(BEING CONTINUED FROM 14/10/14)
VI.I Simulation environment
The simulations were performed using Geant4 version 9.5. The simulation reference was default
Geant4 aluminium. Material definitions included correction factors, derived from the first
measurement campaign, to estimate impurities on the layers.
The layer thicknesses that were studied are the following:
· Aluminium with thickness of 1 and 2 mm
· Tungsten of 50 and 100 μm
· Stainless Steel of 50 μm
· Prepreg of 215 μm
The thicknesses and compositions of the prepreg and steel layers were optimized to the results of the
first test campaign measurements with 20 MeV protons.
The incoming particles and energy regions that were studied are:
· Protons of 1-100 MeV
· Electrons of 1-20 MeV
· Gamma of 1-20 MeV
The effect of the materials in terms of absorbed energy and dose was estimated by using a scoring
layer of silicon with thickness of 300 μm. This was placed right after the samples in the simulation
Secondary radiation produced with the spacecraft material by trapped protons and interactions with
cosmic rays provide an important, and at times,dominant radiation environment for the instruments.
Energy loss measurements will not provide information about the type of radiation that the
components will experience behind the shielding layers. Therefore, in the simulations, in addition to studying the energy loss of the particles, overall radiation dose is measured in a layer behind the sample under studies. This is done in scoring layer of
Silicon with thickness of 300 μm. The material and the thickness were selected to represent the standard
electronics components inside a spacecraft.
The number of incoming particles was set to 10.000 to reduce the required CPU time for the calculations.
Simulations of interactions of protons were made in the energy region of 1-100 MeV. Most of the
effect the thin shielding materials have to the incoming radiation will occur in the lower end of the
selected energy region. Therefore particle energies higher than 100 MeV were not included in the
simulation models. Though particles with higher energies contribute to Single Event Upsets (SEU),
their effect to overall dose experienced by the components is very small due to the small number of
these types of particles.
Figure 10 shows simulated energy loss of protons traversing layers of different materials. As expected,
the high Z material, Tungsten, has the highest effect on the kinetic energy of the incoming protons. At
lower energies, layers made from Prepreg have equal effect on the particle energies as 50 μm of Tungsten.
Overall, the effect of a Prepreg layer is better or equal as 50 μm layer of Steel.
Figure 10: Energy loss of protons traversing through sample layers. Tungsten 50 and 100 describe the thickness of the layers in microns.
When the dose of the incoming radiation is compared, it can be seen in Figure 11, that since the
100 μm layer of Tungsten has the highest effect on proton energy, it also suffers from highest dose
coming from secondary particles. However, this is only visible on lower energies since at higher
energies the rate of secondary particles is less affected by the properties of a thin layer. This is due
to the drop in probability of interactions.
Figure 11: Dose in Silicon scoring layer after the samples from proton beams
Though most of the incoming electrons have kinetic energy in the region of 1-7 MeV, the studies
were expanded up to 20 MeV. This was done to study the effects of secondary particles that are produced in
the proton interactions.
From Figure 12 it can be seen that only the layers with high Z materials have effect on incoming
electron energy. The improvement between the two Tungsten samples can be seen to be directly
proportional to the material thickness.
Figure 12: Energy loss of the electrons traversing the sample materials.
Figure 13 shows the measured dose on the scoring layer after the samples.
Figure 13: Dose on the scoring layer from electron beams of various energies.
(TO BE CONTINUED)