Table of Contents ECSS Model Page
Background Information Geant4 tools
Geant4 source particles

Table of contents

  1. Overview
  2. Incident particles
  3. Energy spectrum
  4. Angular distribution
  5. References

Overview

The incident (source) particles, their energy spectrum and angular distribution can be specified in terms of the Geant4 General Particle Source (gps). These settings are used as an input by Multi-Layered Shielding Simulation (Mulassis), Geant4-based Microdosimetry Analysis Tool (GEMAT), Geant4 Radiation Analysis for Space (Gras) and Planetocosmics.

Incident particles

The first thing that the user must do is to specify the environment. The two options are, . For a user defined environment the following options for the energy spectrum are available: .

The energy spectrum selection for the mission based environment depends on which SPENVIS radiation model has run before. If the trapped radiation models or the Solar particle mission fluences models have run then the options are and 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 type of incident particles can be selected with the menu . Only when selecting the ion option, additional inputs are required. These consist of:

The first three quantities for the definition of the ion are integers, the fourth one is a real number.

Note that the type of incident particles is related to the environment selection and only electron and proton can be available when a mission based environment is chosen.

Energy spectrum

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.

Advanced users can make the simulation of the trapped particle or long-term solar particle spectra more efficient by adding the option for energy biasing. The large range of the environmental particle spectra can make it improbable that the higher energy/low flux particles of the spectrum will be simulated. To counter this and improve the efficiency of the simulation, energy biasing can be used, which increases the probalility of the low flux particles being generated. This is particularly useful when the spectrum is soft or the shielding is thick. Because of Bremstrahlung generation, the results for electrons can be misleading.

Depending on the choice of energy spectrum, different parameters have to be given to fully define the spectrum.

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: .

Mono-energetic distribution

For a mono-energetic energy distribution the only two values needed are the energy and the intensity. These default to 100 MeV and 1.0 particles (cm-2) (s-1).

Linear distribution

A linear distribution takes the form:
y = A E + B

The energy E is expressed in MeV, and the default values for A and B are 1.0 and 0.0 respectively. The minimum and the maximum energy range must also be specified (these values default to 0.0 MeV and 100.0 MeV).

Power law distribution

The power law distribution takes the form:
y = A E alpha

In this case, the exponent alpha has to be defined by the user (the default value is -1.0). The constant A has the same function as the source strength parameter defined above. In this case, the minimum and maximum energy to be considered have to be defined as well (the default values being 1.0 MeV and 100.0 MeV respectively).

Exponential distribution

The exponential distribution takes the form:
y = A e E/E0

The exponent E0 has to be defined (the default value is 1.0 MeV). As in the previous case, the value of the constant A has the same function as the source strength parameter defined above. Again, the minimum and maximum energy to consider in the spectrum have to be given (the default values being 0.0 MeV and 100.0 MeV respectively).

User-defined spectrum

In this case, a 'text area' is opened, where a user-defined spectrum can be entered. This can be done by typing into the area, or using 'copy and paste' from another application. The energies entered in the spectrum should be in ascending order. The minimum and maximum energy are defined by the first, respectively the last, entry.

Angular distribution

1D angular distribution

The angular distribution for the 1D problems (e.g. Mulassis) can be defined using . In the case of "omnidirectional" or "point source", the minimum and maximum zenith angle to consider can be changed by the user, and the units include cm-2. If the "parallel-beam" option is selected and the geometry is planar slab, the user can also change the incident angle from the default normal incident case.

3D angular distribution

For the 3D problems (e.g. Planetocosmics) one can specify the angular distribution using the following options: . For the "isotropic" and "cosine-law" distributions users can provide the half opening angle and maximum angle respectively.

References

  1. Geant4 General Particle Source (gps)

Last update: Fri, 20 Apr 2018