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 recorded in a separated macro file (GPS macro file) and subsequently used as an input for Multi-Layered Shielding Simulation (MULASSIS), Geant4-based Microdosimetry Analysis Tool (GEMAT) and Geant4 Radiation Analysis for Space (GRAS) in the form of a separated Geant4 macro file.

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 proton and electron fluxes, the Solar particle mission fluences or the Galactic cosmic ray (GCR) fluxes 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. For the GCRs option, the user inputs for ions are reduced to the atomic number and the isotope of interest.

Note that the type of incident particles is related to the environment selection. Therefore, when a mission based environment is chosen only electron (trapped particles), proton (trapped particles, long-term solar particles) or ion (GCR particles) are available.

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, long-term solar particle or GCR 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.

Note that it is assumed that all of the particle spectra are omnidirectional, or have been integrated over 4π, 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, long-term solar particles and GCRs models, as well as 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 (i.e energy values and the corresponding differential flux/fluences). 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.

SPENVIS mission average spectrum

It has been already mentioned that outputs from the SPENVIS trapped particle, long-term solar particles or GCRs models can be converted into GPS inputs for specifying the energy spectrum of the incident particle (mission based environment). More specifically, the output differential fluence/flux spectrum from the above models is used. Finally, appropriate transformations are applied in order to convert the differential fluences or fluxes in terms of particles cm-2 (s-1) MeV-1 when needed.

Angular distribution

The angular distribution can be defined using the following options: . In the case of an omnidirectional or a point source angular distribution, the minimum and maximum zenith angle to consider can be changed by the user, and the units include cm-2 in the case of an 1D simulation e.g. MULASSIS. 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.

For a mission based environment the omnidirectional angular distribution is recommended.

Normalisation factors

Based on the user’s input for the energy spectrum and the angular distribution two normalisation factors are calculated and recorded in the Geant4 macro file specifying the source.

The first normalisation factor n1 is related to the number of particles in the energy range for the different flux/fluence spectra. In the following table the formulas of n1 for all the possible types of incident particle spectra in SPENVIS are provided:

Energy distribution Normalisation factor
Mono-energetic
Linear
Power law
Exponential
User defined or
SPENVIS spectrum

where I(E0) is the fluence/(flux) intensity and F denotes the integral fluence/(flux). For the User defined or SPENVIS generated spectrum, the normalisation factor is calculated by simply subtracting the fluence/flux at the maximum energy in the spectrum from the fluence/flux at the minimum energy.

Next, the normalisation factors n2 related to the angular distribution are provided:

Angular distribution Normalisation factor
Omnidirectional
Point source
Parallel beam

Note that the one over 4π factor in the above formulas is because of the assumption that all of the particle spectra are omnidirectional, or have been integrated over 4π.

The two normalisation factors n1 and n2 are recorded as aliases in the generated macro file namely NORM_FACTOR_SPECTRUM and NORM_FACTOR_ANGULAR respectively.

Finally, note that SPENVIS is taking into account any unit transformation necessary in order to ensure that the normalisation factor is calculated in terms of particles per cm -2.

References

  1. Geant4 General Particle Source (gps)
  2. Geant4 MULASSIS manual

Last update: Fri, 27 Apr 2018