Geant4 - an Object-Oriented Toolkit for Simulation in HEP
Xray_SiliconPoreOptics
P.Dondero (1), R.Stanzani (1)
Apr 2023
1. Swhard S.r.l, Genoa (GE), Italy.
Contacts: paolo.dondero@cern.ch, ronny.stanzani@cern.ch
Acknowledgements: example developed within the ESA AREMBES Project, Contract n.
4000116655/16/NL/BW. Valentina Fioretti provided the simplified mass model, as
described in [1].
Xray_SiliconPoreOptics is an example of the application of Geant4 in a space
environment.
The geometry used in this example represents a single reflective pore used to
simulate on a smaller scale the effect of the millions of pores forming the
mirror of the ATHENA Silicon Pore Optics (SPO), as described in [1].
The main purpose of the simulation is the estimation of the induced residual
background at the pore exit caused by proton scattering at grazing angles
(<1deg).
Reflection steps inside the pore and relevant information are saved on a .root
file for subsequent analysis [2]. For execution time optimization purposes, only
particle steps respecting specific conditions (e.g. reflection length and volume
name) are stored.
An example of ROOT-based analysis of the output file is included
("./analysis/analysis.C") and can be used to obtain basic data representations.
Xray_SiliconPoreOptics implements a physics list dedicated to space radiation
interactions, developed within the ESA AREMBES Project for the ATHENA mission,
called Space Physics List (SPL).
The example shows a way to optimize the simulation's execution time
and output size by selectively saving data based on specific combined conditions
(e.g. position, eventID and process name).
NOTE: in a multiple-run session, the last run always overrides the root file.
1 - GEOMETRY
The geometry is given in the GDML format, and consists of a single Silicon pore
aligned to the ideal optics symmetry axis of the SPO [1], i.e., the Z-axis of
the Geant4 reference system. The pore has the following parameters:
- length: ~203.0 mm
- pore entrance size: ~0.83x0.61 mm
- pore thickness: 0.17 mm
Three volumes (DummyEntrance, DummyExit and DummySphere) are used to save the
state of the particles as they pass.
2 - INPUT FLUX
100keV protons are emitted with a Cosine-law distribution from a planar surface
(same dimensions of the pore) at 1mm above the entrance, within a cone of +-1 deg
aperture, as described in [1].
3 - PHYSICS LIST
This example implements a dedicated physics list called "Space Physics List",
developed within the ESA AREMBES Project. This physics list has been designed
focusing on the ATHENA physics processes, but contains high precision
models that can be used in a more general space application.
In details, this physics list provides a custom electromagnetic part combined
with the QBBC hadronic physics list.
In addition, the G4EmStandardSS Physics List is used to simulate the single
scattering inside the pore, as it is associated to a specific region
from the macro file.
In general, the use of SS only in selected regions allows the simulation to
reduce CPU consumption in the majority of the volumes and be very accurate in
the desired ones.
The default production cuts are selected for all volumes, i.e. 1mm.
4 - HOW TO RUN THE EXAMPLE
Compile code and execute Xray_SiliconPoreOptics in 'batch' mode from the macro
file:
./XraySiliconPoreOptics run01.mac
For this example, the multi-thread (MT) capability of Geant4 is enabled by
default.
To specify the desired number of threads, the user can use the command
"/run/numberOfThreads" in "run01.mac". To show the output from a single thread
in the terminal, the user can use the "/control/cout/ignoreThreadsExcept
{THREADNUM}" command.
5 - STEPPING
Within the "SteppingAction" class relevant information about the particle's
state are stored in Tuples [2], defined in the "HistoManager" class.
The tuples contain the following information:
1. event ID
2. volume name
3. track ID
4. coordinates (x,y,z)
5. angles (theta, phi)
6. process name
7. parent ID
8. the number of internal reflections whenever the particle reaches one of the
dummy volumes defined above.
6 - ANALYSIS
Xray_SiliconPoreOptics provides an analysis macro example (analysis.C) to
visualize data in the following representations:
- a histogram for the normalized efficiency for Theta and Phi;
- a pie chart for the number of reflections inside the pore.
The normalized efficiency serves to observe the angular distribution of the
exiting protons, normalized over the total entering particles. A proton is
selected if it enters the first volume (pore entrance), exits from the second
empty volume (pore exit) and enters the sphere at the detector side (the
hemisphere below the pore). No pore interaction is required.
The pie chart reports the number of reflections with the highest probability.
7 - VISUALISATION
The visualisation manager is set via the G4VisExecutive class in the main()
function in xray_SiliconPoreOptics.cc.
The initialisation of the drawing is done via a set of /vis/ commands in the
macro vis.mac. This macro is automatically read from the main function when
the example is used in interactive running mode.
References
[1] Fioretti V et al. "The Geant4 mass model of the ATHENA Silicon Pore Optics
and its effect on soft proton scattering", Space Telescopes and Instrumentation
2018: Ultraviolet to Gamma Ray. Vol. 10699. SPIE, 2018.
[2] BRUN, René, et al. "The ROOT Users Guide". CERN, root.cern.ch">http://root.cern.ch, 2003.
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SEE ALSO: README.txt
