Advanced Example Xray_SiliconPoreOptics


xray_pores example image
Figure 1: Geometry of the simulation set-up.

Responsible Geant4 Collaborator: Paolo Dondero, Swhard S.r.l, Genova, Italy.

Contributors: Ronny Stanzani (Swhard S.r.l, Genova, Italy)

Short description

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 primary purpose of the simulation is to estimate the induced residual background at the pore exit caused by proton scattering at grazing angles (< 1 deg). Reflection steps inside the pore and relevant information are saved in 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 how 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).

Geometrical set-up

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 through the geometry.

Primary particles and physics list

100 keV protons are emitted with a cosine-law distribution from a planar surface (exact pore dimensions) at 1 mm above the entrance, within a cone of 1 deg aperture, as described in [1]. This example implements a dedicated physics list called Space Physics List, developed within the ESA AREMBES Project. This physics list has been designed to model the ATHENA physics processes but contains high-precision models that can be used in other space applications. This physics list provides a custom electromagnetic component combined with the QBBC hadronic physics list. In addition, the G4EmStandardSS Physics List is used to simulate the single scattering inside the pore and it is associated with a specific region in the macro file, input to the simulation. In general, using SS only in selected regions allows the simulation to reduce CPU consumption in most volumes and be very accurate in the desired ones. The default production cuts are selected for all volumes, i.e., 1 mm.

Simulation output

Xray_SiliconPoreOptics provides a macro analysis example (analysis.C) to plot data in the following representations:

  • A histogram for the normalized efficiency for Theta and Phi and
  • a pie chart for the number of reflections inside the pore.

The Normalized efficiency is calculated with the angular distribution of the exiting protons, divided by the total number of 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.


[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,, 2003.


Example developed within the ESA AREMBES Project, Contract n. 4000116655/16/NL/BW.

Last updated: 26/05/2023 by S. Guatelli