Geant4 optics Giovanni Santin ESA / ESTEC and RheaTech Ltd On behalf of the Geant4 collaboration Ecole Geant4 Annecy, 18-21 and 25-28 Nov 2008 Slides adapted from previous tutorials and talks by Peter Gumplinger, TRIUMF (coordinator of the developments on processes involving
Geant4 optics. Giovanni Santin ESA / ESTEC and RheaTech Ltd On behalf of the Geant4 collaboration Ecole Geant4 Annecy, 18-21 and 25-28 Nov 2008 Slides adapted from previous tutorials and talks by Peter Gumplinger, TRIUMF - PowerPoint PPT Presentation
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Geant4 optics
Giovanni SantinESA / ESTEC and RheaTech Ltd
On behalf of the Geant4 collaboration
Ecole Geant4 Annecy, 18-21 and 25-28 Nov 2008
Slides adapted from previous tutorials and talks by Peter Gumplinger, TRIUMF
(coordinator of the developments on processes involving optical photons)
Giovanni Santin - General Particle Source (GPS) - Ecole Geant4 2008, Annecy 2
Outline
Introduction Optical processes
– Processes producing photons– Processes undergone by photons
Optical properties in material property tables Examples
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Optical photons
Physically optical photons should be covered by the electromagnetic category, but– optical photon wavelength is >> atomic spacing– treated as waves no smooth transition between optical and gamma particle classes
G4OpticalPhoton: wave like nature of EM radiation
G4OpticalPhoton <=|=> G4Gamma– New particle type– No smooth transition
G4Cerenkov can limit the Step by:– User defined average maximum number of photons to be generated during a step– New: User defined maximum allowed change in beta = v/c in % during the step.– New: A definite step limit when the track drops below the Cerenkov threshold
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Scintillation process
Number of photons generated proportional to the energy lost during the step
Emission spectrum sampled from empirical spectra Isotropic emission Uniform along the track segment With random linear polarization Emission time spectra with one exponential decay time
constant
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G4ScintillationProcess implementation details
Scintillation material has a characteristic light yield The statistical yield fluctuation is either broadened due to impurities for
doped crystals or narrower as a result of the Fano Factor Suspend primary particle and track scintillation photons first
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Rayleigh ScatteringG4OpRayleigh
Elastic scattering including polarization of initial and final photons– The scattered photon direction is perpendicular to the new photon
polarization in such a way that the final direction, initial and final polarization are all in one plane
The diff. cross section is proportional to cos2() where is the angle between the initial and final photon polarization
Rayleigh scattering attenuation coefficient is calculated for “Water” material following the Einstein-Smoluchowski formula, but in all other cases it must be provided by the user:
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Wavelength shifting Handled by G4OpWLS
– initial photon is killed, one with new wavelength is created– builds it own physics table for mean free path
User must supply:
– Absorption length as function of photon energy– Isotropic emission– With random linear polarization– Emission spectra parameters as function of energy– Possible time delay between absorption and re-emission
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Boundary processes
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Boundary interactionsOptical photons as particles
Geant4 demands particle-like behavior for tracking:
thus, no “splitting” event with both refraction and
reflection must be simulated by at least two events
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Boundary interactions
Handled by G4OpBoundaryProcess
– refraction
– reflection User must supply surface
properties using G4OpticalSurface models
Boundary properties– dielectric-dielectric
– dielectric-metal
– dielectric-black material Surface properties:
– polished
– ground
– front- or back-painted, ...
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G4BoundaryProcessImplementation Details
A ‘discrete process’, called at the end of every step Never limits the step (done by the transportation) Sets the ‘forced’ condition Logic such that
– preStepPoint: is still in the old volume– postStepPoint: is already in the new volume
so information is available from both
Post-step point
Step
Boundary
Pre-step point
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Surface ConceptSplit into two classes
Conceptual class: G4LogicalSurface(in the geometry category) holds
– pointers to the relevant physical or logical volumes– pointer to a G4OpticalSurface
These classes are stored in a table and can be retrieved by specifying:– an ordered pair of physical volumes touching at the surface
[G4LogicalBorderSurface]• in principle allows for different properties depending on which direction the photon arrives
– or a logical volume entirely surrounded by this surface [G4LogicalSkinSurface]
• useful when the volume is coded by a reflector and placed into many volumes • limitation: only one and the same optical property for all the enclosed volume’s sides)
Physical class: G4OpticalSurface (in the material category) keeps information about the physical properties of the surface itself
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G4OpticalSurface Set the simulation model used by the boundary process:
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Optical surface types Dielectric - Dielectric
Depending on the photon’s wave length, angle of incidence, (linear) polarization, and (user input) refractive index on both sides of the boundary:
a) total internal reflectedb) Fresnel refractedc) Fresnel reflected
Dielectric – MetalThe photon cannot be transmitted.
a) absorbed (detected), with probability estimated according to the table provided by the user
b) reflected
Dielectric – Black materialA black material is a tracking medium for which the user has not defined any optical property. The photon is immediately absorbed undetected
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Surface effects POLISHED: In the case where the surface between two bodies is perfectly polished,
the normal used by the G4BoundaryProcess is the normal to the surface defined by:– the daughter solid entered; or else– the solid being left behind
GROUND: The incidence of a photon upon a rough surface requires choosing the angle, a, between a ‘micro-facet’ normal and that of the average surface.
The UNIFIED model assumes that the probability of micro-facet normals that populates the annulus of solid angle sin()d will be proportional to a gaussian of SigmaAlpha:theOpSurface -> SetSigmaAlpha(0.1); [rad]
In the GLISUR model this is indicated by the value of polish; when it is <1, then a random point is generated in a sphere of radius (1-polish), and the corresponding vector is added to the normal. The value 0 means maximum roughness with effective plane of reflection distributed as cos.theOpSurface -> SetPolish(0.0);
The ‘facet normal’ is accepted if the refracted wave is still inside the original volume.
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Microfacets
The assumption is that a rough surface is a collection of ‘microfacets’
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In cases (b) and (c), multiple interactions with the boundary are possible within the process itself and without the need for relocation by the G4Navigator.
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Csl: Reflection prob. about the normal of a micro facet Css: Reflection prob. about the average surface normal Cdl: Prob. of internal Lambertian reflection Cbs: Prob. of reflection within a deep grove with the ultimate
result of exact back scattering.
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The G4OpticalSurface also has a pointer to a G4MaterialPropertiesTable
In case the surface is painted, wrapped, or has a cladding, the table may include the thin layer’s index of refraction
This allows the simulation of boundary effects both – at the intersection between the medium and the surface layer and – at the far side of the thin layer
all within the process itself and without invoking the G4Navigator– the thin layer does not have to be defined as a G4 tracking volume
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Logic in G4OpBoundaryProcess:PostStepDoIt
Make sure:– the photon is at a boundary (StepStatus = fGeomBoundary)– the last step taken is not a very short step (StepLength>=kCarTolerance/2) as it can happen upon reflectionELSE do nothing and RETURN
If the two media on either side are identical do nothing and RETURN
If the original medium had no G4MaterialPropertiesTable defined kill the photon and RETURNELSE get the refractive index
Get the refractive index for the medium on the other side of the boundary, if there is one
See, if a G4LogicalSurface is defined between the two volumesif so get the G4OpticalSurface which contains physical surface parameters
Default to glisur model and polished surface
If the new medium had a refractive index, set the surface type to ‘dielectric-dielectric’ELSEIF get the refractive index from the G4OpticalSurfaceELSE kill the photon
Use (as far as it has the information) G4OpticalSurface to model the surface ELSE use Default
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As a consequence
1) For polished interfaces, no ‘surface’ is needed if the index of refraction of the two media is defined
2) The boundary process implementation is rigid about what it expects the G4Navigator does upon reflection on a boundary
3) G4BoundaryProcess with ‘surfaces’ is only possible for volumes that have been positioned by using placement rather than replica or touchables
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Examples
Some user applications
N06 – examples/novice/N06
Liquid Xenon – examples/extended/optical/LXe
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Sample of user applications
Courtesy H.Araujo (Imperial College London & UK Dark Matter Collaboration)
Courtesy A. Etenko, I. Machulin - Kurchatov Institute
G.Santin, HARP Cerenkov, CERN
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ExampleN06
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Summary Optical processes handle
– the reflection, refraction, absorption, wavelength shifting and scattering of long-wavelength photons, and
– the productions of photons by scintillation, Cerenkov and transition radiation The simulation may commence with the propagation of a charged particle and end
with the detection of the ensuing optical photons on photo sensitive areas, all within the same event loop
Documentationhttp://cern.ch/geant4 User support Application Developers Guide Optical photon processeshttp://cern.ch/geant4 User support Physics reference manual Optical photons