Presented by M G Pia (CERN, INFN)

Authors:

P Truscott & F Lei

(Defence Evaluation & Research Agency, UK)

C Ferguson & R Gurriaran (University of Southampton, UK)

P Nieminen & E Daly

(ESA /ESTEC)

J Apostolakis, S Giani, M G Pia (CERN, Switzerland)

L Urban (KFKI Hungary)

M Maire (LAPP, France)

Principal Sources of Radiation in theNear-Earth Environment

Solar Protons & Heavier Ions(up to 100 MeV/nuc)

X-rays, -rays

Trapped Particlesprotons (~1 MeV - 100’s MeV)electrons (~1 keV - several MeV)

Galactic Cosmic Rays(protons & heavier nuclei)

~1MeV/nuc - 100’s GeV/nuc

Extra-Solar System-rays, X-rays

The EnvironmentSpacecraft are required to operate in a severe, energetic radiation environment comprising cosmic-ray protons and heavier nuclei, trapped protons and electrons that form the Van Allen radiation belts, and solar particles emitted during flare events and coronal mass ejections. The interactions of these particles in spacecraft materials not only attenuates the radiation, but may also give rise to secondary particles (e.g. protons, neutrons, bremsstrahlung) that may have higher fluxes and/or greater effects than the primaries.

The EffectsThe radiation environment can give rise to a range of deleterious effects in spacecraft microelectronics, solar arrays, sensors and specialised (mission-specific) instruments such as -ray detectors:

• Total ionising dose

• Atomic displacement (bulk) damage

• Single event effects (SEE), in which ionisation by single particles can temporarily upset or destroy integrated circuits

• Induced radioactivity and enhanced background rates in sensors

Applications for Radiation Transport ModelsSpacecraft instrument design/operation for mitigation of effects: Radiation transport models may be used to predict the effects of the environment on the systems making up the spacecraft platform and payload, and to allow designers to develop and optimise hardening strategies. The planning of spacecraft operations must take into consideration variations in the environment during the mission (such as solar particle events) and radiation transport models help quantify the threat to instruments and the effectiveness of possible operational measures.

Trouble-shooting: Such tools are also valuable for trouble-shooting in-flight anomalies experienced in spacecraft that have already been launched, and for assessing possible mitigation strategies.

Optimise detector/sensor designs: Simulation of radiation transport is necessary for optimising the design of radiation detectors, such as those used for -ray or X-ray missions.

Data interpretation: The interpretation of data from such instruments may also be critically dependent upon model results, e.g. predictions of induced X-ray yields from asteroids/moons are required to derive composition from detected X-ray fluxes.

Spacecraft Missions and Radiation Effects

Space Science & Astrophysics

Manned space flight

Civil communications

• Specification of total dose, NIEL, SEE environment and required tolerance of service module and payload

• Scheduling spacecraft operations• Analysis of operational anomalies

• Specification of radiation environment for humans and shielding requirements

• Scheduling crew operations (EVAs)

Analysis of solar system bodies:Instrument design, operation and analysis of returned data

System operation (redistribution of services)

• Sensor design:• Detector optimisation /

background reduction• Instrument operation• Analysis of returned data

Geant4 Overview and Features (see CHEP 2000 Paper 140):– The result of a world-wide collaboration of ~100 scientists and computer

engineers from 40 institutes

– Latest release is Geant4.1.0 issued 8th Dec 99

– Monte Carlo simulation for nuclei, hadrons, leptons and bosons in 3D

– Object Oriented design (implemented in C++)

– Geant4 Adaptive GUI available

– Excellent facilities for visualisation (essential for geometry debugging)

– Comprehensive range of physical processes already implemented as well as planned

– “Fast simulation” mode - response of a volume may be parameterised based on empirical or simulation data

– Facilities for event biasing

– Geometry can be constructed from solid simple volumes or breps

• Defined by user-written C++

• STEP interpreter for geometry input from CAD tools

Applications of Geant4 Physical Processes

Hadron-nucleon or hadron-nuclear

Hadron-nucleon or hadron-nuclear

ParameterisedParameterised

Parton-string (>5GeV)Parton-string (>5GeV)

Kinetic (10MeV - 10GeV)Kinetic (10MeV - 10GeV)

QMD modelsQMD models

Pre-compound (2-100 MeV)Pre-compound (2-100 MeV)

Low-energy neutron (thermal - 20 MeV)

Low-energy neutron (thermal - 20 MeV)

Isotope productionIsotope production

Cosmic ray nuclei and secondaries

Cosmic ray nuclei and secondaries Trapped protons

and secondaries

Trapped protons and secondaries

Secondary neutrons, including atmospheric/planetary albedo neutrons

Secondary neutrons, including atmospheric/planetary albedo neutrons

Induced radioactive background calculations

Induced radioactive background calculations

Nuclearde-excitation

Nuclearde-excitation

Evaporation (A>16)Evaporation (A>16)

Fermi break-up (A16)Fermi break-up (A16)

Fission (A65)Fission (A65)

Photo-evaporation (ENSDF)Photo-evaporation (ENSDF)

Treatment for seondaries from cosmic ray nuclei and trapped protons, esp. important in calculation of single event effects (microdosimetry)

Treatment for seondaries from cosmic ray nuclei and trapped protons, esp. important in calculation of single event effects (microdosimetry)

Radioactive decay* (ENSDF)Radioactive decay* (ENSDF) Induced and natural radioactive backgrounds

Induced and natural radioactive backgrounds

Multi-fragmentationMulti-fragmentation

Applications of Geant4 Physical Processes

Electromagnetic*Electromagnetic*

IonisationIonisation

Multiple scatteringMultiple scattering

-ray production-ray production

BremsstrahlungBremsstrahlung

annihilationannihilation

Photo-electric effectPhoto-electric effect

Important for treatment of SEE (microdosimetry from nuclear recoil and evaporation prods)

Important for treatment of SEE (microdosimetry from nuclear recoil and evaporation prods)

Trapped electron effectsTrapped electron effects

Compton scatteringCompton scattering

Pair-productionPair-production

Atomic relaxationAtomic relaxation Induced and natural radioactive backgrounds

Induced and natural radioactive backgrounds

Rayleigh scatteringRayleigh scattering

L o w -E ne rg y E M

R a d ioa c tive D e cay

N e w P h ys ics

G e n era l S ou rce P a rtic le M o d u le

N e w F un c tio ns

G e an t4 K ern a l

C A D Fro n t-E nd T o o l

In te rfa ce T o o ls

S e c to r S h ie ld in g A n a lys is T o o l

G e n e ra l T o o ls

E S A /G ea n t4 P ro je ct

Space Specific GEANT4 ModulesClearly Geant4 offers a very comprehensive environment to specify a geometry, perform particle tracking, and model a wide range of physical interaction processes. Furthermore the object-oriented design allows relatively straightforward extension of the toolkit through class inheritance.

The particular requirements of spacecraft radiation effects studies have led the European Space Agency to sponsor the development of a number of space-specific components. These are now discussed.

MGA for Geant4 STEP InterfaceThe CAD STEP (Standard for Exchange of Product Data - ISO 10303) interface provides an efficient method of defining spacecraft system geometries for Geant4, especially since the use of CAD tools is widespread in the aerospace industry. In addition, a number of commercial CAD tools already have STEP interfaces.

However, the protocol (AP203) of STEP does not allow the association of materials information with each volume. The Materials and Geometry Association (MGA) tool is a Java-based utility to attribute material information and visualisation properties with volumes in a STEP file. This can be achieved manually by the user through the Graphical User Interface, or automatically if the CAD engineer uses pre-defind meta-data information in the PRODUCT records for each volume in the STEP file. The user can draw upon a database of standard spacecraft materials, or create her own materials by defining elemental and nuclide composition. Once this association is complete, both the STEP and MGA files are read by Geant4 to obtain a complete description of the geometry.

Evaluation of CAD ToolsCAD Product: Suitability for G4 based

ProEngineer (Parametric) yes

Euclid (Matra Datavision) yes (with translator)

Catia (IBM Dassault) yes

MicroStation (Bentley) translator released ?

AutoCAD (AutoDesk) yes (R14.01 +)

I-deas (SDRC) yes

MGA Data Entry WindowsVolume, material and Vis attribute

association

Creation of internal databases (materials and colours)

Materials specification

window

CAD Tool

Database of pre-defined space materials and pre-

defined colours.

MGA file:Materials definitioncolourassociation

Creation of internal databases (materials

and colours)

colours)

Materials definitions

Visualisation definitions

Association: VolumesMaterialsColours

of :

STEP file

MGA application

User defined volume names

Off the shelf CAD application

Low-Energy Electromagnetic InteractionsPreviously, the minimum energies for accurate -ray and electron transport in Geant3 and in EGS and ITS was 10 keV and 1 keV respectively. A key requirement identified by ESA for a general space radiation tool was that it treat X-ray fluorescence from the surfaces of asteroids and moons. This necessitates cut-offs of 250 eV. To achieve this, new processes (for low-energy Compton, Rayleigh, photo-electric effect, Bremsstrahlung, ionisation and fluorescence) have been introduced which utilise data parameterisations to evaluated data from Lawrence Livermore National Laboratory (EPDL97, EEDL and EADL).

It is planned to also extend the low-energy EM physics for positrons to below 1 keV, as well as treat Auger electron production.

The physics for ionisation caused by low-energy hadrons, ions has been extended using parameterisations to particle range and stopping power data from Ziegler and ICRU, permitting accurate tracking down to 1 keV. Accurate treatment of these physical processes is essential for simulating single event effects in microelectronics as a result of proton- or neutron-nuclear interactions. A similar extension is in progress for antiprotons.

Low-Energy Electromagnetic Interactions

Radioactive Decay ModuleLong-term (>1s) radioactive decay induced by spallation interactions can represent an important contributor to background levels in spaceborne -ray and X-ray instruments, as the ionisation events that result often occur outside the time-scales of any veto pulse. The Radioactive Decay Module (RDM) treats the nuclear de-excitation following prompt photo-evaporation by simulating the production of , -, +, and anti-, as well as the de-excitation -rays. The model can follow all the descendants of the decay chain, applying, if required, variance reduction schemes to bias the decays to occur at user-specified times of observation. The branching ratio and decay scheme data are based on the Evaluated Nuclear Structure Data File (ENSDF), and the existing Geant4 photo-evaporation model is used to treat prompt nuclear de-excitation following decay to an excited level in the daughter nucleus. (Atomic de-excitation following nuclear decay is treated by the Geant4 EM physics processes.)

The RDM has applications in the study of induced radioactive background in space-borne detectors and the determination of solar system body composition from radioactive - ray emission.

Sector Shielding Analysis Tool It is often possible to obtain a first-order estimate of the radiation dose received within a spacecraft as a function of location using shield distribution data for that location in conjunction with dose-versus-depth information. The Sector Shielding Analysis Tool can provide shield distribution data using particle tracking facilities in Geant4. The so-called “geantino” particle is used to determine the path-lengths between material boundaries for rays emanating from a user-defined point. The

user is able to define the limits of the solid angle analysed (the direction window) based on an arbitrary co-ordinate system, and then sub-divide divisions in this solid angle. Geantino rays can be sampled randomly within each sub-division so that the shielding can be assessed as a function of and . Analysis output can be provided as a function of material in units of g/cm2, cm or radiation lengths, or for overall thickness irrespective of material type.

General Source Particle ModuleThe space radiation environment is often quite complex in energy and angular distribution, and requires sophisticated sampling algorithms. The General Source Particle Module (GSPM) allows the user to define his source particle distribution (without the need for coding) in terms of the following:

• Spectrum : linear, exponential, power-law, black-body, or piece-wise linear (or logarithmic) fit to data

• Angular : unidirectional, isotropic, cosine-law, or arbitrary (user-defined)

• Spatial sampling : from simple 2D or 3D surfaces, such as discs, spheres, boxes, cylinders

The GSPM also provides the option of biasing the sampling distribution. This is advantageous, for example, for sampling the area of a spacecraft where greater sensitivity to radiation effects is expected (e.g. where radiation detectors are located) or increasing the number of high-energy particles simulated, since these may produce greater numbers of secondaries.

Summary Geant4 is a new-generation toolkit for Monte Carlo

particle simulation Unlike other codes Geant4 has been developed to

provide comprehensive particle simulation in a single tool, but due to its OO design, it permits easy extension of physics modelled …. if required

The Geant4 Toolkit has wide applications including not only HEP, but also space and medicine

Geant4 fulfils the functions required for space radiation effects studies and will be the basis of ESA’s next generation of spacecraft radiation shielding tools

Visit Our Web Sites:

ESA/DERA/UoS Spacecraft Radiation Shielding and Effects:http://www.estec.esa.nl/wmwww/WMA/research/Shielding_tools.html http://www.space.dera.gov.uk/space_env/geant_mn.html

Geant4 collaboration at CERN:http://wwwinfo.cern.ch/asd/geant4/geant4.html