Exotic Physics¶
The Geant4 toolkit has recently been extended to include “exotic physics”. This covers the area of phonon propagation and crystal channelling. These two domains are applicable for Dark Matter experiments (phonon excitation) and beam extraction and collimation (crystal channelling). The framework within Geant4 is similar in that a macroscopic periodic crystal lattice is required for both and wave functions are propagated within the medium (rather than discrete particles as in the case of conventional Geant4). Contained here is a brief description of how to modify a Geant4 application to include the crystal as both a material and a geometry (plane orientations).
Physics¶
For a more complete description and understanding the user is referred to the extended examples category “exoticphysics” and the references therein.
Material¶
The implementation of solid-state processes in Geant4 requires the
addition of two important features, the crystal unit cell with all its
parameters and the support for other data required by the processes. The
extended data for a material is stored in a class derived from the
virtual class G4VMaterialExtension
. The G4ExtenededMaterial
class collects the pointers to concrete instances of
G4VMaterialExtension
. The G4CrystalExtension
class is a derived
class of G4VMaterialExtension
and collects information on the
physics properties of a perfect crystal. In particular, the class
contains a pointer to a G4CrystalUnitCell
object, the elasticity
tensor, a map of G4CrystalAtomBase
objects associated with a
G4Element
and a vector of G4AtomicBond
. The
G4CrystalUnitCell
class collects information on the mathematical
description of the crystal unit cell, i.e. the sizes and the angles of
the unit cell, the space group, the Bravais lattice and the lattice
system, and methods for the calculation of the volume in the direct and
reciprocal space, the spacing between two planes, the angle between two
planes, and for the filling of the reduced elasticity tensor. The
G4CrystalExtension
constructor takes as argument a pointer to a
G4Material
object and has to be registered to the
G4ExtendedMaterial
to which it is attached. The
G4CrystalAtomBase
class stores the position of atoms in the crystal
unit cell. Since the G4CrystalAtomBase
class is mapped to a
G4Element
in the G4CrystalMaterial
, each G4Element
should
have an associated G4CrystalAtomBase
. The G4AtomicBond
class
contains information on the atomic bond in the crystal. For each
instance of the class two G4Elements
have to be specified as well as
the atom number in the G4CrystalAtomBase
associated to the
G4Element
.
Geometry¶
The G4LogicalCrystalVolume
accepts only a pointer to a
G4CrystalExtension
in its constructor and stores the definition of
the orientation of the crystalline structure with respect to the solid
to which it is attached. By convention, the crystal < 100 > direction is
by default set parallel to the {[1,0,0]} direction in the Geant4
reference system, and the < 010 > axis lays on the plane which contains
the [1,0,0] and [0,1,0] directions in the Geant4 reference system.