--------------------------------------------------------------------------------
=========================================================
Geant4 - an Object-Oriented Toolkit for Simulation in HEP
=========================================================
Xray_TESdetector
------------------
P.Dondero (1), R.Stanzani (1)
Dec 2022
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. This example is a reduced mass model of the
Athena X-IFU instrument based on an early configuration which is no more
applicable for any evaluation. Simone Lotti provided the simplified mass
model and background derived from those used in [1].
--------------------------------------------------------------------------------
Xray_TESdetector is an example of the application of Geant4 in a space
environment. It represents an x-ray detector derived from the X-IFU, the
X-ray spectrometer designed and developed by the European Space Agency (ESA)
for use on the ATHENA telescope.
The detector is a Transition-edge sensor (TES) composed of 317 Bismuth pixels
arranged in a hexagonal shape and its setup includes different layers of
shielding, filters and support.
The main purpose of the simulation is the estimation of the particle radiation
background impacting on the detector. For execution time optimization purposes,
only particle steps respecting specific conditions (e.g. hit selected
volumes close to the detector) are stored on a .root file [2].
An example of ROOT-based analysis of the output file is included
("./analysis/analysis.C") and can be used to obtain basic plots and histograms.
Xray_TESdetector implements a physics list dedicated to space radiation interactions,
developed within the ESA AREMBES Project for the ATHENA mission, called Space
Physics List (SPL).
Technically, this example shows how to manage a complex geometry obtained
with advanced detector construction features (e.g., boolean operations,
parameterisation).
In addition, 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 DEFINITION
The geometry consists of a simplified version of the X-IFU detector and is composed of
the following:
- the TES array, the backscattering (BSC) and the
Anti-coincidence detector (ACD);
- the structural elements supporting the detector (e.g. the cage underneath
it);
- the thermal shieldings;
- the structural elements of the cryostat chamber;
- a hollow Aluminum sphere schematizing the satellite.
Detector parameters:
- Detector thickness: 3 um
- Number of pixels: 317
- Detector's shape: regular hexagon
- Hexagon's apothem: 8.593 mm
The default geometry is constructed in DetectorConstruction class.
Alternatively, a GDML file is provided (xray_TESdetector.gdml).
The position of each pixel is defined by a list of coordinates (x,y)
contained in "pixelpos.txt".
2 - PARTICLE SOURCE
The radiation field is composed of galactic cosmic rays (GCR) protons with a
flux estimated for the L1/L2 Lagrangian points, as described in [1].
The energies range from 10 MeV to 100 GeV, and the particles are isotropically
generated on the surface of a sphere surrounding the geometry and randomly
launched toward its interior. The detector is placed in the center of the
sphere and the sphere's radius is chosen to avoid intersections with geometry
elements.
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 volumes near the detector, where high precision in the scattering description
is needed, the use of Single Scattering (SS) model is reccomended, as shown in
the "run01.mac", through the SetEmRegion command.
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 near the
detector.
The default production cuts are selected for all volumes, i.e. 1mm.
4 - HOW TO RUN THE EXAMPLE
Compile code and execute the example in 'batch' mode from the macro file:
./XrayTESdetector run01.mac
to launch it with the DetectorConstruction, or:
./XrayTESdetector run02.mac
to launch it by using the provided GDML.
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".
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. parent ID
7. pixel number (from the TES array)
8. step energy deposit
9. step number
10. initial kinetic energy
11. kinetic energy
12. particle name
13. pre and post-step names
14. creator process name
Tuples are filled with the informations listed above in two cases:
- when a new particle is generated (both primaries and secondaries);
- when the particles reach the volumes next to the detector and the
detector itself.
6 - ANALYSIS
xray_TESdetector provides an analysis macro example (analysis.C) with several
predefined histograms:
- Average energy deposit per pixel (1D);
- Energy deposit on the detector (2D);
- Particle count per pixel (1D);
- Spectra of the primaries on the detector (1D);
- Total spectra on the detector (1D);
- distribution of the particles on the detector (1D).
The first three are used to qualitatively check how the interactions are
distributed on the detector pixels and what is the average deposit per pixel
and particle. The 2D histogram for the Energy deposit on the detector shows
the shape of the detector on the XY plane.
The spectrum histograms are used to observe the following:
- the initial energies of the particles (at launch or generation);
- the energy deposit on the detector;
- the energy of the step before the impact on the pixel.
Those information are the starting point to assess the background
composition and intensity on the detector, and thus optimize the
detector shielding and background rejection techniques.
Histograms are managed by the "analysis.C" file.
7 - VISUALISATION
The visualization manager is set via the G4VisExecutive class
in the main() function in xray_TESdetector.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] S. Lotti, S. Molendi, C. Macculi, V. Fioretti, L. Piro et al., "Review of
the Particle Background of the Athena X-IFU Instrument", The Astrophysical
Journal, 2021.
[2] BRUN, René, et al. "The ROOT Users Guide". CERN, http://root.cern, 2003.
Gabriele Cosmo