Geant 4 optics Giovanni Santin ESA ESTEC and

Geant 4 optics Giovanni Santin ESA / ESTEC and Rhea. Tech Ltd On behalf of the Geant 4 collaboration Ecole Geant 4 Annecy, 18 -21 and 25 -28 Nov 2008 Slides adapted from previous tutorials and talks by Peter- Gumplinger, TRIUMF Giovanni Santin General Source (GPS) - Ecole (coordinator of the developments. Particle of processes involving optical photons)

Outline Introduction n Optical processes n – Processes producing photons – Processes undergone by photons Optical properties in material property tables n Examples n Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 2

Optical photons n Physically optical photons should be covered by the electromagnetic category, Introduction but Basic functioning – optical photon wavelength is >> atomic spacing – treated as waves no smooth transition between optical and gamma particle classes n G 4 Optical. Photon: wave like nature of EM radiation n G 4 Optical. Photon <=|=> G 4 Gamma Position, angular & energy distributions Examples – New particle type – No smooth transition particle. Gun->Set. Particle. Definition(G 4 Optical. Photon: : Optical. Photon. Definition()); /gps/particle opticalphoton /gun/particle opticalphoton n Define a (spin) vector for the photon, added as data member to the G 4 Dynamic. Particle description class aphoton->Set. Polarization(ux, uy, uz); // unit vector!!! /gps/polarization ux uy uz /gun/polarization ux uy uz Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 3

Optical properties associated to G 4 Material n Optical properties can be specified as properties table in G 4 Material – reflectivity, transmission efficiency, dielectric constants, surface properties n Photon spectrum properties also defined in G 4 Material – scintillation yield, time structure (fast, slow components) n Properties are expressed as a function of the photon’s momentum n New: Some of the properties previously part of the optical processes are now part of the G 4 Material. Properties. Table const G 4 int NUMENTRIES = 32; G 4 double photmom[NUMENTRIES] = {2. 034*e. V, ……, 4. 136*e. V}; G 4 double rindex[NUMENTRIES] = {1. 3435, ……, 1. 3608}; G 4 double absorption[NUMENTRIES] = {344. 8*cm, ……, 1450. 0*cm}; G 4 Material. Properties. Table *MPT = new G 4 Material. Properties. Table(); MPT -> Add. Property(“RINDEX”, photmom, rindex, NUMENTRIES}; MPT -> Add. Property(“ABSLENGTH”, photmom, absorption, NUMENTRIES}; G 4 Nist. Manager* man = G 4 Nist. Manager: : Instance(); G 4 Material* water = man->Find. Or. Build. Material("G 4_WATER"); water -> Set. Material. Properties. Table(MPT); Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 4

Processes producing optical photons n Optical photons are produced by the following Geant 4 processes: – G 4 Cerenkov – G 4 Scintillation – G 4 Transition. Radiation n Classes located in processes/electromagnetic/xrays n Warning: these processes generate optical photons without energy conservation Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 5

Cerenkov Process n Cerenkov light occurs when a charged particle moves through a medium faster than the medium’s group velocity of light. n Photons are emitted on the surface of a cone, and as the particle slows down: the cone angle decreases the emitted photon frequency increases and their number decreases a) b) c) Cerenkov photons have inherent polarization perpendicular to the cone’s surface All these properties are described by the G 4 Cerenkov process n Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 6

G 4 Cerenkov Some implementation details n Cerenkov photon origins are distributed rectilinear over the step even in the presence of a magnetic field – Users must limit the max step size in order to accurately model the emission position n Cerenkov photons are generated only in media where the user has provided an index of refraction n An average number of photon is calculated for the wavelength interval in which the index of refraction is given – The actual number of emitted photons is then statistically sampled n New: Cerenkov photon number varies linearly with velocity (no longer uniformly distributed along the step) Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 7

G 4 Cerenkov User options n Suspend primary particle and track Cerenkov photons first – True: e. g. to avoid particle stack becoming too large with secondary photons – False: e. g. to avoid tracking all photons if the event is globally not interesting n Set the (max) average number of Cerenkov photons per step – The actual number generated in any given step will be slightly different because of the statistical nature of the process n G 4 Cerenkov 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 n example Expt. Physics. List: #include “G 4 Cerenkov. hh” G 4 Cerenkov* the. Ckov. Process = new G 4 Cerenkov(“Cerenkov”); the. Ckov. Process -> Set. Track. Secondaries. First(true); G 4 int Max. Num. Photons = 300; the. Ckov. Process->Set. Max. Num. Photons. Per. Step(Max. Num. Photons); the. Ckov. Process->Set. Max. Beta. Change. Per. Step(10. 0); Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 8

Scintillation process n n n Number of photons generated proportional to the energy lost during the step Emission spectrum sampled from one (or two) empirical spectra Isotropic emission Uniform along the track segment With random linear polarization Emission time spectra with one (or two) exponential decay time constants (fast/slow) Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 9

G 4 Scintillation Process implementation details n n n 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 Option: suspend primary particle and track scintillation photons first Example physics list: #include “G 4 Scintillation. hh” G 4 Scintillation* the. Scint. Process = new G 4 Scintillation(“Scintillation”); the. Scint. Process -> Set. Track. Secondaries. First(true); the. Scint. Process -> Set. Scintillation. Yield. Factor(0. 2); the. Scint. Process -> Set. Scintillation. Excitation. Ratio(1. 0); Note n n n The ‘Yield. Factor’ allows for different scintillation yields depending on the particle type In such case, separate scintillation processes must be attached to the various particles Same for the ratio between fast and slow components Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 10

G 4 Scintillation details: material properties #include “G 4 Material. hh // Liquid Xenon material G 4 Element* element. Xe = new G 4 Element(“Xenon”, ” Xe”, 54. , 131. 29*g/mole); G 4 Material* LXe = new G 4 Material (“LXe”, 3. 02*g/cm 3, 1, k. State. Liquid, 173. 15*kelvin, 1. 5*atmosphere); LXe -> Add. Element(element. Xe, 1); const G 4 int NUMENTRIES = 9; G 4 double LXe_PP[NUMENTRIES] = {6. 6*e. V, 6. 7*e. V, 6. 8*e. V, 6. 9*e. V, 7. 0*e. V, 7. 1*e. V, 7. 2*e. V, 7. 3*e. V, 7. 4*e. V}; G 4 double LXe_SCINT[NUMENTRIES] = {0. 000134, 0. 004432, 0. 053991, 0. 241971, 0. 398942, 0. 000134, 0. 004432, 0. 053991, 0. 241971}; G 4 double LXe_RIND[NUMENTRIES] = { 1. 57, 1. 57}; G 4 double LXe_ABSL[NUMENTRIES] = { 35. *cm, 35*cm }; G 4 Material. Properties. Table* LXe_MPT = new G 4 Material. Properties. Table(); LXe_MPT LXe_MPT -> -> Add. Property(“FASTCOMPONENT”, LXe_PP, LXe_SCINT, NUMENTRIES); Add. Property(“RINDEX”, LXe_PP, LXe_RIND, NUMENTRIES); Add. Property(“ABSLENGTH”, LXe_PP, LXe_ABSL, NUMENTRIES); Add. Const. Property (“SCINTILLATIONYIELD”, 100. /Me. V); Add. Const. Property(“RESOLUTIONSCALE”, 1. 0) Add. Const. Property(“FASTTIMECONSTANT”, 45. *ns); Add. Const. Property(“YIELDRATIO”, 1. 0); LXe -> Set. Material. Properties. Table (LXe_MPT); Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 11

Processes undergone by optical photons n Optical photons undergo: – – bulk absorption Rayleigh scattering wavelength shifting refraction and reflection at medium boundaries n Classes located in processes/optical n Geant 4 keeps track of polarization – but not overall phase no interference n Example N 06 at examples/novice/N 06 Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 12

G 4 Op. Absorption n Bulk absorption – uses photon attenuation length from material properties to get mean free path – photon is simply killed after a selected path length G 4 double Photon. Energy[n. Entries] = {6. 6*e. V, 6. 7*e. V, 6. 8*e. V, 6. 9*e. V, 7. 0*e. V, 7. 1*e. V, 7. 2*e. V, 7. 3*e. V, 7. 4*e. V}; G 4 double Abs. Length[n. Entries] = {0. 1*mm, 0. 2*mm, 0. 3*mm, 0. 4*cm, 1. 0*cm, 10. 0*cm, 1. 0*m, 10. 0*m}; MPT->Add. Property(“ABSLENGTH”, Photon. Energy, Abs. Length, NUMENTRIES}; Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 13

Rayleigh Scattering G 4 Op. Rayleigh n 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 n The diff. cross section is proportional to cos 2(a) where a is the angle between the initial and final photon polarization n 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: MPT -> Add. Property(“RAYLEIGH”, Photon. Energy, Scattering, NUMENTRIES}; Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 14

Wavelength shifting n Handled by G 4 Op. WLS – initial photon is killed, one with new wavelength is created – builds it own physics table for mean free path n 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 Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 15

Wavelength-shifting: example code #include “G 4 Material. hh const G 4 int n. Entries = 9; G 4 double Photon. Energy[n. Entries] = {6. 6*e. V, 6. 7*e. V, 6. 8*e. V, 6. 9*e. V, 7. 0*e. V, 7. 1*e. V, 7. 2*e. V, 7. 3*e. V, 7. 4*e. V}; G 4 double RIndex. Fiber[n. Entries] = {1. 6, 1. 6}; G 4 double Abs. Fiber[n. Entries] = {0. 1*mm, 0. 2*mm, 0. 3*mm, 0. 4*cm, 1. 0*cm, 10. 0*cm, 1. 0*m, 10. 0*m}; G 4 double Emission. Fiber[n. Entries] = {0. 0, 0. 1, 0. 5, 1. 0, 5. 0, 10. 0}; G 4 Material* WLSFiber; G 4 Material. Properties. Table* MPTFiber = new G 4 Material. Properties. Table(); MPTFiber->Add. Property(“RINDEX”, Photon. Energy, RIndex. Fiber, n. Entries); MPTFiber->Add. Property(“WLSABSLENGTH”, Photon. Energy, Abs. Fiber, n. Entries); MPTFiber->Add. Property(“WLSCOMPONENT”, Photon. Energy, Emission. Fiber, n. Entries); MPTFiber->Add. Const. Property (“WLSTIMECONSTANT”, 0. 5*ns); WLSFiber->Set. Material. Properties. Table (MPTFiber); Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 16

Boundary processes Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 17

Boundary interactions Optical photons as particles Geant 4 demands particle-like behavior for tracking: n thus, no “splitting” n event with both refraction and reflection must be simulated by at least two events n Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 18

Boundary interactions n Handled by G 4 Op. Boundary. Process n – dielectric-dielectric – dielectric-metal – refraction – Reflection n User must supply surface properties using G 4 Optical. Surface models Boundary properties n Surface properties: – polished – ground – front- or back-painted, . . . Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 19

G 4 Boundary. Process Implementation Details A ‘discrete process’, called at the end of every step n Never limits the step (done by the transportation) n Sets the ‘forced’ condition n Logic such that n – pre. Step. Point: is still in the old volume – post. Step. Point: is already in the new volume so information is available from both Boundary Step Pre-step point Post-step point Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 20

Surface Concept: G 4 Logical. Surface and G 4 Optical. Surface Split into two classes n Geometrical class: G 4 Logical. Surface (in the geometry category) holds – – pointers to the relevant physical or logical volumes pointer to a G 4 Optical. Surface These classes are stored in a table and can be retrieved by specifying: – an ordered pair of physical volumes touching at the surface [G 4 Logical. Border. Surface] • – or a logical volume entirely surrounded by this surface [G 4 Logical. Skin. Surface] • • n in principle allows for different properties depending on which direction the photon arrives 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: G 4 Optical. Surface (in the materials category) keeps information about the physical properties of the surface itself Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 21

G 4 Optical. Surface n Set the simulation model used by the boundary process: – GLISUR-Model: original G 3 model – UNIFIED-Model: adopted from DETECT (TRIUMF) enum G 4 Optical. Surface. Model {glisur, unified}; n Set the type of interface: enum G 4 Optical. Surface. Type { dielectric_metal, dielectric_dielectric}; n Set the surface finish: enum G 4 Optical. Surface. Finish { polished, // smooth perfectly polished surface polishedfrontpainted, // polished top-layer paint polishedbackpainted, // polished (back) paint/foil ground, // rough surface groundfrontpainted, // rough top-layer paint groundbackpainted // rough (back) paint/foil }; Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 22

Optical surface types n 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: total internal reflected b) Fresnel refracted c) Fresnel reflected a) Dielectric – Metal The photon cannot be transmitted. a) absorbed (detected), with probability estimated according to the table provided by the user b) reflected n Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 23

Surface effects n POLISHED: In the case where the surface between two bodies is perfectly polished, the normal used by the G 4 Boundary. Process is the normal to the surface defined by: – the daughter solid entered; or else – the solid being left behind n 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. n The UNIFIED model assumes that the probability of micro-facet normals that populates the annulus of solid angle sin(a)da will be proportional to a gaussian of Sigma. Alpha: the. Op. Surface -> Set. Sigma. Alpha(0. 1); [rad] n 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(a). the. Op. Surface -> Set. Polish(0. 0); n The ‘facet normal’ is accepted if the refracted wave is still inside the original volume. Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 24

Microfacets n The assumption is that a rough surface is a collection of ‘microfacets’ Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 25

n In cases (b) and (c), multiple interactions with the boundary are possible within the process itself and without the need for relocation by the G 4 Navigator. Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 26

n n 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. Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 27

The G 4 Optical. Surface also has a pointer to a G 4 Material. Properties. Table n In case the surface is painted, wrapped, or has a cladding, the table may include thin layer’s index of refraction n 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 G 4 Navigator – the thin layer does not have to be defined as a G 4 tracking volume Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 28

Example G 4 Logical. Volume * volume_log; G 4 VPhysical. Volume * volume 1; G 4 VPhysical. Volume * volume 2; // Surfaces G 4 Optical. Surface* Op. Water. Surface = new G 4 Optical. Surface(“Water. Surface”); Op. Water. Surface -> Set. Model(glisur); Op. Water. Surface -> Set. Type(dielectric_metal); Op. Water. Surface -> Set. Finish(polished); G 4 Logical. Border. Surface* Water. Surface = new G 4 Logical. Border. Surface(“Water. Surface”, volume 1, volume 2, Op. Water. Surface); G 4 Optical. Surface * Op. Air. Surface = new G 4 Optical. Surface(“Air. Surface”); Op. Air. Surface -> Set. Model(unified); Op. Air. Surface -> Set. Type(dielectric_dielectric); Op. Air. Surface -> Set. Finish(ground); G 4 Logical. Skin. Surface * Air. Surface = new G 4 Logical. Skin. Surface(“Air. Surface”, volume_log, Op. Air. Surface); Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 29

Example (2) G 4 Optical. Surface * Op. Water. Surface = new G 4 Optical. Surface(“Water. Surface”); Op. Water. Surface -> Set. Model(unified); Op. Water. Surface -> Set. Type(dielectric_dielectric); Op. Water. Surface -> Set. Finish(groundbackpainted); Const G 4 int NUM = 2; G 4 double pp[NUM] = {2. 038*e. V, 4. 144*e. V}; G 4 double specularlobe[NUM] = {0. 3, 0. 3}; G 4 double specularspike[NUM] = {0. 2, 0. 2}; G 4 double backscatter[NUM] = {0. 1, 0. 1}; G 4 double rindex[NUM] = {1. 35, 1. 40}; G 4 double reflectivity[NUM] = {0. 3, 0. 5}; G 4 double efficiency[NUM] = {0. 8, 1. 0}; G 4 Material. Properties. Table *SMPT = new G 4 Material. Properties. Table(); SMPT SMPT -> -> -> Add. Property(“RINDEX”, pp, rindex, NUM); Add. Property(“SPECULARLOBECONSTANT”, pp, specularlobe, NUM); Add. Property(“SPECULARSPIKECONSTANT”, pp, specularspike, NUM); Add. Property(“BACKSCATTERCONSTANT”, pp, backscatter, NUM); Add. Property(“REFLECTIVITY”, pp, reflectivity, NUM); Add. Property(“EFFICIENCY”, pp, efficiency, NUM); Op. Water. Surface -> Set. Material. Properties. Table(SMPT); Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 30

Logic in G 4 Op. Boundary. Process: Post. Step. Do. It n Make sure: – – the photon is at a boundary (Step. Status = f. Geom. Boundary) the last step taken is not a very short step (Step. Length>=k. Car. Tolerance/2) as it can happen upon reflection ELSE do nothing and RETURN n If the two media on either side are identical do nothing and RETURN n If the original medium had no G 4 Material. Properties. Table defined kill the photon and RETURN ELSE get the refractive index n Get the refractive index for the medium on the other side of the boundary, if there is one n See, if a G 4 Logical. Surface is defined between the two volumes if so get the G 4 Optical. Surface which contains physical surface parameters n Default to glisur model and polished surface n If the new medium had a refractive index, set the surface type to ‘dielectric-dielectric’ ELSEIF get the refractive index from the G 4 Optical. Surface ELSE kill the photon n Use (as far as it has the information) G 4 Optical. Surface to model the surface ELSE use Default Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 31

As a consequence 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 G 4 Navigator does upon reflection on a boundary 3) G 4 Boundary. Process with ‘surfaces’ is only possible for volumes that have been positioned by using placement rather than replica or touchables 1) Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 32

Examples n Some user applications n N 06 – examples/novice/N 06 n Liquid Xenon – examples/extended/optical/LXe Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 33

Sample of user applications Courtesy A. Etenko, I. Machulin - Kurchatov Institute Courtesy H. Araujo (Imperial College London & UK Dark Matter Collaboration) G. Santin, HARP Cerenkov, CERN Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 34

Example. N 06 n examples/novice/N 06 Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 35

Liquid Xenon extended example Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 36

Summary n Optical processes handle – the productions of photons by scintillation, Cerenkov and transition radiation and – the reflection, refraction, absorption, wavelength shifting and scattering of long-wavelength photons n 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 n Documentation http: //cern. ch/geant 4 User support Application Developers Guide Optical photon processes http: //cern. ch/geant 4 User support Physics reference manual Optical photons n Examples – examples/novice/N 06 – examples/extended/optical/LXe n Forum http: //cern. ch/geant 4 User support User forum Processes Involving Optical Photons Giovanni Santin - General Particle Source (GPS) - Ecole Geant 4 2008, Annecy 37