stemaxPas = 1.0;
+  float deemaxPas = 0.1;
+  float epsilPas = 1.0E-4;
+  float stminPas = 0.0;
+
+  // local ids 10-15: chosen to not clash with FT3's own createMaterials() (1, 3)
+  matmgr.Material("FT3", 10, "Silicon", 28.0855, 14, 2.33, 0, 0);
+  matmgr.Medium("FT3", 10, "Silicon", 10, 0, ifield, fieldm, tmaxfdSi, stemaxSi, deemaxSi, epsilSi, stminSi);
+  siliconMed = matmgr.getTGeoMedium("FT3", 10);
+  siliconMat = siliconMed->GetMaterial();
+
+  matmgr.Material("FT3", 11, "Copper", 63.546, 29, 8.96, 0, 0);
+  matmgr.Medium("FT3", 11, "Copper", 11, 0, ifield, fieldm, tmaxfdPas, stemaxPas, deemaxPas, epsilPas, stminPas);
+  copperMed = matmgr.getTGeoMedium("FT3", 11);
+  copperMat = copperMed->GetMaterial();
+
+  // Kapton: C22 H10 N2 O5, by weight fraction
+  float aKapton[4] = {12.0107, 1.00794, 14.0067, 15.999};
+  float zKapton[4] = {6., 1., 7., 8.};
+  float wKapton[4] = {0.5641, 0.2564, 0.0513, 0.1282};
+  matmgr.Mixture("FT3", 12, "Kapton", aKapton, zKapton, 1.346, 4, wKapton);
+  matmgr.Medium("FT3", 12, "Kapton", 12, 0, ifield, fieldm, tmaxfdPas, stemaxPas, deemaxPas, epsilPas, stminPas);
+  kaptonMed = matmgr.getTGeoMedium("FT3", 12);
+  kaptonMat = dynamic_cast<TGeoMixture*>(kaptonMed->GetMaterial());
 
   // TODO: Check with Rene the exact type of carbon fiber
-  carbonFiberMat = new TGeoMaterial("FT3_Carbon", 12.0107, 6, 1.8);
-  carbonFiberMed = new TGeoMedium("FT3_Carbon", 6, carbonFiberMat);
-
-  // Epoxy: C18 H19 O3
-  auto* itsEpoxy = new TGeoMixture("FT3_Epoxy", 3);
-  itsEpoxy->AddElement(itsC, 18);
-  itsEpoxy->AddElement(itsH, 19);
-  itsEpoxy->AddElement(itsO, 3);
-  itsEpoxy->SetDensity(2.186);
-
-  epoxyMed = new TGeoMedium("FT3_Epoxy", 4, itsEpoxy);
+  matmgr.Material("FT3", 13, "Carbon", 12.0107, 6, 1.8, 0, 0);
+  matmgr.Medium("FT3", 13, "Carbon", 13, 0, ifield, fieldm, tmaxfdPas, stemaxPas, deemaxPas, epsilPas, stminPas);
+  carbonFiberMed = matmgr.getTGeoMedium("FT3", 13);
+  carbonFiberMat = carbonFiberMed->GetMaterial();
+
+  // Epoxy: C18 H19 O3, by atom count (negative nlmat)
+  float aEpoxy[3] = {12.0107, 1.00794, 15.999};
+  float zEpoxy[3] = {6., 1., 8.};
+  float wEpoxy[3] = {18., 19., 3.};
+  matmgr.Mixture("FT3", 14, "Epoxy", aEpoxy, zEpoxy, 2.186, -3, wEpoxy);
+  matmgr.Medium("FT3", 14, "Epoxy", 14, 0, ifield, fieldm, tmaxfdPas, stemaxPas, deemaxPas, epsilPas, stminPas);
+  epoxyMed = matmgr.getTGeoMedium("FT3", 14);
   epoxyMat = epoxyMed->GetMaterial();
 
-  AluminumMat = new TGeoMaterial("Aluminum", 26.98, 13, 2.7);
-  AluminumMed = new TGeoMedium("Aluminum", 5, AluminumMat);
+  matmgr.Material("FT3", 15, "Aluminum", 26.98, 13, 2.7, 0, 0);
+  matmgr.Medium("FT3", 15, "Aluminum", 15, 0, ifield, fieldm, tmaxfdPas, stemaxPas, deemaxPas, epsilPas, stminPas);
+  AluminumMed = matmgr.getTGeoMedium("FT3", 15);
+  AluminumMat = AluminumMed->GetMaterial();
+
   LOG(debug) << "FT3Module: done initialize_materials";
 }