Gabriele Cosmo
153 lines
6.1 KiB
Plaintext
153 lines
6.1 KiB
Plaintext
-------------------------------------------------------------------
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=========================================================
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Geant4 - an Object-Oriented Toolkit for Simulation in HEP
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=========================================================
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ParticleGun
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-----------
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History:
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10-06-2010 : Makoto Asai - merge into one example
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13-05-2010 : Michel Maire - create as three examples
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This example demonstrates 5 ways of the usage of G4ParticleGun shooting
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primary particles in different cases. These are
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0) uniform particle direction in a given solid angle
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1) Generate several vertices and particles per event
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2) Show how to sample a tabulated function (eg. energy spectrum)
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3) Divergent beam in an arbitrary direction
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4) Shooting primaries in spherical coordinates with rotation matrix.
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These usages can be chosen by a UI command
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/gunExample/selectGunAction <actionID>
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where <actionID> corresponds to above cases.
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1- Geometry construction
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---------------------
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It is a simple box which represents an 'infinite' homogeneous medium.
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2- Physics list
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------------
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PhysicsList.cc defines only geantino and transportation process.
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3- Primary generator
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-----------------
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There are 5 concrete primary generator action classes
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(PrimaryGeneratorActionN, N=0,1,2,3,4) which can be used as an independent sample
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code.
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PrimaryGeneratorAction is the class which uses and switches between these
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5 concrete action classes. Each concrete generator action shoots geantinoes
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in a distribution decribed below.
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3.0- uniform particle direction in a given solid angle
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-------------------------------------------------
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spherical angles (alpha,psi) respective to z axis
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Histograms 5,6 show momentum direction in master frame.
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3.1- Generate several vertices and particles per event
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-------------------------------------------------
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particle 1 : a geantino uniformly randomized on a cylinder surface.
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particle 2 and 3 : symetric to particle 1.
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In addition, time_zero of each event is randomized.
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3.2- Show how to sample a tabulated function (energy spectrum)
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---------------------------------------------------------
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Energy is sampled from a tabulated function defined in InitFunction().
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The function is assumed positive, linear per segment, continuous.
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Two sampling methods are illustrated : RejectAccept() and InverseCumul()
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(see Particle Data book, Monte Carlo techniques).
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Histogram 1 shows generated energy spectrum.
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3.3- Divergent beam in an arbitrary direction with rotation matrix
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-------------------------------------------------------------
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A geantino uniformly randomized around a given direction (theta, phi).
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One wants to limit particle direction uniformly around this direction.
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First, one generates momentum direction in the master frame (eg. World).
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AlphaMax = opening angle around z axis.
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Then one rotates momentum in local frame, using rotateUz() function.
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RotateUz() transforms uz to newUz. It is composition of two simple rotations:
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theta around oy, then phi around oz (non commutative).
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http://proj-clhep.web.cern.ch/proj-clhep/manual/UserGuide/VectorDefs/node49.html
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Histograms 5,6 show momentum direction in local frame.
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3.4- Shooting primaries in spherical coordinates with rotation matrix
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----------------------------------------------------------------
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a geantino uniformly randomized within a spherical shell.
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a) Vertex position
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One wishes to shoot uniformly within a spherical shell.
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One works in spherical coordinates. One uses inverse cumulative method with
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analytical formulae.
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Histograms 2,3,4 demonstrate uniform distribution of vertex position.
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b) Momentum direction
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One wants to limit particle direction uniformly within (alphaMin, alphaMax).
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First, one generates momentum direction in the master frame (eg. World).
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Then, one rotates momentum in vertex_position frame, using rotateUz() function.
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RotateUz() transforms uz to ur. It is composition of two elementary rotations:
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theta around oy, then phi around oz (non commutative).
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http://proj-clhep.web.cern.ch/proj-clhep/manual/UserGuide/VectorDefs/node49.html
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Histograms 5,6 show momentum direction in vertex_position frame.
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4- Visualisation
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-------------
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Visualization Manager is set in the main().
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Initialisation of the drawing is done via the commands
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/vis/.. in the macro vis.mac. This macro is automatically read from the main
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in case of interactive running mode.
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5- How to start ?
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--------------
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- execute particleGun in 'batch' mode from macro files
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% ParticleGun run1.mac
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- execute particleGun2 in 'interactive mode' with visualization
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% particleGun
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....
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Idle> ---> type your commands. For instance:
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Idle> /gunExample/selectGunAction 1
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Idle> /run/beamOn 10
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....
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Idle> exit
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6- Histograms
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----------
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particleGun produces several 1D histograms which are saved as
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particleGun.root by default.
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1 : energy spectrum dN/dE = f(E)
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2 : vertex position: radial distr dN/dv = f(r)
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3 : vertex position: cos(theta)
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4 : vertex position: phi
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5 : particle direction in local frame: cos(alpha)
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6 : particle direction in local frame: psi
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Please note that histogram 1 will be filled only if you use
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PrimaryGeneratorAction2, histos 5,6 with PrimaryGeneratorAction0 and 3
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and 2 through 6 will be filled with PrimaryGeneratorAction4.
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The histograms are managed by the HistoManager class and its Messenger.
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The histos can be individually activated with the command :
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/analysis/h1/set id nbBins valMin valMax unit
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where unit is the desired unit for the histo (MeV or keV, deg or mrad, etc..)
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One can control the name of the histograms file with the command:
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/analysis/h1/setFileName name (default particleGun)
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It is possible to choose the format of the histogram file : root (default),
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xml, csv, by using namespace in HistoManager.hh
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It is also possible to print selected histograms on an ascii file:
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/analysis/h1/setAscii id
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All selected histos will be written on a file name.ascii (default gunExample)
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