1. Detector Description – basic concepts
Geant4: Detector description module
Detector Description – basic concepts
The full set of lecture notes of this Geant4 Course is available at

2. Geant4: Detector description module
Part I The Basics
Part II Logical and physical volumes
Part III Solids, touchables
Part IV Optimisation technique &
Advanced features

3. Detector Description: the Basics
PART 1
Detector Description: the Basics

4. Describe your detector
Geant4: Detector description module
Describe your detector
Derive your own concrete class from G4VUserDetectorConstruction abstract base class.
Implementing the method Construct():
Modularize it according to each detector component or sub-detector:
Construct all necessary materials
Define shapes/solids required to describe the geometry
Construct and place volumes of your detector geometry
Define sensitive detectors and identify detector volumes which to associate them
Associate magnetic field to detector regions
Define visualization attributes for the detector elements
The main class defining the detector geometry is derived from G4VUserDetectorConstruction.
It is advised to modularize its design according to the geometry components (I.e. define a class for each geometry component), especially for complex geometry structures.
Definition of attributes, like sensitive detectors, local fields or visualization are optional.
G.Cosmo, Detector Description - Geant4 Course

5. Creating a Detector Volume
Geant4: Detector description module
Creating a Detector Volume
Solid
Logical-Volume
Physical-Volume
Start with its Shape & Size
Box 3x5x7 cm, sphere R=8m
Add properties:
material, B/E field,
make it sensitive
Place it in another volume
in one place
repeatedly using a function
So you can re-use
i) one solid for 10 or 100 or any number of logical volumes.
ii) one logical volume for 100 or 1000 physical volumes.
Solids are geometrical entities. They can also be used to describe sensitive volumes, ...
G.Cosmo, Detector Description - Geant4 Course

6. Define detector geometry
Geant4: Detector description module
Define detector geometry
Three conceptual layers
G4VSolid -- shape, size
G4LogicalVolume -- daughter physical volumes,
material, sensitivity, user limits, etc.
G4VPhysicalVolume -- position, rotation
G4Box
G4Tubs
G4VSolid
G4VPhysicalVolume
G4Material
G4VSensitiveDetector
G4PVPlacement
G4PVParameterised
G4VisAttributes
G4LogicalVolume
Possible attributes associated to a G4LogicalVolume:
Magnetic field
Material
If sensitive, associated sensitive detector
Quality of geometry optimization (smartless)
Visualization attributes
User limits (maximum allowed step, track length, minimum kinetic energy, …) …
G.Cosmo, Detector Description - Geant4 Course

7. Define detector geometry
Basic strategy
G4VSolid* pBoxSolid =
new G4Box(“aBoxSolid”, 1.*m, 2.*m, 3.*m);
G4LogicalVolume* pBoxLog =
new G4LogicalVolume( pBoxSolid, pBoxMaterial,
“aBoxLog”, 0, 0, 0);
G4VPhysicalVolume* aBoxPhys =
new G4PVPlacement( pRotation,
G4ThreeVector(posX, posY, posZ),
pBoxLog, “aBoxPhys”, pMotherLog,
0, copyNo);
A unique physical volume which represents the experimental area must exist and fully contains all other components
The world volume
G.Cosmo, Detector Description - Geant4 Course

8. Detector Description: Logical and Physical Volumes
PART II
Detector Description:
Logical and Physical Volumes

9. Geant4: Detector description module
G4LogicalVolume
G4LogicalVolume(G4VSolid* pSolid, G4Material* pMaterial,
const G4String& name, G4FieldManager* pFieldMgr=0,
G4VSensitiveDetector* pSDetector=0,
G4UserLimits* pULimits=0,
G4bool optimise=true);
Contains all information of volume except position:
Shape and dimension (G4VSolid)
Material, sensitivity, visualization attributes
Position of daughter volumes
Magnetic field, User limits
Shower parameterisation
Physical volumes of same type can share a logical volume.
The pointers to solid and material must be NOT null
Once created it is automatically entered in the LV store
It is not meant to act as a base class
It is possible to deactivate optimisation (voxelisation) of the geometry for a given logical volume.
Optimisation is built by default, if the flag “optimise” is set to false, the whole hierarchy of volumes identified by the logical volume will not be optimised. Note that optimisation is always applied for logical volumes whose physical volumes are parameterised volumes.
G.Cosmo, Detector Description - Geant4 Course

10. G.Cosmo, Detector Description - Geant4 Course
G4VPhysicalVolume
G4PVPlacement Placement = One Volume
A volume instance positioned once in a mother volume
G4PVParameterised Parameterised = Many Volumes
Parameterised by the copy number
Shape, size, material, position and rotation can be parameterised, by implementing a concrete class of G4VPVParameterisation.
Reduction of memory consumption
Currently: parameterisation can be used only for volumes that either a) have no further daughters or b) are identical in size & shape.
G4PVReplica Replica = Many Volumes
Slicing a volume into smaller pieces (if it has a symmetry)
G.Cosmo, Detector Description - Geant4 Course

11. Geant4: Detector description module
Physical Volumes
repeated
placement
Placement: it is one positioned volume
Repeated: a volume placed many times
can represent any number of volumes
reduces use of memory.
Replica
simple repetition, similar to G3 divisions
Parameterised
A mother volume can contain either
many placement volumes OR
one repeated volume
G.Cosmo, Detector Description - Geant4 Course

12. Geant4: Detector description module
G4PVPlacement
G4PVPlacement(G4RotationMatrix* pRot,
const G4ThreeVector& tlate,
G4LogicalVolume* pCurrentLogical,
const G4String& pName,
G4LogicalVolume* pMotherLogical,
G4bool pMany,
G4int pCopyNo);
Single volume positioned relatively to the mother volume
In a frame rotated and translated relative to the coordinate system of the mother volume
Three additional constructors:
A simple variation: specifying the mother volume as a pointer to its physical volume instead of its logical volume.
Using G4Transform3D to represent the direct rotation and translation of the solid instead of the frame
The combination of the two variants above
Adopting the constructor using the pointer to the logical volume of the mother, is a very natural way of defining a physical volume, and is especially useful when creating sub-detectors: the mother volumes is not placed until a later stage of the assembly program.
The G4Transform3D argument should be constructed by:
i) First rotating it to align the solid to the system of reference of its mother volume, and
ii) Then placing the solid at the location Transform3D.getTranslation(), with respect to the origin of the system of coordinates of the mother volume.
This is useful for the people who prefer to think in terms of moving objects in a given reference frame.
G.Cosmo, Detector Description - Geant4 Course

13. Parameterised Physical Volumes
Geant4: Detector description module
Parameterised Physical Volumes
User written functions define:
the size of the solid (dimensions)
Function ComputeDimensions(…)
where it is positioned (transformation)
Function ComputeTransformations(…)
Optional:
the type of the solid
Function ComputeSolid(…)
the material
Function ComputeMaterial(…)
Limitations:
Applies to simple CSG solids only
Daughter volumes allowed only for special cases
Very powerful
Consider parameterised volumes as “leaf” volumes
For parameterised volumes the mother volume must be always specified, therefore the world volume can never be a parameterised volume.
All methods in G4VPVParameterisation provides an argument (integer) for specifying the copy number of the parameterisation instance under consideration.
Only parameterised volumes in which all the solids have the same size, currently allows the addition of daughter volumes.
G.Cosmo, Detector Description - Geant4 Course

14. Uses of Parameterised Volumes
Geant4: Detector description module
Uses of Parameterised Volumes
Complex detectors
with large repetition of volumes
regular or irregular
Medical applications
the material in animal tissue is measured
cubes with varying material
G.Cosmo, Detector Description - Geant4 Course

15. Geant4: Detector description module
G4PVParameterised
G4PVParameterised(const G4String& pName,
G4LogicalVolume* pCurrentLogical,
G4LogicalVolume* pMotherLogical,
const EAxis pAxis,
const G4int nReplicas,
G4VPVParameterisation* pParam);
Replicates the volume nReplicas times using the parameterisation pParam, within the mother volume
The positioning of the replicas is dominant along the specified Cartesian axis
If kUndefined is specified as axis, 3D voxelisation for optimisation of the geometry is adopted
Represents many touchable detector elements differing in their positioning and dimensions. Both are calculated by means of a G4VPVParameterisation object
Alternative constructor using pointer to physical volume for the mother
G.Cosmo, Detector Description - Geant4 Course

16. Parameterisation example - 1
Geant4: Detector description module
Parameterisation example - 1
G4VSolid* solidChamber = new G4Box("chamber", 100*cm, 100*cm, 10*cm);
G4LogicalVolume* logicChamber =
new G4LogicalVolume(solidChamber, ChamberMater, "Chamber", 0, 0, 0);
G4double firstPosition = -trackerSize + 0.5*ChamberWidth;
G4double firstLength = fTrackerLength/10;
G4double lastLength = fTrackerLength;
G4VPVParameterisation* chamberParam =
new ChamberParameterisation( NbOfChambers, firstPosition,
ChamberSpacing, ChamberWidth,
firstLength, lastLength);
G4VPhysicalVolume* physChamber =
new G4PVParameterised( "Chamber", logicChamber, logicTracker,
kZAxis, NbOfChambers, chamberParam);
Example of parameterisation along the Z axis (kZAxis)
NbOfChambers - Number of chambers
firstPosition - Z of center of first chamber
ChamberSpacing - Z spacing of centers
ChamberWidth - Width of chamber
firstLength - initial length
lastLength - final length
Use kUndefined for activating 3D voxelisation for optimisation
G.Cosmo, Detector Description - Geant4 Course

17. Parameterisation example - 2
Geant4: Detector description module
Parameterisation example - 2
class ChamberParameterisation : public G4VPVParameterisation {
public:
ChamberParameterisation( G4int NoChambers, G4double startZ,
G4double spacing, G4double widthChamber,
G4double lenInitial, G4double lenFinal );
~ChamberParameterisation();
void ComputeTransformation (const G4int copyNo,
G4VPhysicalVolume* physVol) const;
void ComputeDimensions (G4Box& trackerLayer, const G4int copyNo,
const G4VPhysicalVolume* physVol) const; : }
Example of parameterisation along the Z axis (kZAxis)
G.Cosmo, Detector Description - Geant4 Course

18. Parameterisation example - 3
Geant4: Detector description module
Parameterisation example - 3
void ChamberParameterisation::ComputeTransformation
(const G4int copyNo, G4VPhysicalVolume* physVol) const {
G4double Zposition= fStartZ + (copyNo+1) * fSpacing;
G4ThreeVector origin(0, 0, Zposition);
physVol->SetTranslation(origin);
physVol->SetRotation(0); }
void ChamberParameterisation::ComputeDimensions
(G4Box& trackerChamber, const G4int copyNo,
const G4VPhysicalVolume* physVol) const
G4double halfLength= fHalfLengthFirst + copyNo * fHalfLengthIncr;
trackerChamber.SetXHalfLength(halfLength);
trackerChamber.SetYHalfLength(halfLength);
trackerChamber.SetZHalfLength(fHalfWidth);
Example of parameterisation along the Z axis (kZAxis)
G.Cosmo, Detector Description - Geant4 Course

19. Replicated Physical Volumes
Geant4: Detector description module
Replicated Physical Volumes
repeated
The mother volume is sliced into replicas, all of the same size and dimensions.
Represents many touchable detector elements differing only in their positioning.
Replication may occur along:
Cartesian axes (X, Y, Z) – slices are considered perpendicular to the axis of replication
Coordinate system at the center of each replica
Radial axis (Rho) – cons/tubs sections centered on the origin and un-rotated
Coordinate system same as the mother
Phi axis (Phi) – phi sections or wedges, of cons/tubs form
Coordinate system rotated such as that the X axis bisects the angle made by each wedge
The elements positions are calculated by means of a simple linear formula, and the elements completely fill the containing mother volume.
If a G4PVReplica is positioned inside a given mother, it must be the only daughter volume for that mother.
During tracking, translation and rotation of each replica object are computed according to the current active replication.
G.Cosmo, Detector Description - Geant4 Course

20. Geant4: Detector description module
a daughter volume to be replicated
mother volume
G4PVReplica
G4PVReplica(const G4String& pName,
G4LogicalVolume* pCurrentLogical,
G4LogicalVolume* pMotherLogical,
const EAxis pAxis,
const G4int nReplicas,
const G4double width,
const G4double offset=0);
Alternative constructor: using pointer to physical volume for the mother
An offset can only be associated to a mother offset along the axis of replication
Features and restrictions:
Replicas can be placed inside other replicas
Normal placement volumes can be placed inside replicas, assuming no intersection/overlaps with the mother volume or with other replicas
No volume can be placed inside a radial replication
Parameterised volumes cannot be placed inside a replica
The width of the replica refers to its extent along the axis of replication.
G.Cosmo, Detector Description - Geant4 Course

21. Replica – axis, width, offset
Cartesian axes - kXaxis, kYaxis, kZaxis
offset shall not be used
Center of n-th daughter is given as
-width*(nReplicas-1)*0.5+n*width
Radial axis - kRaxis
width*(n+0.5)+offset
Phi axis - kPhi
offset
width
G.Cosmo, Detector Description - Geant4 Course

22. Geant4: Detector description module
Replication example
G4double tube_dPhi = 2.* M_PI;
G4VSolid* tube =
new G4Tubs("tube", 20*cm, 50*cm, 30*cm, 0., tube_dPhi*rad);
G4LogicalVolume * tube_log =
new G4LogicalVolume(tube, Ar, "tubeL", 0, 0, 0);
G4VPhysicalVolume* tube_phys =
new G4PVPlacement(0,G4ThreeVector(-200.*cm, 0., 0.*cm),
"tubeP", tube_log, world_phys, false, 0);
G4double divided_tube_dPhi = tube_dPhi/6.;
G4VSolid* divided_tube =
new G4Tubs("divided_tube", 20*cm, 50*cm, 30*cm,
-divided_tube_dPhi/2.*rad, divided_tube_dPhi*rad);
G4LogicalVolume* divided_tube_log =
new G4LogicalVolume(divided_tube, Ar, "div_tubeL", 0, 0, 0);
G4VPhysicalVolume* divided_tube_phys =
new G4PVReplica("divided_tube_phys", divided_tube_log, tube_log,
kPhi, 6, divided_tube_dPhi);
Example of replication in Phi
G.Cosmo, Detector Description - Geant4 Course

23. Divided Physical Volumes
Geant4: Detector description module
Divided Physical Volumes
Implemented as “special” kind of parameterised volumes
Applies to CSG-like solids only (box, tubs, cons, para, trd, polycone, polyhedra)
Divides a volume in identical copies along one of its axis (copies are not strictly identical)
e.g. - a tube divided along its radial axis
Offsets can be specified
The possible axes of division vary according to the supported solid type
Represents many touchable detector elements differing only in their positioning
G4PVDivision is the class defining the division
The parameterisation is calculated automatically using the values provided in input
G.Cosmo, Detector Description - Geant4 Course

24. Geant4: Detector description module
PART III
Detector Description:
Solids & Touchables

25. Geant4: Detector description module
G4VSolid
Abstract class. All solids in Geant4 derive from it
Defines but does not implement all functions required to:
compute distances to/from the shape
check whether a point is inside the shape
compute the extent of the shape
compute the surface normal to the shape at a given point
Once constructed, each solid is automatically registered in a specific solid store
G.Cosmo, Detector Description – Geant4 Course 24

26. Geant4: Detector description module
Solids
Solids defined in Geant4:
CSG (Constructed Solid Geometry) solids
G4Box, G4Tubs, G4Cons, G4Trd, …
Analogous to simple GEANT3 CSG solids
Specific solids (CSG like)
G4Polycone, G4Polyhedra, G4Hype, …
BREP (Boundary REPresented) solids
G4BREPSolidPolycone, G4BSplineSurface, …
Any order surface
Boolean solids
G4UnionSolid, G4SubtractionSolid, …
G.Cosmo, Detector Description – Geant4 Course 25

27. Geant4: Detector description module
CSG: G4Tubs, G4Cons
G4Tubs(const G4String& pname, // name
G4double pRmin, // inner radius
G4double pRmax, // outer radius
G4double pDz, // Z half length
G4double pSphi, // starting Phi
G4double pDphi); // segment angle
G4Cons(const G4String& pname, // name
G4double pRmin1, // inner radius -pDz
G4double pRmax1, // outer radius -pDz
G4double pRmin2, // inner radius +pDz
G4double pRmax2, // outer radius +pDz
G.Cosmo, Detector Description - Geant4 Course

28. Specific CSG Solids: G4Polycone
Geant4: Detector description module
Specific CSG Solids: G4Polycone
G4Polycone(const G4String& pName,
G4double phiStart,
G4double phiTotal,
G4int numRZ,
const G4double r[],
const G4double z[]);
numRZ - numbers of corners in the r,z space
r, z - coordinates of corners
Additional constructor using planes
Constructor by planes:
G4Polycone( const G4String& pName,
G4double phiStart,
G4double phiTotal,
G4int numZPlanes,
const G4double zPlane[],
const G4double rInner[],
const G4double rOuter[])
G.Cosmo, Detector Description - Geant4 Course

29. Geant4: Detector description module
BREP Solids
BREP = Boundary REPresented Solid
Listing all its surfaces specifies a solid
e.g. 6 squares for a cube
Surfaces can be
planar, 2nd or higher order
elementary BREPS
Splines, B-Splines,
NURBS (Non-Uniform B-Splines)
advanced BREPS
Few elementary BREPS pre-defined
box, cons, tubs, sphere, torus, polycone, polyhedra
Advanced BREPS built through CAD systems
In picture:
G4double zval[6] = { -4000*mm, -3500*mm, -3500*mm, -1500*mm, -1500*mm, -1000*mm };
G4double rmin[6] = { 500*mm, 500*mm, 500*mm, 500*mm, 500*mm, 500*mm };
G4double rmax[6] = { 500*mm, 2000*mm, 1000*mm, 1000*mm, 2000*mm, 500*mm };
G4BREPSolidPCone *Solid =
new G4BREPSolidPCone( "BREPSolidPCone", 0.0*rad, twopi*rad, 6, zval[0], zval, rmin, rmax );
G.Cosmo, Detector Description - Geant4 Course

30. BREPS: G4BREPSolidPolyhedra
Geant4: Detector description module
BREPS: G4BREPSolidPolyhedra
G4BREPSolidPolyhedra(const G4String& pName,
G4double phiStart,
G4double phiTotal,
G4int sides,
G4int nZplanes,
G4double zStart,
const G4double zval[],
const G4double rmin[],
const G4double rmax[]);
sides - numbers of sides of each polygon in the x-y plane
nZplanes - numbers of planes perpendicular to the z axis
zval[] - z coordinates of each plane
rmin[], rmax[] - Radii of inner and outer polygon at each plane
In picture:
#define VECSIZE 6
G4int sides = 5;
G4double rmin[VECSIZE] = { 100*mm, 100*mm, 100*mm, 100*mm, 100*mm, 100*mm };
G4double rmax[VECSIZE] = { 700*mm, 800*mm, 400*mm, 400*mm, 600*mm, 500*mm };
G4double zval[VECSIZE] = { 3500*mm, 3800*mm, 4000*mm, 4300*mm, 4500*mm, 5000*mm };
G4BREPSolidPolyhedra *sphedra =
new G4BREPSolidPolyhedra( "sphedra", pi/2*rad, 2*pi*rad, sides, VECSIZE, zval[0], zval, rmin, rmax);
G4LogicalVolume *lvphedra =
new G4LogicalVolume( sphedra, Al, "lvphedra" );
G4PVPlacement *pvphedra =
new G4PVPlacement( 0, G4ThreeVector(), "pvphedra1", lvphedra, worldPhysic, false, 0 );
G.Cosmo, Detector Description - Geant4 Course

31. Geant4: Detector description module
Boolean Solids
G4UnionSolid
G4IntersectionSolid
G4SubtractionSolid
Solids can be combined using boolean operations:
G4UnionSolid, G4SubtractionSolid, G4IntersectionSolid
Requires: 2 solids, 1 boolean operation, and an (optional) transformation for the 2nd solid
2nd solid is positioned relative to the coordinate system of the 1st solid
Example:
G4Box box(“Box", 20, 30, 40);
G4Tubs cylinder(“Cylinder”, 0, 50, 50, 0, 2*M_PI); // r: > 50
// z: > 50
// phi: 0 -> 2 pi
G4UnionSolid union("Box+Cylinder", &box, &cylinder);
G4IntersectionSolid intersect("Box Intersect Cylinder", &box, &cylinder);
G4SubtractionSolid subtract("Box-Cylinder", &box, &cylinder);
Solids can be either CSG or other Boolean solids
Note: tracking cost for the navigation in a complex Boolean solid is proportional to the number of constituent solids
Passive transformation: a rotation matrix/translation is provided to transform the coordinate system of the 1st solid to the coordinate system of the 2nd solid. The final transformation in this case is NOT copied, therefore, once defined and used it must not be modified.
Active transformation: a transformation (G4Transform3D) is first created to move the 2nd solid to the correct position. The transformation in this case is stored (copied) in the final boolean solid.
G.Cosmo, Detector Description - Geant4 Course

32. How to identify a volume uniquely?
Geant4: Detector description module
How to identify a volume uniquely?
Need to identify a volume uniquely
Is a physical volume pointer enough?
NO!
Touchable 5 4 1 2 3
pPV
Later
Step
A physical volume can represent many actual volumes in a detector, due to:
replica, parameterisation
repetition of mother logical volume in different places in the hierarchy
To identify a volume uniquely, we must ‘anchor’ it to create a touchable.
A touchable may have information on all levels of the hierarchy, from the world volume to the current physical volume.
Touchable
G.Cosmo, Detector Description - Geant4 Course

33. Geant4: Detector description module
What can a touchable do ?
All generic touchables can reply to these queries:
positioning information (rotation, position)
GetTranslation(), GetRotation()
Specific types of touchable also know:
(solids) - their associated shape: GetSolid()
(volumes) - their physical volume: GetVolume()
(volumes) - their replication number: GetReplicaNumber()
(volumes hierarchy or touchable history):
info about its hierarchy of placements: GetHistoryDepth()
At the top of the history tree is the world volume
modify/update touchable: MoveUpHistory(), UpdateYourself()
take additional arguments
MoveUpHistory(num) moves the current pointer of the touchable to point “num” levels up in the history tree.
Therefore, num=1 means moving to point to the mother of the current volume.
The state of the touchable is changed by calling this method.
G.Cosmo, Detector Description - Geant4 Course

34. Benefits of Touchables in track
Geant4: Detector description module
Benefits of Touchables in track
Permanent information stored
to avoid implications with a “live” volume tree
Full geometrical information available
to processes
to sensitive detectors
to hits A1 A2
G.Cosmo, Detector Description - Geant4 Course

35. Geant4: Detector description module
Touchable - 1
G4Step has two G4StepPoint objects as its starting and ending points. All the geometrical information of the particular step should be got from “PreStepPoint”
Geometrical information associated with G4Track is basically same as “PostStepPoint”
Each G4StepPoint object has:
position in world coordinate system
global and local time
material
G4TouchableHistory for geometrical information
Copy-number, transformations
Handles (or smart-pointers) to touchables are intrinsically used. Touchables are reference counted
Since release 4.0 touchables are reference counted.
the lifetime of a single touchable object is assured for the whole lifetime of the application
memory management of the created touchables is performed in an automatic way
G.Cosmo, Detector Description - Geant4 Course

36. Geant4: Detector description module
Touchable - 2
G4TouchableHistory has information of geometrical hierarchy of the point
G4Step* aStep = ..;
G4StepPoint* preStepPoint = aStep->GetPreStepPoint();
G4TouchableHandle theTouchable =
preStepPoint->GetTouchableHandle();
G4int copyNo = theTouchable->GetReplicaNumber();
G4int motherCopyNo = theTouchable->GetReplicaNumber(1);
G4ThreeVector worldPos = preStepPoint->GetPosition();
G4ThreeVector localPos = theTouchable->GetHistory()->
GetTopTransform().TransformPoint(worldPos);
In the example code, it is shown how to use touchables for:
Retrieve the copy number of the current volume
Retrieve the copy number of the mother volume
Retrieve the absolute position in global world coordinates
Transform the position to local coordinates
G.Cosmo, Detector Description - Geant4 Course

37. G.Cosmo, Detector Description - Geant4 Course
Copy numbers
Suppose a calorimeter is made of 4x5 cells
and it is implemented by two levels of replica.
In reality, there is only one physical volume object for each level. Its position is parameterized by its copy number
To get the copy number of each level, suppose what happens if a step belongs to two cells
CopyNo = 0 1 2 3 4
CopyNo = 1
CopyNo = 2
CopyNo = 3
Remember geometrical information in G4Track is identical to "PostStepPoint". You cannot get the collect copy number for "PreStepPoint" if you directly access to the physical volume
Use touchable to get the proper copy number, transform matrix,…
G.Cosmo, Detector Description - Geant4 Course