Table of Contents ECSS Model Page
Background Information Geant4 tools
Multi-layered shielding simulation software

Table of contents

  1. Overview of MULASSIS
  2. MULASSIS in the SPENVIS environment
  3. Geometry definition
  4. Material definition
  5. Source particles
  6. Physics models
  7. Cuts-in-range
  8. Normalisation factor
  9. Analysis parameters
    1. Fluence
    2. Non ionising dose
    3. Total ionising dose/energy deposition
    4. Pulse-height spectrum
  10. Results
  11. References

Overview of MULASSIS

MULASSIS allows the definition of a multi-layered, one-dimensional shield and incident particle source, and using the Geant4 toolkit simulates radiation transport through the geometry, treating electromagnetic and nuclear interactions. Upon completion of the simulation, the software will provide the following information as a function of geometry layer:

This version, which has been integrated into the SPENVIS web-site, allows the user to execute MULASSIS remotely without the need to install Geant4 and the MULASSIS code on his/her local computer. However, users preferring a local version of this tool can download the source code from the ESA European Space Software Repository[1] website.

The latest version of MULASSIS installed in SPENVIS is v1.23 compiled with geant4-09-05-patch-02.

MULASSIS in the SPENVIS environment

The SPENVIS interface to MULASSIS simplifies the process of defining run parameters via a number of input pages (see below) that a user can access from a table on the main page of the model. In addition, some information on the status and a short summary of the user input is also displayed.

Users can go to any particular input page and enter their settings. After they are done, they can hit the button and return to the main model page. Once they are satisfied with all their input they can use 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.

As the model uses a Monte-Carlo simulation-based code, execution times can be very long. In order to guarantee the consistency between the different models available in the SPENVIS system (e.g. particle spectrum vs. total ionising dose), the user project is 'blocked' while running the multi-layered shielding simulation, but navigation remains possible. The execution is limited to ten minutes of CPU-time on the simulation machine. If the MULASSIS run exceeds this limit, the simulation will be terminated and intermediate results returned to the user.

Geometry definition

The user can choose the geometry he/she wants to use with the selection menu .

The default geometry is a single planar slab geometry with 26 layers (the boundaries are equivalent to the default SHIELDOSE thicknesses, but the geometry as a whole isn't). The material for the default geometry is Aluminium.

When a user defined geometry is chosen, the page includes a table with the choice of a geometry, and the number of layers to use. The next lines of the table are created dynamically, depending on the number of layers. For each layer, the user can define the material to use, the thickness (with units), and the colour to use in the graphical representation. For the material, there are four defaults defined: Vacuum, Air, Aluminium and Silicon. Pressing the button in the header of the table opens the material definition page and allows the user to choose other materials or define new materials.

The next option is to get some graphical representation of the chosen geometry. This can be made in two formats, i.e. PostScript or VRML (available only if number of layers is less than ten), with or without 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.

Material definition

Users can employ the material definition tool to either specify their own material or make use of the predefined lists.

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 MULASSIS main macro.

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.

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.

More specifically in MULASSIS one can group the shield layers into different regions and apply different cuts to each region. 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.

The normalisation factor

In general, all the simulation results are divided by the number of the primary events. The normalisation factor for the SPENVIS implementation of MULASSIS 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).

Finally, 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.

Analysis parameters

Four different types of analysis are available:

The user can make only one analysis at a time. For more analyses new runs are needed.

Fluence analysis

The fluence analysis parameters allow the user to control the measurement of particle fluence spectra at the boundaries between the different layers. 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 in energy and angle: The units of fluence used in the MULASSIS output may be selected as particles/cm2-bin or particles/m2-bin according to the selection box: . The type of fluence measurement can be selected as :

Non ionizing dose (NID)

The NID can be calculated in units of based on the fluence results and NIEL coefficients from CERN [Vasilescu and Lindstroem], JPL [Dale et al., 1993, Holland] or JPL/NRL/NASA 2003 [Proton: Jun et al., Messenger et al.; Neutron: ASTM, Messenger et al., Griffin; Electron: Summers et al., Messenger]. For NIEL calculations involving the CERN, JPL, or JPL/NRL/NASA 2003 Silicon coefficients, the resulting NIEL is only calculated for silicon - indeed the JPL coefficients are only applicable to incident protons, where as neutron, electron and pion NIEL is included using the CERN/ROSE data. The JPL/NRL/NASA 2003 Gallium Arsenide and JPL/NRL/NASA 2003 Indium Phosphate options permit the user to calculate NIEL in these materials. It is the users responsibility to select the appropriate set of coefficients for the material and layer under investigation.

Energy deposition/total ionizing dose (TID)

Energy deposition can be calculated for each of the layers in units of . As with the particle fluence, the results are normalised to the fluence normalisation factor (the planar fluence of particles crossing the surface of the shield, calculated internally).

Pulse-height spectrum (PHS)

The energy depositions from each incident particle and its secondaries can be logged as a function of layer to determine the pulse-height spectra of energy depostion events. Such data may be used, for example, to predict the spectra in planar silicon detectors. Like the fluence spectra, the user can control the binning scheme for energy-deposition scale to be . The default binning scheme is linear from 0 MeV to 1 MeV over 100 bins. The pulse-height energy deposition spectra are normalised to the fluence normalisation factor, and are in units of events in each energy bin per cm2 of the layer.

Results

MULASSIS produces the files listed in the table below. A description of the format of the files can be brought up by clicking on their description in the table.

The report file spenvis_mlr.txt contains details of the inputs and outputs. The log file spenvis_mlp.txt records the output from MULASSIS to stdout and stderr. The output file spenvis_mlo.txt containing tabulated results (fluence, NID, dose or PHS) for the selected analysis. The graphics files spenvis_ml.wrl and spenvis_ml.eps show the 3D geometry or the shield cross section.

Output files generated by MULASSIS
File name Description
spenvis_mlr.txt Report file
spenvis_mlp.txt Log file
spenvis_mlo.txt Outputs for the selected analysis type
spenvis_mul.wrl VRML representation of the geometry
spenvis_mul.eps Cross section view of the geometry

To generate plots, select the plot type(s), options and graphics format, and click the button. The current page will be updated with the newly generated plot files.

References

  1. ESA European Space Software Repository/MULASSIS
  2. Geant4 website
  3. Geant4 Physics Reference Manual
  4. ROSE NIEL coefficients
  5. Non-Ionizing Energy Loss
  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. 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. Ferrari, A., ATLAS TDR 1997, and private communication 1997.
  9. Griffin, P. J. et al., SAND92-0094, Sandia Nat. Lab 1993, Private communication 1996.
  10. Holland, A., private communication.
  11. Huhtinen, M., and P. A. Aarnio, NIM A 335, 580, 1993, and private communication.
  12. Jun, I., 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.
  13. Konobeyev, A., J. Nucl. Mater., 186, 117, 1992.
  14. Messenger, S. R., NRL, Private communication, c.f. Mott differential scattering.
  15. Messenger, S. R., 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.
  16. Messenger, S. R., 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.
  17. Summers, G. P., 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.
  18. Vasilescu, A. 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