Table of Contents ECSS Model Page
Background Information Radiation sources and effects
Sector Shielding Analysis Tool (SSAT)

The Sector Shielding Analysis Tool (SSAT) performs ray tracing from a user defined point within the 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 utilises 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 for a given point within the geometry. Please consult the SSAT website for a full description of SSAT and downloads of binaries and source code.

On the first input page, the geometry file source is specified, together with the shielding and dose distribution parameters. The second input page contains the parameters for the ray tracing. When the input forms have been completed, a Geant4 macro is generated. Pressing the button will start the calculation and bring up the "Results" page.

Geometry and shielding parameters

Geometry definition

SSAT can accept an external geometry defined in Geometry Definition Markup Language (GDML) format. GDML provides a mechanism for defining detector geometries in XML and importing those geometry definitions into Geant4 geometries. GDML consists of an XML schema definition and a C++ object library that can read an XML file (compliant to the GDMLschema) and set up the geometry defined in Geant4. By default, SSAT uses the GDML file generated by the geometry definition tool.
Advanced users have the option to upload a GDML file.

Shielding distribution

The shield binning information can be defined in three ways: With the latter two options, the shielding units have to be specified.

Dose curve upload

The format of the uploaded dose curve (dose/depth curve that is used with the calculated shield distribution to compute the expected dose) should be an ASCII file with each line consisting of two values separated by comma. The first value corresponds to the depth and the units have to be consistent with the one requested for the histogram output (e.g. [g/cm2], [cm], etc.). The units for the dose can be in any user selected units (provided on the SPENVIS template e.g. [rad]) and the calculated dose will be in the same units.

Ray tracing parameters

Source position and angular window

The position of the geantino source is specified in Cartesian world coordinates. When a GDML file generated by the geometry definition tool is used, the geantino source location is set automatically to the detector location.

SSAT uses polar angles theta and phi in order to specify the shielding as a function of the look direction.

In order to provide maximum flexibility, advanced users are allowed to define the orientation of the polar coordinate system with respect to the geometry Cartesian system, by specifying vectors Vref,1 and Vref,2 which lie in plane P (see figure below) and where:
Based on this coordinate system (or the default coordinate system), the user is required to specify a direction window within which the shielding is to be sampled. This direction window is determined by the polar coordinate limits. The direction window is sub-divided into azimuthal and polar intervals. The intervals in azimuth are always linear, whilst for the polar coordinate, the user may apply the following binning intervals: Geantino directions are sampled randomly within each of the subdivisions of the direction window to determine the mean shielding level and standard deviation of the shielding within that interval (each individual geantino track also contributes to information about the total shielding level, i.e. to determine the fraction of the direction window for which the shielding lies within a given interval). The sampling distribution for geantinos within the subdivision may be isotropic or cosine law.

Propagation modes

The shielding analysis performed by SSAT involves projecting a series of geantino particles from the point of interest inside the geometry which then follow straight line paths until they exit the geometry. This analysis, referred to as the slant analysis (see figure below), is conditional to the particles in the real environment not being significantly affected by multiple scattering and therefore following nearly straight paths.
Where particles are more strongly affected by scattering processes, it is recommended in the ECSS documentation and the procedures followed by some major European spacecraft manufacturers that the path length is determined along the normal to the surface at which the particle crosses. These alternative approaches actually represent two extremes for particle propagation: the former is for rectilinear propagation and the latter more appropriate to propagation by diffusion.

In SSAT, three path length calculation modes have been implemented based on the way the geatinos are propagated through the geometry setup. Advanced users may select which is the appropriate one to use:

  1. Slant: original rectilinear propagation (default).
  2. Normal: normal propagation for steps above a certain size.
  3. Weighted: rectilinear propagation but path length is weighted by entrance and exit angles.
The user defined parameter in mode 2 represents the maximum path length (in units of g cm-2) for which slant propagation may be applied (related to the mean free path between multiple scattering events), so that the slant method can be used where any intervening shielding is very thin, and the normal method is applied where the shielding is more significant.

Visualization

In order to check the geometry definition, an option is available to run SSAT without performing the ray tracing and shield binning. In this mode, a number of checks is performed on the geometry. The output from these checks is written to the log file.

SSAT produces a VRML representation of the GDML geometry. The user has the option to include the ray traces in this visualization. This option is only available when Visualization plus calculation has been selected.

Macro file

When all inputs have been defined, a Geant4 macro is generated. This macro file can be used to run SSAT on SPENVIS, or it can be downloaded for further editing and running on a local copy of SSAT.

Results

SSAT produces the files listed in the table below. The log file spenvis_ssp.txt contains the input parameters and SSAT runtime information. The macro file spenvis_ssa.g4mac contains the macro commands for the SSAT run. The GDML file spenvis_ssa.gdml contains the GDML representation of the geometry. It is a copy of either the GDML file produced by the geometry definition tool, or of the GDML file uploaded by the user. The output file spenvis_sso.txt contains the 2-D and 1-D shielding distributions. The plot file spenvis_ssa.wrl contains a VRML representation of the geometry.

Output files generated by SSAT
File name Description
spenvis_ssp.txt Log file
spenvis_ssa.gdml GDML description of the geometry
spenvis_sso.txt 2-D and 1-D shielding distributions
spenvis_ssa.wrl VRML representation of the GDML geometry

Last update: Mon, 12 Mar 2018