Geant4 in the Space Environment: Tools and Applications G. Santin, R. Nartallo, P. Nieminen, H. Evans, F. Lei, P. R. Truscott, D. Heynderickx, B. Quaghebeur, C. S. Dyer and E. Daly Abstract-- Radiation shielding analysis is a crucial process in the spacecraft and space instrument development cycle. A number of dedicated tools have been developed in the last decades for the study of the effects of radiation on materials and instruments in space. We present here a set of detailed analyses based on Monte Carlo radiation transport simulations, and new tools for space applications, both based on the Geant4 simulation toolkit, developed as part of the European Space Agency (ESA) activities in the Geant4 collaboration. Finally we present some activities initiated by ESA for the creation of a Geant4 space users’ community. I.
INTRODUCTION
years the use of Monte Carlo (MC) simulations INhasrecent rapidly increased in the space domain, with applications ranging from instrument and detector response verification to radiation shielding optimization, component effects, support of scientific studies, and analysis of biological effects. Radiation shielding analysis is a crucial process in the spacecraft and space instrument development cycle. A number of dedicated tools have been developed in the last decades for the study of the effects of radiation on materials and instruments in space. We present here a set of applications and new tools for space based on the Geant4 simulation toolkit, developed as part of the European Space Agency (ESA) activities in the Geant4 collaboration. MC studies in space related applications have in many cases features and requirements specific to the simulation of spacecraft and space instruments. Some new activities have been triggered by ESA to gather and foster the Geant4 space users’ community.
Manuscript received October 29, 2003. G. Santin and H. Evans are with RHEA System SA, and currently based at the European Space Agency, Space Environment and Effects Section, ESTEC, Noordwijk 2200AAG, The Netherlands (e-mail:
[email protected]). R. Nartallo is with RHEA System SA, 2 avenue Einstein, B-1348 LouvainLa-Neuve, Belgium and was based at the European Space Agency, Space Environment and Effects Section. P. Nieminen and E. Daly are with the European Space Agency, Space Environment and Effects Section, ESTEC, Noordwijk 2200AG, The Netherlands. F. Lei, P. R. Truscott and C. S. Dyer are with QinetiQ, Farnborough, England.
II. THE ROLE OF GEANT4 IN THE SPACE DOMAIN Geant4 is a toolkit for the simulation of particle transport in matter [1]. While developed in the context of High Energy Physics (HEP) experiments, it is currently used also in nuclear physics, space applications, medical physics, astrophysics and radioprotection. Geant4 is at present the standard Monte Carlo code used by ESA for detailed radiation transport analyses. The description of the physics processes ranges from thermal energies (for neutrons) to PeV energies. It includes new process implementations (the hadronic cascade models [2][3] are a noticeable example) as well as the re-write in C++ of existing models, like HETC [4] or GHEISHA [5] in the hadronic sector, or part of the Penelope models [6] in the electromagnetic one. Evaluated databases are used for the cross sections wherever available. There are several examples of the application of the Geant4 hadronic processes in the space domain. It is worth mentioning that elastic and inelastic hadronic processes are of particular importance for the evaluation of radiation damage to components, or that secondary neutron production is a key issue to obtain reliable estimates of the dose expected for manned missions. The Low-Energy extension of the electromagnetic processes is interesting for future space missions, with examples in the study of the elemental composition of the Mercury surface through X-ray fluorescence, in the new space-borne X and gamma ray telescopes, with the possibility of polarized radiation analysis, or in the evaluation of risks to human exposure to space radiation due to nano-scale effects of energetic particles at the cellular and DNA molecule level. The Radioactive Decay Module (RDM) can be of high interest in the context of low background experiments to assess the detector sensitivity in complete simulations when realistic experimental set-ups are included in the environment model. With the increase of the overall mass of the spacecrafts, the importance of secondary particles has increased. Geant4 studies are expected to give more reliable estimates of radiation penetration after thick shields than tools based on a look-up table approach (such as SHIELDOSE [7]), which are usually faster than a complete Monte Carlo description but approximate.
Fig. 1. Geant4 model of the GAIA geometry. Detail of the Service and Payload modules with the thin thermal tent cover and the optical windows.
Fig. 2. Visualization of the GAIA simulation: 2.0 MeV proton tracks impinging on the CCD surface, entering through the thermal cover windows.
The General Particle Source (GPS) eases the allows the definition of the space environment primary particle sources with a large set of predefined angular, energy and vertex position distributions and the additional possibility to input user ones. The geometry can be defined either with C++ coding, or through dedicated graphic interfaces (like the Geant4 Geometry Editor, GGE) or with an XML description. Handles for interfaces to external packages are offered, like in the case of visualization, where a number of options are available (e.g. OpenGL, VRML, and other internal specific tools). The extensibility of the toolkit, the perspective of a long term development and support and the possible interface to external packages are other key features in the development of space engineering tools based on Geant4. III.
SPACE MISSION SIMULATION
A. GAIA and solar events in the L2 orbit Many future ESA mission studies see the L2 Lagrangian location as providing benefits in terms of stability, launch cost, thermal characteristics and communications. The GAIA mission, successor to ESA's Hipparcos satellite (1989-93), will
Fig. 3. JPL’91 Proton differential fluence energy spectrum for the 6 year GAIA mission. The “Bastille” solar event, which started on July 14th 2000, has also been used as an example of single solar event analysis. For the transient effect studies the CRÈME’96 models have been used instead.
Fig. 4. Detection of different particle species at the GAIA CCD surface. Secondary particle production is predominant at very high energies. At very low energies protons can still enter through the windows of the thermal cover.
Fig. 5. Same plot as Fig. 4, but for a different geometrical model, in which side panels protect the detector from the side and a thin (2 mm) glass covers the CCD surface. This shielding configuration reduces considerably the dose in the detector by stopping the lower part of the spectrum.
perform a multi-epoch survey of the central regions of galaxies of the sky with high spatial resolution and multi-color photometry from an orbit around the Sun, in the L2 point. The L2 location, outside the geomagnetic shielding at a distance of 1.5 million kilometers in the anti-Sun direction, is not affected by the trapped particle belts. Still, cosmic rays and solar event particles can reach the spacecraft and have an impact on the mission duration. A study of the effects of the radiation environment with a realistic spacecraft geometry description (in terms of shapes and materials) must be performed in order to evaluate the
ability of the particles of the environment to reach sensitive regions of the spacecraft, and the effects caused once they reach key elements like electronic circuits, detectors or optical devices. A complete description of the geometry model has been developed with accurate description of shielding materials and thickness. Fig. 1 shows a detail of the upper payload module, which hosts the main scientific detectors, a large assembly of CCD modules in the focal plane of the telescope. Radiation effects to the mirrors or to the large CCD arrays have to be fully evaluated and shielding optimized. Both prompt and cumulative effects have been considered [8]. Rates of protoninduced events on the CCD surface have been estimated during hard solar events. Detector degradation has been addressed with Total Ionizing and Non-Ionizing Dose analyses. An example of the input spectra used for the data analysis is given in Fig. 3: the JPL’91 statistical model [9] for prediction of high-energy solar proton fluences with a confidence level of 90% has been used to assess the detector shielding design. The energy distribution of the protons in the so-called “Bastille day” event (that started on July 14th, 2000), as recorded by the NOAA GOES detectors, has been used as an example of single solar event spectrum and intensity. Fig. 4 shows the detection of primary and secondary particles in the CCD. The passage of primary particles through the optical windows causes the non-zero low energy plateau. High fluxes of particles of a few MeV (whose simulation is shown in Fig. 2) can therefore enter the payload and stop in the thin active layer of the focal plane detector, causing considerable damage in terms of cumulative degradation. The direct exposure of the CCD could be prevented with additional side protections. Shielding modification evaluation is currently underway. While the thickness of the payload cover could be limited for spacecraft mass constraints, other options that foresee a glass cover on the CCD surface, certainly effective in terms of degradation reduction (the suppression of a large part of the spectrum is apparent in Fig. 5, and the total dose in the CCD is a factor 10 lower), might cause disturbances in the telescope optical performance.
Fig. 6. The XMM-Newton EPIC MOS focal plane consisting of 7 MOS type CCD detectors.
Fig. 7. XMM-Newton telescope geometry, showing the X-ray baffle, mirrors and the grating. Other external and internal baffles, filters and telescope enclosure were also simulated (not shown).
Fig. 8. Efficiency curves as a function of initial proton energy showing the proportion of simulated protons incident on the telescope aperture that reach the EPIC and RGS instruments at the focal plane of the XMM-Newton telescope.
B. Low energy protons on XMM An X-ray telescope focuses X-ray photons by low-angle scattering from concentric mirror “shells”. It has been found that protons of energies in the range of tens of keV to a few MeV can scatter at grazing angles through the mirror shells and reach the focal plane [10]. These protons will not suffer significant energy losses, as they are mostly “reflected” by interaction with the electron plasma cloud on the mirror surfaces [11][12]. These protons, because of their low energy, can deposit a high non-ionizing dose in unshielded CCD detectors leading to a degradation in their Charge Transfer Efficiency, as was experienced by the CCDs on the Chandra Xray observatory [13]. The full Chandra and XMM-Newton geometries were implemented (example shown in Fig. 7) in a complete particle transport simulation based on Geant4 [10]. The model was used extensively to characterize the effects of low-angle proton scattering on the two X-ray missions [10][12][14][15]. Tuning of simulation parameters and validation of the results obtained was done against experimental data [16]. An efficiency parameter was derived to characterize the propagation of scattered protons through the telescope optics
g/cm2) for a given point within the geometry. It produces distributions of shielding material and thickness as viewed from a given point within the configuration as a function of direction from that location. This approach is highly useful for calculating the absorbed radiation dose, and for finding optimal shielding geometries. Fig. 9 shows the application of SSAT to the shield study for the GAIA mission [8]. The analysis is performed for both the global shielding and the individual contribution from the single materials. In the top figure the apertures in the shielding due to the optical windows in the thermal cover are clearly visible, while in the bottom part the main mirrors and their support cause the peaks in the Silicon Carbide shielding distribution.
Fig. 9. Example of the application of the SSAT to the shielding study for the GAIA mission. On the top the total shield for all materials, on the bottom only for the Silicon Carbide payload elements.
[10]. Fig. 8 shows the efficiency with which protons of various energies reach the focal plane instruments on XMMNewton. The value of efficiency obtained for the on-axis CCD camera on Chandra was comparable to that obtained for the EPIC camera on XMM-Newton, which indicated that a similar fluence of damaging protons as experienced by Chandra’s detectors would reach the EPIC CCDs [10][14]. Protective measures were adopted during radiation belt passages, which prevented displacement damage of the EPIC detectors from occurring. IV. THE NEW SIMULATION TOOLS One of the aims of ESA involvement in the Geant4 collaboration is to develop a set of radiation effects analysis tools based on the Geant4 toolkit to replace/supplement tools currently employed in ESA projects and missions. A. The Sector Shielding Analysis Tool The Sector Shielding Analysis Tool (SSAT) [17] performs ray tracing from a user-defined point within a Geant4 geometry to determine shielding levels (i.e. the fraction of solid angle for which the shielding is within a defined interval) and shielding distribution (the mean shielding level as a function of look direction). To achieve this the tool utilizes the fictitious geantino particle, which undergoes no physical interactions, but flags boundary crossings along its straight trajectory. Knowledge of the positions of these boundary crossings together with the density of the material through which the particle has passed can be used to profile the shielding (in
B. The CAD Front-End Tool The availability of the STEP interface in Geant4 is particularly advantageous since the use of professional CAD tools is now commonplace in the aerospace industry. However, the protocol within ISO 10303 (AP203) does not allow assignment within the STEP file of Monte Carlo-related materials information with geometry bodies. The Materials and Geometry Association (MGA) tool is a Java-based utility that has been developed to obviate the need for the user to write Geant4 code to perform this function. MGA provides a graphical user-interface (GUI) that allows the user to draw upon material definitions from an existing database of common spacecraft materials, or to define new materials in terms of elemental or nuclear composition [18]. As well as physical properties, visualization attributes may also be assigned to the materials. The association between volumes in the STEP file can be performed manually through the MGA graphical environment, or automatically if the CAD engineer uses pre-defined meta-data information in the PRODUCT records of the STEP file. Once this association is complete, Geant4 can read both the MGA interface file and the STEP file to obtain a complete description of the geometry. C. Mulassis The Multi-Layered Shielding Simulation Software (MULASSIS) [19] is a MC simulation based tool for dose and particle fluence analysis associated with the use of radiation shields. Users can define the shielding and detector geometry as planar or spherical layers, with the material in each layer defined by its density and elemental/isotopic composition. Incident particles can be any Geant4 particles; these include protons, neutrons, electrons, gammas, alphas and light ions. There is a wide choice for their initial energy and angular distribution. In addition, radiation spectra produced by SPENVIS can be inputted when the tool is used within this system. For the physics description, the user can choose from a list of eight simulation modules each using a pre-selected and selfconsistent set of Geant4 physics models for electromagnetic (Standard or Low-Energy), hadronic and low-energy neutron
transport processes. Users can also apply material and particle dependent cuts to speed up the simulation. Users can carry out fluence, total ionizing dose and Pulse Height Spectrum (PHS) analysis for any layer in the geometry. Fluence can be tallied into energy distribution histograms as a function of particle type and particle angular direction. NonIonizing Energy Loss (NIEL) analysis can be performed for layers of silicon material only, as the required NIEL coefficients are not available for other material at the moment. The histograms are output in Comma Separated Values (CSV) format so they can be easily input into other analysis and plotting tools. 1) The MULASSIS integration into SPENVIS MULASSIS has been integrated into the ESA SPENVIS system [20], thus making it one of the tools SPENVIS users can choose for their radiation analysis. SPENVIS MULASSIS web-pages create a macro file using inputs from the user. MULASSIS and the Geant4 toolkit operate on a separate PC/Linux server which is http linked to the SPENVIS server. The macro file is passed to the MULASSIS server and the simulation starts. Upon completion the results are sent back to the SPENVIS server and made available to the user. The full MULASSIS functionality is available via the SPENVIS user interface. In addition the radiation environment as evaluated by other SPENVIS models can be used as inputs for the MULASSIS simulation, thus making it very easy to obtain the modified radiation spectra behind a specific shield defined by the user. V. THE GEANT4 SPACE USERS’ COMMUNITY MC studies in space related applications have in many cases features and requirements specific to the simulation of spacecraft and space instruments. ESA has triggered some activities to foster the Geant4 space users’ community. Dedicated events are being periodically organized with the aim of gathering the world experts working in the field and eliciting the requirements in various aspects, which include physics models, geometry modeling, visualization, or interfaces to external tools.
performances of the Geant4 physics and geometry description and the possibilities of interfaces to external packages, make Geant4 a valuable simulation toolkit for the description of the radiation transport in matter in the context of space studies. VII. REFERENCES [1] [2] [3] [4] [5] [6]
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VI. CONCLUSIONS We presented the use of the Geant4 simulation toolkit for space applications. Simulations of the radiation environments and effects in complete space missions have been shown, with the examples of the impact of solar event particles on GAIA and the low energy protons on the XMM CCD’s. We have shown the development of new Geant4-based tools for space radiation shielding and effects analysis. Within the context of ESA ongoing and future projects other tools are currently under development, like a Geant4-based tool for microscopic NIEL studies in electronic devices. The original features of the new tools and the results obtained with the full simulations, together with the good
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