Sampling calorimeter (ECAL + HCAL).

Description

The Calorimeter subsystem builds a layered ECAL + HCAL stack. Each calorimeter module is a tile in an nx × ny grid; each tile is built from a sequence of layer types that interleave absorber plates, scintillator bars and HPL fibre layers.

All parameters — layer sequences, thicknesses, module tiling — are constexpr constants in CalorimeterConstants.h (namespace SHiPGeometry::Calo).

Geometry tree

/SHiP/calorimeter (Air, 6000 × 7000 × 2900 mm — the fixed subsystem envelope)
 └─ for each (mx, my) in [0..nx-1] × [0..ny-1]:
      └─ <layer_env_MXxYy>           (Air envelope, kPlateXY × kPlateXY × layer_thickness)
            └─ layer body            (Lead / Iron / WidePVT / ThinPS / HPL)
                  └─ ...             (fibres or bars, depending on layer type)

Layer types

The kEcalLayers / kHcalLayers sequences in CalorimeterConstants.h are arrays of the LayerCode enum:

Enumerator	Layer type
`WidePVT_H`	WidePVT bar layer, bars along X (H orientation)
`WidePVT_V`	WidePVT bar layer, bars along Y (V orientation)
`ThinPS_H`	ThinPS bar layer, bars along X (H orientation)
`ThinPS_V`	ThinPS bar layer, bars along Y (V orientation)
`FibreHPL_Y`	HPL fibre layer, fibres along Y
`FibreHPL_X`	HPL fibre layer, fibres along X
`Absorber`	Absorber plate (Lead in ECAL section, Iron in HCAL)
`AirGap`	Air gap (no volume placed, just advances Z cursor)

Materials

Material	Density	Composition	Notes
Air	1.29 mg/cm³	already in catalog	container & layer envelopes
Lead	11.34 g/cm³	Pb	ECAL absorber plates
Iron	7.87 g/cm³	Fe	HCAL absorber plates
PVT	1.032 g/cm³	C₉H₁₀, mass-fraction-normalised	bar scintillator
Polystyrene	1.05 g/cm³	C₈H₈, mass-fraction-normalised	HPL fibre core

Lead, PVT and Polystyrene are added by this subsystem; Air and Iron were already present in the central SHiPMaterials catalog.

Validation

The parameters are validated at compile time by static_asserts in CalorimeterConstants.h:

kModuleNX and kModuleNY must be strictly positive.
The total Z extent of the ECAL sequence + kGapEcalHcal + the HCAL sequence (kTotalStackZ, 2600 mm) must fit inside the container's full Z extent (2 × kContainerHalfZ, 2900 mm).
The tiled modules' half-footprint in X and Y — centre to the outer edge of the outermost module — must fit inside the container's X/Y half-extents (kContainerHalfX / kContainerHalfY).
kFiberCoreDiameter cannot exceed kFiberDiameter.
kPlateXY must be a whole number of bars at both bar pitches.

A violation fails the build with a descriptive message.

Tests

test_calorimeter.cpp exercises:

CalorimeterBuilds — the factory returns a non-null GeoPhysVol of the expected envelope size.
CalorimeterHasChildren — the container holds exactly kModuleNX × kModuleNY × kVolsPerModule children (588), so a dropped or duplicated layer placement cannot pass unnoticed.
TotalStackZMatchesReference — Calo::kTotalStackZ matches the hand-computed reference, guarding the layer-sequence transcription.

Status

[x] ECAL + HCAL layer sequencer
[x] Bar scintillator layers (CaloBarLayer)
[x] HPL fibre layers (CaloFibreHPLayer)
[x] Lead, PVT, Polystyrene materials
[x] Module-array tiling
[x] Compile-time envelope-fit validation
[ ] Verification against a reference GDML

TODO

Cross-check fibre and bar dimensions against the SHiP TDR once the optimisation phase fixes the design.
Reconsider whether the ECAL and HCAL sequences should be unified into a single sequence with an explicit ECAL/HCAL marker, once the HCAL detector choice (own design vs. LHCb HCAL) is finalised.