More specifically, this tool allows the user to define a multi-layered, one-dimensional shield, incident particle source and physics scenario. Then, it uses MULASSIS[1] to simulate radiation transport through the geometry, treating electromagnetic and nuclear interactions, in order to calculate the flux/fluence behind the shielding. MULASSIS is executed remotely without the need to install Geant4[2] and the MULASSIS code on a local computer.
In turn, the resulting flux/fluence is used in combination with the appropriate conversion coefficients[4] and provides the following information:
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.
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.
Note that the following inputs are used for generating the MULASSIS macro file.
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 planar slabspherical 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.
Users can employ the material definition tool to either specify their own material or make use of the predefined lists.
The energy spectrum selection depends on which SPENVIS radiation model has run before. If the trapped radiation models or the solar particle mission fluence models have run then the options are Trapped proton, Trapped electron or Solar proton respectively.
Next, the user then to give the number of incident particles he wants to simulate in the Monte-Carlo run. As the total time for the run is limited, this number should be chosen as small as possible but large enough to provide statistically meaningful results. As a guideline, users should first make a run with a limited number of incident particles. When the results seem to make sense, a new run with more particles can be made to improve the statistics.
Warning: The particle track visualisation will be disabled when the number of particle is greater than 100!
The next part defines various parameters related to the energy and angular spectrum of the incident particles. The different options for the energy spectrum depend on the selection of the environment and the energy spectrum in the previous section. Note that the definition of the energy spectrum is not available when geantino is selected as the source particle.
It is assumed that all of the particle spectra are "omnidirectional", or have been integrated over 4PI, i.e. there are no units for sr-1. The particle spectra can be either flux or fluence spectra. For this reason, the unit (s-1) is placed between brackets. The units for other terms depend on the angular distribution (see below).
As the spectra obtained from the trapped particle, the solar proton models, and the user-defined spectrum consist of pointwise data, these can be interpolated for other energies using several interpolation methods. The methods available are: linearpower-law exponentialcubic spline.
More specifically in MULASSIS, advanced users can group the shield layers into different regions and apply different cuts to each region. It can be a single cut for gamma, electron and positron productions, or different cuts for each type of the three particles.
The energy binning scheme has been defined as a user-defined binning scheme consisting of fifty seven bins from 0 to 1000 GeV. The edges of each bin have been chosen in such a way that the binning scheme covers the same energy range that was used for calculating the conversion coefficients.
For the angle binning scheme the default option is used i.e. linear binning scheme from 0 to 180 degrees comprising of two bins (to monitor forward propagation and backward propagation at the boundary). Note that for the purpose of this application we are interested only on the forward propagation.
The units of fluence used to generate the MULASSIS output is selected as particles/cm2 per bin. Finally, the fluence/flux is assumed to be omni-directional.
The user can make only one analysis at a time. For more analyses new runs are needed.
Finally, it is the responsibility of the user to ensure the consistency between the definition of the radiation source, the shielding geometry and the analysis parameters (i.e. appropriate conversion coefficient tables).
The isotropic irradiation scenario requires the use of a spherical geometry. Note that the spherical geometry MUST be used only in combination with isotropic irradiation (an error is generated otherwise). In addition, the last layer of this spherical geometry should be made of vacuum and its thickness should correspond to the radius of the cavity.
This is not a typical space radiation environment scenario. However, the symmetry of the ICRU sphere allows one to calculate the effect of isotropic irradiation in terms of ambient dose equivalent[5], [6]. For space radiation environment applications, one could use this option when the distance between the shielding and the target is sufficient.
The report file spenvis_smr.txt contains details of the inputs and outputs. The output file spenvis_smo.txt containing tabulated results (effective dose or ambient dose equivalent) for the selected analysis. The MULASSIS output file spenvis_smlo.txt containing tabulated results for fluence/flux behind the shielding. The graphics files spenvis_ml.wrl and spenvis_ml.eps show the 3D geometry or the shield cross section.
spenvis_smr.txt
spenvis_smo.txt
spenvis_smlo.txt
spenvis_ml.wrl
spenvis_ml.eps
spenvis_smr.html
spenvis_mul.g4mac
spenvis_mlo.txt
spenvis_mul.wrl
spenvis_mul.eps
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.