PIXE simulation with Geant4 - eROSITA example
M. G. Pia, G. Weidenspointner, M. Augelli, L. Quintieri, P. Saracco, M. Sudhakar, A. Zoglauer,
PIXE simulation with Geant4
IEEE Trans. Nucl. Sci., vol. 56, no. 6, pp. 3614-3649, Dec. 2009
eROSITA is an astronomical X-ray full-sky survey mission on-board the Spectrum-X-Gamma space mission. The launch is planned for end of 2012.
The main scientific goals of eROSITA are a systematic detection and analysis of all obscured accreting Black Holes in nearby galaxies and of ten thousand galaxy clusters. Furthermore the nature of dark energy and dark matter will be investigated.
Geant4 PIXE software has been successfully applied to the optimization of the graded shielding of the eROSITA mission.
The purpose of the passive shielding is two-fold:
This background noise consists of both fluorescence lines and continuum background due to Bremsstrahlung photons and rays from cosmic-ray interactions.
Wafer including 4 eROSITA pnCCDs
The event timing accuracy of current imaging Si detectors for X-ray astronomy (photon energy range 0.1–15 keV) is limited by the signal integration time of these devices. For such detectors, an active anti-coincidence system cannot be used for background reduction because discarding complete readout frames correlated with an anti-coincidence signal would result in unacceptable dead time.
Detector triggers due to primary cosmic-ray particles can be discriminated in imaging detectors due to their high energy deposit and their pixel image pattern. However, interactions of primary cosmic-ray particles in the detector and surrounding passive material give rise to secondary X-rays and charged particles. These may in turn lead to detector triggers within the accepted energy range.
Such triggers contribute to the instrumental background noise, because they cannot be distinguished from valid events due to cosmic X-ray photons that were focused by the telescope mirror system onto the detector.
The production of secondary photons and particles by the ubiquitous cosmic radiation is inevitable, but graded Z shielding permits the shifting of the energy of secondaries from atomic deexcitation to low values.
In a graded Z shield cosmic-ray induced fluorescence X rays produced in an outer, higher Z layer of the shield are absorbed in an inner, lower Z layer. Subsequent atomic deexcitation, for instance following photoionization, in this inner layer gives rise to fluorescence photons and Auger electrons with energies that are lower than the energies of the deexcitation particles from the outer layer; in addition, there will be relatively more deexcitation electrons than X-rays. If the effective charge number Z of the innermost shield layer is sufficiently low, ionization results in the generation of mainly Auger electrons with energies below 1 keV, which can easily be stopped in a thin passivation layer on top of the detectors. Ionization can also create rare fluorescence X rays of similarly low energy.
A first set of graded Z shield designs was studied by Monte
Carlo simulation, using the prototype PIXE software together with Geant4
versions 9.1-patch 01.
The detector chip was placed inside a box-shaped shield Three shielding configurations were studied:
A model for the detector response, taking into account Fano statistics (and hence the energy resolution) and detector noise, was then applied to obtain a simulated data sets. These simulated data were further processed with algorithms that have been developed to eliminate charged particle tracks or invalid pixel image patterns and to select and reconstruct events that are most likely due to single photons; these procedures are identical to those applied to actual satellite data.
The resulting spectra, which represent the average background in a detector pixel, are shown below.
1) Cu shielding
Strong Cu K_alpha and K_beta lines, originating from PIXE, are visible
2) Cu-Al shielding
Cu lines are suppressed, but Al K_alpha line is visible
3) Cu-Al-B4C shielding
Al X-rays are absorbed; the innermost layer is not a significant source of fluorescence
Further details about the simulation for the optimisation of eROSITA shielding can be found in
M. G. Pia, G. Weidenspointner, M. Augelli, L. Quintieri, P. Saracco, M. Sudhakar,
PIXE simulation with Geant4, IEEE Trans. Nucl. Sci., vol. 56, no. 6, pp. 3614-3649, Dec. 2009
A simplified version of the eROSITA simulation is available as the Geant4 eRosita advanced example.
|The simulated set-up is defined in eRositaDetectorConstruction.
The Cu block is located at position x = y = z = 0 cm.
Its dimensions in x, y, and z are 0.5 cm * 0.5 cm * 3 cm.
The CCD, represented by a slab of Si, is positioned at x = z = 0 cm and y = 2 cm.
The CCD dimensions in x, y, z and are 4 cm * 450 mu_m * 4 cm.
The vertex and initial momenta of the protons are defined in eRositaPrimaryGeneratorAction.
The vertex is at x = 0 cm, y = 2.25 cm, and z = 4 cm.
The initial direction of the protons is given by the vector (0.0, -0.5, -1.0).
The initial kinetic energy of the protons is 150 MeV.
The K_alpha1, K_alpha2 and K_beta1 characteristic lines of Cu are clearly visibile in the histogram at 8.0071, 7.98467 and 8.86534 keV.
The peak at approximately 1.8 keV corresponds to fluorescence emission in the Si detector itself.
To compile the example:
>> cd eRosita
If the G4WORKDIR environment variable and has been defined, an executable named eRosita will be generated in $G4WORKDIR/bin/$G4SYSTEM.
To run the example without visualisation:
>> cd $G4WORKDIR/bin/$G4SYSTEM
Copy the eRosita/eRosita.in file to this directory.
The input file eRosita.in defines a simulation with 1000 primary protons of 100 MeV energy.
Run the simulation with: eRosita eRosita.in > eRosita.out
To run the example with visualisation:
>> cd $G4WORKDIR/bin/$G4SYSTEM
Copy eRosita/vis.mac to to this directory and run the simulation with: eRosita vis.mac
The macro file vis.mac utilizes the DAWN visualization driver to display the simulation of 100 protons with 100 MeV energy.
An ASCII output file TrackerPhotonEnergy.out is created; it lists the energy deposited by photons in the detector. However, this file may be empty in case the generated protons do not produce any fluorescence photons that reach the tracker.