Gabriele Cosmo
173 lines
7.7 KiB
Plaintext
173 lines
7.7 KiB
Plaintext
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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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Xray_TESdetector
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------------------
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P.Dondero (1), R.Stanzani (1)
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Dec 2022
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1. Swhard S.r.l, Genoa (GE), Italy.
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Contacts: paolo.dondero@cern.ch, ronny.stanzani@cern.ch
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Acknowledgements: example developed within the ESA AREMBES Project,
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Contract n. 4000116655/16/NL/BW. This example is a reduced mass model of the
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Athena X-IFU instrument based on an early configuration which is no more
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applicable for any evaluation. Simone Lotti provided the simplified mass
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model and background derived from those used in [1].
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Xray_TESdetector is an example of the application of Geant4 in a space
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environment. It represents an x-ray detector derived from the X-IFU, the
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X-ray spectrometer designed and developed by the European Space Agency (ESA)
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for use on the ATHENA telescope.
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The detector is a Transition-edge sensor (TES) composed of 317 Bismuth pixels
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arranged in a hexagonal shape and its setup includes different layers of
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shielding, filters and support.
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The main purpose of the simulation is the estimation of the particle radiation
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background impacting on the detector. For execution time optimization purposes,
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only particle steps respecting specific conditions (e.g. hit selected
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volumes close to the detector) are stored on a .root file [2].
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An example of ROOT-based analysis of the output file is included
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("./analysis/analysis.C") and can be used to obtain basic plots and histograms.
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Xray_TESdetector implements a physics list dedicated to space radiation interactions,
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developed within the ESA AREMBES Project for the ATHENA mission, called Space
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Physics List (SPL).
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Technically, this example shows how to manage a complex geometry obtained
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with advanced detector construction features (e.g., boolean operations,
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parameterisation).
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In addition, the example shows a way to optimize the simulation's execution time
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and output size by selectively saving data based on specific combined conditions
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(e.g. position, eventID and process name).
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NOTE: in a multiple-run session, the last run always overrides the root file.
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1 - GEOMETRY DEFINITION
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The geometry consists of a simplified version of the X-IFU detector and is composed of
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the following:
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- the TES array, the backscattering (BSC) and the
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Anti-coincidence detector (ACD);
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- the structural elements supporting the detector (e.g. the cage underneath
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it);
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- the thermal shieldings;
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- the structural elements of the cryostat chamber;
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- a hollow Aluminum sphere schematizing the satellite.
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Detector parameters:
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- Detector thickness: 3 um
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- Number of pixels: 317
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- Detector's shape: regular hexagon
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- Hexagon's apothem: 8.593 mm
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The default geometry is constructed in DetectorConstruction class.
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Alternatively, a GDML file is provided (xray_TESdetector.gdml).
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The position of each pixel is defined by a list of coordinates (x,y)
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contained in "pixelpos.txt".
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2 - PARTICLE SOURCE
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The radiation field is composed of galactic cosmic rays (GCR) protons with a
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flux estimated for the L1/L2 Lagrangian points, as described in [1].
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The energies range from 10 MeV to 100 GeV, and the particles are isotropically
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generated on the surface of a sphere surrounding the geometry and randomly
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launched toward its interior. The detector is placed in the center of the
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sphere and the sphere's radius is chosen to avoid intersections with geometry
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elements.
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3 - PHYSICS LIST
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This example implements a dedicated physics list called "Space Physics List",
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developed within the ESA AREMBES Project. This physics list has been designed
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focusing on the ATHENA physics processes, but contains high precision
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models that can be used in a more general space application.
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In details, this physics list provides a custom electromagnetic part combined
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with the QBBC hadronic physics list.
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In volumes near the detector, where high precision in the scattering description
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is needed, the use of Single Scattering (SS) model is reccomended, as shown in
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the "run01.mac", through the SetEmRegion command.
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The use of SS only in selected regions allows the simulation to reduce CPU
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consumption in the majority of the volumes and be very accurate near the
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detector.
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The default production cuts are selected for all volumes, i.e. 1mm.
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4 - HOW TO RUN THE EXAMPLE
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Compile code and execute the example in 'batch' mode from the macro file:
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./XrayTESdetector run01.mac
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to launch it with the DetectorConstruction, or:
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./XrayTESdetector run02.mac
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to launch it by using the provided GDML.
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For this example, the multi-thread (MT) capability of Geant4 is enabled by
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default. To specify the desired number of threads, the user can use the
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command "/run/numberOfThreads" in "run01.mac".
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5 - STEPPING
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Within the "SteppingAction" class relevant information about the particle's
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state are stored in Tuples [2], defined in the "HistoManager" class.
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The tuples contain the following information:
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1. event ID
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2. volume name
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3. track ID
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4. coordinates (x,y,z)
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5. angles (theta, phi)
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6. parent ID
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7. pixel number (from the TES array)
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8. step energy deposit
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9. step number
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10. initial kinetic energy
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11. kinetic energy
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12. particle name
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13. pre and post-step names
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14. creator process name
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Tuples are filled with the informations listed above in two cases:
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- when a new particle is generated (both primaries and secondaries);
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- when the particles reach the volumes next to the detector and the
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detector itself.
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6 - ANALYSIS
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xray_TESdetector provides an analysis macro example (analysis.C) with several
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predefined histograms:
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- Average energy deposit per pixel (1D);
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- Energy deposit on the detector (2D);
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- Particle count per pixel (1D);
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- Spectra of the primaries on the detector (1D);
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- Total spectra on the detector (1D);
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- distribution of the particles on the detector (1D).
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The first three are used to qualitatively check how the interactions are
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distributed on the detector pixels and what is the average deposit per pixel
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and particle. The 2D histogram for the Energy deposit on the detector shows
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the shape of the detector on the XY plane.
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The spectrum histograms are used to observe the following:
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- the initial energies of the particles (at launch or generation);
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- the energy deposit on the detector;
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- the energy of the step before the impact on the pixel.
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Those information are the starting point to assess the background
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composition and intensity on the detector, and thus optimize the
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detector shielding and background rejection techniques.
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Histograms are managed by the "analysis.C" file.
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7 - VISUALISATION
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The visualization manager is set via the G4VisExecutive class
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in the main() function in xray_TESdetector.cc.
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The initialisation of the drawing is done via a set of /vis/ commands
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in the macro vis.mac. This macro is automatically read from
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the main function when the example is used in interactive running mode.
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References
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[1] S. Lotti, S. Molendi, C. Macculi, V. Fioretti, L. Piro et al., "Review of
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the Particle Background of the Athena X-IFU Instrument", The Astrophysical
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Journal, 2021.
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[2] BRUN, René, et al. "The ROOT Users Guide". CERN, http://root.cern, 2003.
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