Gabriele Cosmo
173 lines
7.0 KiB
Plaintext
173 lines
7.0 KiB
Plaintext
$Id: README 93258 2015-10-14 08:34:50Z gcosmo $
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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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fanoCavity
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----------
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This program computes the dose deposited in an ionization chamber by a
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monoenergetic photon beam.
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The geometry of the chamber satisfies the conditions of charged particle
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equilibrium. Hence, under idealized conditions, the ratio of the dose
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deposited over the beam energy fluence must be equal to the
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mass_energy_transfer coefficient of the wall material.
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E.Poon and al, Phys. Med. Biol. 50 (2005) 681
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I.Kawrakow, Med. Phys. 27-3 (2000) 499
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1- GEOMETRY
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The chamber is modelized as a cylinder with a cavity in it.
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6 parameters define the geometry :
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- the material of the wall of the chamber
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- the radius of the chamber and the thickness of the wall
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- the material of the cavity
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- the radius and the thickness of the cavity
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Wall and cavity must be made of the same material, but with different
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density
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All above parameters can be redifined via the UI commands built in
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DetectorMessenger class
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-----------------
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| wall |
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| ----- |
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| | | |
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| | <-+-----+--- cavity
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------> | | | |
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------> | | | |
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beam -------------------------------- cylinder axis
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------> | | | |
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------> | | | |
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| ----- |
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-----------------
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2- BEAM
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Monoenergetic incident photon beam is uniformly distribued, perpendicular
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to the flat end of the chamber. The beam radius can be controled with an
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UI command built in PrimaryGeneratorMessenger; the default is full wall
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chamber radius.
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Beam regeneration : after each Compton interaction, the scattered photon is
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reset to its initial state, energy and direction. Consequently, interaction
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sites are uniformly distribued within the wall material.
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This modification must be done in the ParticleChange of the final state
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of the Compton scattering interaction. Therefore, a specific model
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(MyKleinNishinaCompton) is assigned to the ComptonScattering process in
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PhysicsList. MyKleinNishinaCompton inherites from G4KleinNishinaCompton;
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only the function SampleSecondaries() is overwritten.
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3- PURPOSE OF THE PROGRAM
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The program computes the dose deposited in the cavity and the ratio
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Dose/Beam_energy_fluence. This ratio is compared to the mass_energy_transfer
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coefficient of the wall material.
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The mass_energy_transfer coefficient needs :
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- the photon total cross section, which is read from the PhysicsTables
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by G4EmCalculator (see EndOfRunAction).
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- the average kinetic energy of charged secondaries generated in the
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wall during the run.
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The program needs high statistic to reach precision on the computed dose.
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The UI command /run/printProgress allows to survey the convergence of
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the kineticEnergy and dose calculations.
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In addition, to increase the program efficiency, the secondary particles
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which have no chance to reach the cavity are immediately killed (see
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StackinAction). This feature can be switched off by an UI command (see
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StackingMessenger).
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The simplest way to study the effect of e- tracking parameters on dose
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deposition is to use the command /testem/stepMax.
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4- PHYSICS
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The physics lists contains the standard electromagnetic processes, with few
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modifications listed here.
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- Compton scattering : as explained above, the final state is modified in
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MyKleinNishinaCompton class.
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In order to make the program more efficient, one can increase the Compton
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cross section via the function SetCSFactor(factor) and its
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associated UI command. Default is factor=1000.
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- Bremsstrahlung : Fano conditions imply no energy transfer via
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bremsstrahlung radiation. Therefore this process is not registered in the
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physics list. However, it is always possible to include it.
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See PhysListEmStandard class.
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- Ionisation : In order to have same stopping power in wall and cavity, one
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must cancel the density correction term in the dedx formula. This is done in
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a specific MollerBhabha model (MyMollerBhabhaModel) which inherites from
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G4MollerBhabhaModel.
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To prevent explicit generation of delta-rays, the default production
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threshold (i.e. cut) is set to 10 km (CSDA condition).
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The finalRange of the step function is set to 10 um, which more on less
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correspond to a tracking cut in water of about 20 keV. See emOptions.
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Once again, the above parameters can be controled via UI commands.
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- Multiple scattering : is switched in single Coulomb scattering mode near
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boundaries. This is selected via EM options in PhysicsList, and can be
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controled with UI commands.
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- All PhysicsTables are built with 100 bins per decade.
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5- HISTOGRAMS
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fanoCavity has several predefined 1D histograms :
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1 : emission point of e+-
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2 : energy spectrum of e+-
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3 : theta distribution of e+-
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4 : emission point of e+- hitting cavity
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5 : energy spectrum of e+- when entering in cavity
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6 : theta distribution of e+- before enter in cavity
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7 : theta distribution of e+- at first step in cavity
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8 : track segment of e+- in cavity
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9 : step size of e+- in wall
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10 : step size of e+- in cavity
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11 : energy deposit in cavity per track
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The histograms are managed by G4AnalysisManager 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/setFileName name (default fanoCavity)
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It is possible to choose the format of the histogram file :
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root(default), xml, hbook. See 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 fanocavity)
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6- HOW TO START ?
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- execute fanoCavity in 'batch' mode from macro files
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% fanoCavity run01.mac
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- execute fanoCavity in 'interactive mode' with visualization
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% fanoCavity
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....
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Idle> type your commands
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....
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Idle> exit
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