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Geant4 - an Object-Oriented Toolkit for Simulation in HEP
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TestEm15
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How to compute and plot the final state of:
- Multiple Scattering
- Gamma Conversion
considered as an isolated processes, see PHYSICS.
For Multiple Scattering, the method is exposed below.
For Gamma Conversion, when G4BetheHeitler5DModel Model is used.
1- GEOMETRY DEFINITION
It is a single box representing a 'semi infinite' homogeneous medium.
Two parameters define the geometry:
- the material of the box,
- the (full) size of the box.
The default geometry (100 m of water) is constructed in
DetectorConstruction, but the above parameters can be changed
interactively via the commands defined in DetectorMessenger.
2- PHYSICS LIST
The physics list contains the standard electromagnetic processes.
In order not to introduce 'artificial' constraints on the step size,
there is no limitation from the maximum energy lost per step.
3- AN EVENT: THE PRIMARY GENERATOR
The primary kinematic consists of a single particle starting at the edge
of the box. The type of the particle and its energy are set in
PrimaryGeneratorAction (1 MeV electron), and can be changed via the G4
build-in commands of ParticleGun class (see the macros provided with
this example).
4- PHYSICS
All discrete processes are inactivated (see provided macros),
so that Multiple Scattering or Gamma Conversion is 'forced' to
determine the first step of the primary particle.
The step size and the final state are computed and plotted.
Then the event is immediately killed.
Multiple Scattering:
The result is compared with the 'input' data, i.e. with the cross
sections stored in the PhysicsTables and used by Geant4.
The stepMax command provides an additional control of the step size of
the multiple scattering.
5- HISTOGRAMS
The test contains 16 built-in 1D histograms, which are managed by
G4AnalysisManager 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, etc..)
(see the macros xxxx.mac).
1 Multiple Scattering. True step length
2 Multiple Scattering. Geom step length
3 Multiple Scattering. Ratio geomSl/trueSl
4 Multiple Scattering. Lateral displacement: radius
5 Multiple Scattering. Lateral displac: psi_space
6 Multiple Scattering. Angular distrib: theta_plane
7 Multiple Scattering. Phi-position angle
8 Multiple Scattering. Phi-direction angle
9 Multiple Scattering. Correlation: cos(phiPos-phiDir)
10 Gamma Conversion. Open Angle * Egamma
11 Gamma Conversion. Log10(P recoil)
12 Gamma Conversion. Phi P recoil angle
13 Gamma Conversion. Phi P plus angle
14 Gamma Conversion. 2 * cos(phiplus + phiminus) Asymmetry
15 Gamma Conversion. E plus / E gamma
16 Gamma Conversion. Phi of Gamma Polarization
The histograms are managed by the HistoManager 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 testem15)
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 testem15)
6- VISUALIZATION
The Visualization Manager is set in the main().
The initialization of the drawing is done via the commands
/vis/... in the macro vis.mac. To get visualization:
> /control/execute vis.mac
The detector has a default view which is a longitudinal view of the
box.
The tracks are drawn at the end of event, and erased at the end of run.
7- HOW TO START ?
execute TestEm15 in 'batch' mode from macro files:
% TestEm15 compt.mac
execute TestEm15 in 'interactive mode' with visualization:
% TestEm15
Idle> control/execute vis.mac
....
Idle> type your commands
....
Idle> exit
8 - MACROS
The examples of macros for Multiple Scattering:
electron.mac muon.mac proton.mac
The example of Gamma Conversion macro:
gamma.mac - gamma to e+ e-
9 - HISTOGRAMS for gamma conversion
10 # Open Angle (rad)* E gamma (MeV)
The most probable value of the e+ e- pair opening angle multiplied by the
photon energy is 1.6 rad*MeV and 338 rad*MeV in case mu+ mu- pair.
See: Olsen, Phys. Rev. 131 (1963) 406.
See also: Fig. 7 of arXiv:1802.08253 and Fig. 6 arXiv:1910.12501.
11 # Log10 ( recoil momentum)
The distribution of the recoil momentum is described by
Jost, Phys. Rev. 80 (1950) 189 (no form factor).
See also Fig. 2 of Astroparticle Physics 88 (2017) 60.
12 # Phi recoil
13 # Phi positron
For linearly polarized incident photons, the distributions should show
a sinusoidal shape with period 180°, for non polarized incident photons,
the distribution of azimuthal angles should be flat.
14 # Asymmetry 2 * cos(phi_+ + phi_-)
For a photon propagating along x, polarized along y,
the average value of ( 2.0 * cos(phi_+ + phi_-) ),
provides a measurement of the polarization asymmetry, A.
Eq. (12) of Nucl. Instrum. Meth. A 729 (2013) 765
The azimuthal angle of the event defined as the bisector angle
of the azimuthal angles of the positron and of the electron,
(phi_+ + phi_-)/2,
provides the optimal measurement of the asymmetry
Astroparticle Physics 88 (2017) 30.
For high-energy photons (E >> 20 MeV), the asymptotic expression for A
can be used for comparison.
Boldyshev, Yad. Fiz. 14 (1971) 1027, Sov.J.Nucl.Phys. 14 (1972) 576.
See also eq. (13) of arXiv:1802.08253
Example : A ~ 0.17 at 100 GeV.
15 # E plus / E gamma
x_+ = E plus / E gamma has a more-or-less flat spectrum that extends
almost from 0. to 1.
See Fig. 16 page 261 of "The Quantum Theory of Radiation", W. Heitler,
3rd edition, 1954.
16 # Phi of Gamma Polarization
The phi of polarization vector after transformation into reference system
defined by gamma direction (z) , gamma polarization (x).
10 - UI COMMANDS
There are two commands to control G4BetheHeitler5DModel:
/process/gconv/conversionType itype
/process/gconv/onIsolated bool
The command:
/process/gconv/conversionType
Allow to force conversion on nuclear or electron
The parameter values
0 - (default) both triplet and nuclear conversion in proportion triplet/nuclear 1/Z
1 - force nuclear conversion
2 - force triplet
The command:
/process/gconv/onIsolated
Allow simulate conversion on isolated particles without screening
The perimeter values:
false - (default) atomic electron screening
true - conversion on isolated particles