This program computes the dose deposited in an ionization chamber by a monoenergetic photon beam. The geometry of the chamber satisfies the conditions of charged particle equilibrium. Hence, under idealized conditions, the ratio of the dose deposited over the beam energy fluence must be equal to the mass_energy_transfer coefficient of the wall material.

E.Poon and al, Phys. Med. Biol. 50 (2005) 681
I.Kawrakow, Med. Phys. 27-3 (2000) 499

GEOMETRY

The chamber is modelized as a cylinder with a cavity in it.

6 parameters define the geometry :

the material of the wall of the chamber
the radius of the chamber and the thickness of the wall
the material of the cavity
the radius and the thickness of the cavity

Wall and cavity must be made of the same material, but with different density

All above parameters can be redifined via the UI commands built in DetectorMessenger class

                    -----------------
                    |               |
                    | wall          |
                    |     -----     |
                    |     |   |     |
                    |     | <-+-----+--- cavity
         ------>    |     |   |     |
         ------>    |     |   |     |
   beam     -------------------------------- cylinder axis
         ------>    |     |   |     |
         ------>    |     |   |     |
                    |     |   |     |
                    |     |   |     |
                    |     -----     |
                    |               |
                    |               |
                    -----------------

BEAM

Monoenergetic incident photon beam is uniformly distribued, perpendicular to the flat end of the chamber. The beam radius can be controled with an UI command built in PrimaryGeneratorMessenger; the default is full wall chamber radius.

Beam regeneration : after each Compton interaction, the scattered photon is reset to its initial state, energy and direction. Consequently, interaction sites are uniformly distribued within the wall material.

This modification must be done in the ParticleChange of the final state of the Compton scattering interaction. Therefore, a specific model (MyKleinNishinaCompton) is assigned to the ComptonScattering process in PhysicsList. MyKleinNishinaCompton inherites from G4KleinNishinaCompton; only the function SampleSecondaries() is overwritten.

PURPOSE OF THE PROGRAM

The program computes the dose deposited in the cavity and the ratio Dose/Beam_energy_fluence. This ratio is compared to the mass_energy_transfer coefficient of the wall material.

The mass_energy_transfer coefficient needs :

the photon total cross section, which is read from the PhysicsTables by G4EmCalculator (see EndOfRunAction).
the average kinetic energy of charged secondaries generated in the wall during the run.

The program needs high statistic to reach precision on the computed dose. The UI command /testem/event/printModulo allows to survey the convergence of the kineticEnergy and dose calculations.

In addition, to increase the program efficiency, the secondary particles which have no chance to reach the cavity are immediately killed (see StackinAction). This feature can be switched off by an UI command (see StackingMessenger).

The simplest way to study the effect of e- tracking parameters on dose deposition is to use the command /testem/stepMax.

PHYSICS

The physics lists contains the standard electromagnetic processes, with few modifications listed here.

Compton scattering : as explained above, the final state is modified in MyKleinNishinaCompton class.

In order to make the program more efficient, one can increase the Compton cross section via the function SetCSFactor(factor) and its associated UI command. Default is factor=1000.
Bremsstrahlung : Fano conditions imply no energy transfer via bremsstrahlung radiation. Therefore this process is not registered in the physics list. However, it is always possible to include it. See PhysListEmStandard class.
Ionisation : In order to have same stopping power in wall and cavity, one must cancel the density correction term in the dedx formula. This is done in a specific MollerBhabha model (MyMollerBhabhaModel) which inherites from G4MollerBhabhaModel.

To prevent explicit generation of delta-rays, the default production threshold (i.e. cut) is set to 10 km (CSDA condition).

The finalRange of the step function is set to 10 um, which more on less correspond to a tracking cut in water of about 20 keV. See emOptions. Once again, the above parameters can be controled via UI commands.
Multiple scattering : is switched in single Coulomb scattering mode near boundaries. This is selected via EM options in PhysicsList, and can be controled with UI commands.
All PhysicsTables are built with 100 bins per decade.

HISTOGRAMS

fanoCavity has several predefined 1D histograms :

1 : emission point of e+-
2 : energy spectrum of e+-
3 : theta distribution of e+-
4 : emission point of e+- hitting cavity
5 : energy spectrum of e+- when entering in cavity
6 : theta distribution of e+- before enter in cavity
7 : theta distribution of e+- at first step in cavity
8 : track segment of e+- in cavity
9 : step size of e+- in wall
10 : step size of e+- in cavity
11 : energy deposit in cavity per track

The histograms are managed by G4AnalysisManager class and its messenger. The histos can be individually activated with the command :

/analysis/h1/set id nbBins  valMin valMax unit

where unit is the desired unit for the histo (MeV or keV, deg or mrad, etc..)

One can control the name of the histograms file with the command:

/analysis/setFileName  name  (default fanoCavity)

It is possible to choose the format of the histogram file : root (default), hdf5, xml, csv, by changing the default file type in HistoManager.cc

It is also possible to print selected histograms on an ascii file:

/analysis/h1/setAscii id

All selected histos will be written on a file name.ascii (default fanocavity)

HOW TO START ?

Execute fanoCavity in 'batch' mode from macro files
```
% fanoCavity   run01.mac
```

Execute fanoCavity in 'interactive mode' with visualization

% fanoCavity
....
Idle> type your commands
....
Idle> exit