Table of Contents ECSS Model Page
Background Information Geant4 tools
Geant4 Radiation Analysis for Space (GRAS)

Table of contents

  1. Overview of GRAS
  2. GRAS in the SPENVIS environment
  3. Source particles
  4. Source geometry definition
  5. Analysis parameters
    1. Fluence
    2. Non ionising energy loss
    3. Total ionising dose/energy deposition
    4. Dose equivalent analysis
    5. Equivalent dose
    6. Path length analysis
    7. Linear energy transfer analysis
    8. Charging analysis
  6. GDML definition
  7. Cuts-in-range
  8. Physics models and production cut-in-range
  9. The normalisation factor
  10. Results
  11. References

Overview of GRAS

GRAS is a Geant4-based tool that provides a general space radiation analysis for 3D geometry models [Santin et al., 2005]. More specifically, GRAS allows the definition of a multi-volume 3D geometry (MULASSIS-like geometries can also be defined) and incident particle source. Then, using the Geant4 toolkit it simulates radiation transport through the geometry, treating electromagnetic and nuclear interactions.

The version that has been integrated into SPENVIS allows the user to execute GRAS remotely without the need to install Geant4 and the GRAS code on his/her local computer. However, users preferring a local version of this tool can download the source code from the ESA/GRAS website.

The latest version of GRAS installed in SPENVIS is v3.1 compiled with geant4-09-05-patch-02.

GRAS in the SPENVIS environment

The SPENVIS interface to GRAS simplifies the process of defining the run parameters. The user can access a number of input pages (see below) from a table on the main page of the model. On the same table, information about the status and a short summary for each input are also displayed.

Users can go to any particular input page and enter their settings. Note that some input is obligatory. After they have provided their data on a particular input page, they can press the button and return to the main model page. Once they are satisfied with all their input, they can press on the button to generate the macro file. Pressing the button will start the calculation and bring up the results page.

Advanced users have the option to input a number of fine-tuning parameters.

Since the model uses a Monte-Carlo simulation-based code, execution time can be very long. In order to guarantee consistency between the different models available in the SPENVIS system, the user project is 'blocked' while running the GRAS simulation. Nevertheless, navigation remains possible. The excution is limited to ten minutes of CPU-time on the simulation server. If the GRAS run exceeds this limit, the simulation will be terminated and intermediate results returned to the user.

There are two execution modes available for the SPENVIS GRAS implementation: . For the MULASSIS execution mode the user is referred to the MULASSIS help page since the input pages are the same. However, note that some of the input for the two models might differ (e.g. different analysis types available for MULASSIS and GRAS).

The following sections describe the user input for the GDML execution mode.

Source particles

Users can specify the incident particle, its energy spectrum and angular distribution using the “source particles” template that is common for all Geant4 tools in SPENVIS.

This information is recorded in a separated macro file (GPS macro file) that is executed inside the GRAS main macro.

Source geometry definition

The source can be any of the following types:

The user can locate the source anywhere inside the GDML geometry by selecting a particular GDML volume (see later) and providing the Cartesian coordinates with respect to the centre of that volume. For a point or a disk source, the user can also specify where the source is pointing to by selecting another volume and providing again the Cartesian coordinates with respect to the centre of that volume. Note that the two reference volumes can coincide. When the source is assumed to be a disk or a sphere, an additional input for the source radius is required. The Cartesian coordinates and the source radius can be defined in terms of [mm], [cm] or [m].

The default geometry source is a point source located inside the volume “world” at x=0, y=0 and z=100 [mm] pointing at the centre of “world”. For a mission based environment and a GDML execution mode, a spherical source is recommended. However, it is the responsibility of the user to position correctly the source with respect to the GDML geometry.

Finally, it is possible to produce a graphical representation of the chosen geometry. This can be generated in two formats, i.e. PostScript or VRML, with or without the visualisation of the particle tracks. Note that no particle tracks would be represented in the graphics file if more than hundred primary particles are requested for the simulation.

Analysis parameters

Eight different analysis types are available:

The user can select only one analysis type at a time.

In addition, a common analysis module is automatically inserted by GRAS (no user action is required) and its results are output at the end of the simulation.

For fluence and NIEL, users can select a number of volume interfaces for the analysis. Then, a count is registered only when the particle crosses the selected boundary in the direction specified by the user (i.e. the order of the volume names). For TID, dose equivalent and equivalent dose the user can specify the number and the name of the volumes where the particular analysis will be performed.

Fluence analysis

The fluence analysis parameters allow the user to control the measurement of particle fluence spectra at the boundaries between the different volumes. The user can select whether the fluence for the source particle, protons, neutrons, electrons, gamma-rays, charged-pions and/or muons (multiple selections are possible) are to be logged and histogrammed. The histogram binning scheme can be defined by the user by providing an energy binning scheme: . The default binning scheme is linear from 0 to 100 MeV comprising 100 bins.

The results are recorded as:

Non ionizing energy loss (NIEL)

The NIEL can be calculated in units of based on the fluence results and NIEL coefficients from ROSE/CERN [Vasilescu and Lindstroem, 2000], JPL/CREME86 (CREME86 JPL Si) [Dale et al., 1993, Holland] or JPL/NRL/NASA 2003 (Silicon, Gallium Arsenide or Indium Phosphide - Proton:[Jun et al., 2003], [Messenger et al., 2003]; Neutron: [ASTM, 2002], [Messenger et al., 2005] and [Griffin]; Electron: [Summers et al., 1993], [Messenger]). For NIEL calculations involving the ROSE/CERN or the JPL/CREME86 the resulting NIEL is only calculated for silicon. Note that the JPL/CREME86 coefficients are only applicable for incident protons [Jun et al., 2003], [Messenger et al, 2003], while the CERN/ROSE data [ROSE NIEL coefficients] can be also used when neutrons, electrons or pions are assumed to be the incident particles. It is the users responsibility to select the appropriate set of coefficients for the material and volume under investigation.

Energy deposition/total ionizing dose (TID)

The total (cumulative) energy deposition can be calculated for each of the selected volumes in units of . The TID analysis also calculates the energy deposited at each event in order to allow the user to get the event pulse height spectrum (PHS).

Dose equivalent

The dose equivalent analysis allows the computation of the total (cumulative) dose equivalent in the selected volumes. The available units are . The dose equivalent calculation takes into account the Relative Biological Effectiveness (RBE) of radiation as a function of particle type and energy using the Quality Factor (QF) [ICRP 60, 1990].

Equivalent dose analysis

The equivalent dose analysis allows the computation of the total (cumulative) equivalent dose in the selected volumes in units of . For the equivalent dose calculation the radiation weighting factor, wR, is used. The user can choose between the values adopted in ICRP 60 [ICRP 60, 1990] or the updated factors given in ICRP 92 [ICRP 92, 2003].

Path length analysis

The path length analysis module computes the total path length in the selected volume(s) i.e. the sum of all step lengths of all particles inside the volume(s). The available units are . In SEU analyses, the path length can be useful for the generation of step-length histograms for later convolution with LET spectra that can be obtained using the LET analysis module. In micro-dosimetric studies it can be used to define an average length of radiation from a given source direction for lineal energy spectra.

Linear energy transfer analysis

The Linear Energy Transfer (LET) analysis module computes the LET spectrum at user a selected volume boundary. The LET is obtained by computing the value of dE/dx for user specified particle type and energy in a given material. The output units are MeV/cm. In SEU analyses the LET spectrum can be used to obtain an estimate of the SEU rate (e.g. by integrating the LET spectrum above a given threshold).

Charging analysis

The charging analysis module computes the total charge balance passing through user defined boundaries. If a particle is passing through the surface in the direction specified by the user, the charge of the particle is added, otherwise it is subtracted. The output units are e.

Common analysis module

The common analysis module performs a basic analysis of the events by counting the number of secondary electrons, positrons and gammas generated and the total number of steps performed by all tracks in the event. The output can be used to monitor the Geant4 tracking and the consequence of the secondary particle production cuts in electromagnetic processes.

GDML definition

Users must define their GDML geometry when the GDML execution mode is selected. This can be done by making use of the “GDML definition tool” implemented in SPENVIS.

Cuts-in-range

Advanced users can define the different cuts-in-range to be used.

The general principles in Geant4 regarding secondary particle production cuts are the following:

  1. Each process has its intrinsic limit(s) to produce secondary particles.
  2. All particles produced (and accepted) will be tracked up to zero range.
  3. Production cuts are assigned to Regions.
A region is a collection of geometry volumes. There is always a default region covering the whole geometry, for which there is a suggested global cut-in-range for gamma, electron and positron productions. The user has the option of changing the global production cuts. It can be a single cut for gamma, electron and positron productions, or different cuts for each type of the three particles.

Volumes that require different cuts from the global ones shall be grouped into regions and each region can be given its own cuts. Again, region cuts-in-range can be defined as a single cut for gamma, electron and positron productions, or different cuts for each type of the three particles. In addition the user can group the GDML geometry volumes into different regions and apply different cuts to each region.

Note that the default is no region cuts-in-range and the default values for the global cuts-in-range length is 1 µm.

Physics models

Space radiation sources range from very low to very high energy and their interactions with the spacecraft sensitive devices and the shielding structures include both electromagnetic and hadronic processes. A number of physics models are available within Geant4 that give an almost complete coverage of the main interaction mechanisms for trapped, solar and cosmic radiation in the spacecraft materials.

More recently, a new Geant4 physics list QBBC has been created dedicated for space applications, radiation biology and radiation protection. It includes combinations of BIC, BIC-Ion, BERT, CHIPS, QGSP and FTFP models and has higher precision than the others for many hadron-ion and ion-ion interactions in a wide energy range [Ivantchenko et al., 2012].

Based on the user’s selection for the incident particle, SPENVIS automatically selects the appropriate physics scenario. For pure EM interactions (e.g. incident gamma or electron) the Geant4 Option 3 (emstandard_opt3) is used. This standard EM physics list is optimised for medical and space applications. Otherwise, the physics list QBBC is used to simulate additional hadronic interactions. Finally, note that for geantinos no physics scenario is required.

The normalisation factor

In general, GRAS offers different normalisation types of the simulation results. Also, by default all the simulation results are divided by the number of the primary events.

The normalisation factor for the SPENVIS implementation of GRAS is calculated using the following formula:

where n1 and n2 are calculated by the SPENVIS Geant4 “source particles tool” and recorded in the generated macro file (NORM_FACTOR_SPECTRUM and NORM_FACTOR_ANGULAR aliases in the GPS macro file). Note that the normalisation factor is calculated in terms of particles per cm -2.

When the GDML mode is selected the user can also specify the geometry of the source. For a disk-like or a spherical source the area of its surface has to be taken into consideration when calculating the normalisation factor. In the SPENVIS implementation this is computed automatically by GRAS using the following macro command: /gras/analysis/setSourceSurfaceType AUTO

For the 1D simulations (MULASSIS mode) and when the geometry refers to a spherical shell the normalisation factor requires an additional term in order to take into consideration the integration over the surface of the source sphere. In other words, one needs to divide the above formula by a factor of 4πR2.

Finally, GRAS can distinguish (internally) between normalising with respect to external flux or current by using the /gras/analysis/setSourceFluenceType macro command and selecting either FLUX or CURRENT respectively. However, the outputs of the Geant4 Monte-Carlo simulations are in general inherently normalised to unit incident current e.g. as in the case of MULASSIS. Therefore, for both GDML and MULASSIS modes the SPENVIS GRAS v3.1 results are normalised to a current going through the primary surface i.e. the CURRENT option is used.

Results

GRAS produces the files listed in the table below. An explanation about the format of the files can be brought up by clicking on their description in the table.

The log file spenvis_gras.g4log records the output from GRAS to stdout and stderr. The output file spenvis_gras.csv containing tabulated results (fluence, NIEL, TID, dose equivalent or equivalent dose) for the selected analysis. The graphics files spenvis_gras.wrl and spenvis_gras.eps show the 3D geometry or the shield cross section.

Output files generated by GRAS
File name Description
spenvis_gras31.g4log Log file
spenvis_gras31.g4mac Macro file
spenvis_gras31.csv Outputs for the selected analysis type
spenvis_gras31_aida.root Output root file for the selected analysis type
spenvis_gras31.wrl VRML representation of the geometry
spenvis_gras31.eps Cross section view of the geometry
spenvis_gras31_aida.ps Output preview of GRAS results (PS)

References

  1. ESA European Space Software Repository/GRAS
  2. Geant4
  3. Geant4 Physics Lists for Electromagnetic Interactions
  4. Geant4 Physics Reference Manual
  5. ROSE NIEL coefficients
  6. ASTM, Standard Practise for Characterizing Neutron Energy Fluence Spectra in Terms of an Equivalent Monoenergetic Neutron Fluence for Radiation Hardness Testing of Electronics, ASTM International Standard E722-94 (re-approved 2002), American Society for Testing Materials.
  7. C. Dale et al., Displacement Damage Effects in Mixed Particle Environments for Shielded Spacecraft CCDs, IEEE Trans. Nucl. Sc., Vol 40, No 6, 1628-1637, 1993.
  8. A. Ferrari, ATLAS TDR 1997, and private communication 1997.
  9. P. J. Griffin et al., SAND92-0094, Sandia Nat. Lab 1993, Private communication 1996.
  10. A. Holland, private communication.
  11. M. Huhtinen, and P. A. Aarnio, NIM A 335, 580, 1993, and private communication.
  12. ICRP 60, 1990 Recommendations of the International Commission of Radiological Protection, Annals of the ICRP, Vol 21, No 1-3, 1991.
  13. ICRP 92, Relative biological effectiveness (RBE), quality factor (Q) and radiation weighting factor (wR), Annals of the ICRP, Vol 33, No 4, 2003.
  14. A. V. Ivantchenko, V. N. Ivanchenko, J.-M. Q. Molina and S. L. Incerti, Geant4 hadronic physics for space radiation environment, Int. J. Radiat. Biol., 88(1-2), 171–175, 2012
  15. I. Jun, M. A. Xapsos, S. R. Messenger, E. A. Burke, R. J. Walters, G. P. Summers, and T. Jordan, Proton nonionizing energy loss (NIEL) for device Applications, IEEE Trans. Nucl. Sci., Vol. 50, 1924-1928, 2003.
  16. A. Konobeyev, J. Nucl. Mater., 186, 117, 1992.
  17. S. R. Messenger, NRL, Private communication, c.f. Mott differential scattering.
  18. S. R. Messenger, E. A. Burke, M. A. Xapsos, G. P. Summers, R. J. Walters, I. Jun, and T. M. Jordan, NIEL for heavy ions: An analytical approach, IEEE Trans. Nucl. Sci., Vol. 50, 1919, 2003.
  19. S. R. Messenger, E. A. Burke, J. Lorentzen, R. J. Walters, J. H. Warner, G. P. Summers, S. L. Murray, C. S. Murray, C. J. Crowley, N. A. Elkouh, The correlation of proton and neutron damage in photovoltaics, Proc. 31st IEEE Photovoltaic Specialists Conf., Lake Buena Vista, FL, Jan 3-7, 2005.
  20. G. Santin et al, GRAS: A general-purpose 3-D modular simulation tool for space environment effects analysis, IEEE Trans. Nucl. Sc., Vol 52, No 6, 2294-2299, 2005.
  21. G. P. Summers, E. A. Burke, P. Shapiro, S. R. Messenger, R. J. Walters, Damage correlations in semiconductors exposed to Gamma, Electron and Proton Radiations, IEEE Trans. Nucl. Sc., Vol. 40, No. 6, 1372, Dec. 1993.
  22. A. Vasilescu and G. Lindstroem, Notes on the fluence normalisation based on the NIEL scaling hypothesis, ROSE/TN/2000-01, 2000.

Last update: Thu, 16 Jun 2022