A Novel Multi-Functional Composite Material (MFCM) for Radiation

A Novel Multi-Functional Composite Material (MFCM) for Radiation Protection
for NASA Spacecraft and Astronauts
Midterm Report
1.0
Introduction
Radiation shielding is a critical issue in space exploration. Space radiation can damage
onboard electronics, can cause parts failure, and can be hazardous to Astronauts. NASA
currently uses aluminum as their primary radiation shielding material. One of the many
dangers posing humans in space flight is that of radiation. There are two main categories
used to describe the risks and issues caused by exposure to these energetic particles. The
first is non-ionizing. Since it doesn’t alter the structure of an atom by knocking out an
electron the only damage that occurs in a human is usually thermal and not as significant.
The second category is ionizing, which is the primary type found in space radiation. The
effects include deterministic effects such as loss of function to an organ or tissue due to
mass amounts of cell death in the area as well as stochastic effects such as cancer and
inheritable genetic damage. This damage is caused by the ion destroying or altering parts
of the cell and DNA structure. A final issue that is noted is that central nervous system
damage can occur, effectively altering the person’s behavior and shortening the lifespan.
All these risks make studying shielding materials important for further and future space
flights.
One of the most significant pieces of radiation in space to understand is that of Galactic
Cosmic Ray’s (GCR’s). The composition is approximately 85% protons, 14% alpha, and
1% heavy ions. These rays are with very high energy, an average of 100 to 1000 MeV’s,
and have been theorized to have been accelerated over a long period of time. GCR’s are
unique to the space environment and thus it is important to test and protect against these
types of particles found in the composition.
2.0
Radiation Protection
2.1 Onboard Electronics and Parts
Onboard electronics can be damaged when exposed to radiation causing failures in
various parts and electrical systems. Electronics on board a spacecraft needs to be
properly shielded from not only the space radiation but from the radiation emitted from
the onboard nuclear reactor. Electrical components are more sensitive to radiation than
others. Components such as transistors, diodes, and integrated circuits are damaged more
than resistors, capacitors, and inductors.
2.2 Radiation Effects on Human Tissue
The first goal was to ascertain which part of the human body was the most sensitive and had the lowest limits to radiation exposure. The second goal was to discover the radiation doses that occurred in various scenarios in order for a proper reference. It was found that the blood forming organs, such as bone marrow has the lowest limit i.e. can withstand only 0.25 Sieverts (Sv) in thirty days or 0.5 annually. The same for eye is only 0.5 Sieverts in thirty days and 1 annually. The radiations doses found for various situations has been compiled in table 1 below. Most of the dosages are quite small and are yearly doses. The limits included provides a reference to compare with how much astronauts receive and allow for a better understanding of overall safe doses of radiation. Table 1: Radiations Doses Found For Various Situations Situation Natural Background Mines Nuclear Fuel Cycle Medical Uranium Mines Air Crew Nuclear Industrial Limit Lowest level that Cancer Risk is Evident Rescue for Fukushima Limit Ramsar, Iran 5 in 100 Fatal Cancer Apollo Mission Low Orbit Space Unshielded Project Mars Trip Shielded (with current technology) Biological damage of Absorbed Dose (mSv) 2.4/yr 3.0/yr 1.0/yr 10-‐20 per test 2-‐10/yr 5-‐9/yr 20/yr 100 250 250/yr 1000 1.2/day 400-‐900 500-‐1000 3.0 Deficiencies with Current Radiation Shields
NASA uses aluminum sheets as their primary radiation shield material. Aluminum can
provide the necessary shield when adequately designed with proper thickness. This
thickness may result in very expensive design for weight sensitive structure like a
spacecraft.
4.0 Current Investigation – Computer Simulation for Space Application
SRIM (The Stopping and Ranging of Ions in Matter) created in 1983 is a computer
program, which allows simulating the ion distribution in a material. The SRIM code is a
result from the work of both J.P. Biersack and J.F. Ziegler [2]. The SRIM program
can simulate various applications such as Ion Stopping and Range in Targets, Ion
Implantation, Sputtering, Ion Transmission, and Ion Beam Therapy. For the current
simulation study, the Ion Stopping and Range in Target code is used. The MFCM is
modeled in SRIM using chemical compositions, physical states, densities, and
thicknesses.
The first material used in the SRIM was lead. Thickness dimension is calculated so that
all the material has same weight per unit area. Each of the ions energy was increased until
the material did not completely stop the ion. The results are compared as given in table 2
below.
Table 2: Ion Energy Level For Different Material With Same Weight Per Unit Area
Hydrogen
Helium (MeV) Co 60 (GeV)
Cs 137 (GeV)
(MeV)
Lead
9
35
2
7
Tantalum
9
35
2
7
MFCM
28
110
6.5
20
Figure 1: Hydrogen shot into the Multifunctional Composite Material (MFCM) at 28
MeV. It shows that the ion completely stops in the 13.06 mm thick material.
Table 2 demonstrates that for every type of ion tested, the MFCM is able to stop
significantly higher energy particles than that of lead and tantalum at the same weight.
The trends suggest that this would be true of every particle and show that the MFCM has
better shielding qualities compared to that of lead.
These results also show that the MFCM would have excellent performance in low orbit
space. Due to the magnetic fields around Earth, the energy levels in low orbit are
significantly lower than outside the Van Allen belt. The energy levels have shown to be
on average closer to 10 MeV rather than on the scale of 1 GeV.
Once outside of low orbit the materials complete stopping ability would decline. All the
heavy ions of the GCR’s would be completely stopped, as would almost all of the alpha
particles. The only problem left would be that of the hydrogen ions. Further testing was
done and yielded that even though the composite material does not stop the ion after an
energy level of 28 MeV, it does slow down the ions. Further testing shows that it would
require a very large increase in thickness to fully stop an energy level of 80 MeV’s,
which does not yet reach the average energy level of GCR’s. Even at three times the
thickness with a water injected polyurethane core, the ion is still shown to penetrate
through the entirety of the material (figure 2).
Figure 2: A Kevlar layer at three times the length of the testing layer of 13.06 mm with a
Hydrogen atom penetrating at 80 MeV’s. The ions go all the way through the material.
The results of this test show that the material is not very good at stopping high-energy
hydrogen atoms, which represent the proton component of the GCR’s. The ions will slow
down through the material, but the tests suggest that the high energy level will still
present some radiation risks. However, since the material blocks almost all of the other
components and performs better than lead, it could still be considered a viable option for
space travel and could significantly reduce the absorbed dosage of astronauts travelling
through space.
5.0 DEMRON Fabric – A Commercial Radiation Shielding Material
Demron has shown to have similar performance levels to lead. In some ion cases the
material performed worse. This material has also shown to potentially be highly effective
in “clinical and homeland security” environments.
Simulation on “DEMRON” does not include significantly high-energy radiation levels
that may be seen in space. The studies also don’t include ions such as alpha and hydrogen
particles, which are main components of the GCR’s. Since overall the material performs
very similar to lead, the data from the lead in SRIM can be used for a strong
approximation of Demron in the situations that were not tested. This means, based on the
tests of lead versus the MFCM, the MFCM performs better than Demron for a space
environment.
Following graphs demonstrated that the thickness required stopping various energy levels up to and over 100 MeV’s when a Hydrogen atom was shot at the material. The materials tested in the SRIM included: Lead, Tantalum, and the MFCM. The three graphs are shown below. Hydrogen Shot into Lead at Varying Energy Levels Thickness (cm) 2.5 2 1.5 1 0.5 0 0 20 40 60 80 100 120 140 Energy (MeV) Figure 3: The above graph shows data collected from an SRIM for a Hydrogen Ion being shot at lead. The energy was varied and the thickness was increased until the ion was completely stopped within the material. 2.5 Hydrogen Shot into Tantalum at Varying Energies Thickness (cm) 2 1.5 1 0.5 0 0 20 40 60 80 100 Energy (MeV) 120 140 160 Figure 4: The above graph shows data collected from an SRIM for a Hydrogen Ion being shot at Tantalum. The energy was varied and the thickness was increased until the ion was completely stopped within the material. Hydrogen Shot into MFCM at 16 Varying Energy Levels Thickness (cm) 14 12 10 8 6 4 2 0 0 20 40 60 Energy (MeV) 80 100 120 Figure 5: The above graph shows data collected from an SRIM for a Hydrogen Ion being shot at the Kevlar polyurethane composite. The energy was varied and the thickness was increased until the ion was completely stopped within the material. The final task was to recreate representative graphs similar to those found in the Demron report. Tantalum and the MFCM were tested with varying thickness to determine the percent transmission of the ion into the material. The elements of Cadmium 109 and Cesium 137 were used for this purpose. It is shown that at equal Dose Transmission thicknesses, tantalum deflects the ion faster due to its high density. The graphs composed are shown below. The Dose Transmission of Cd 109 into Tantalum and MFCM at Varying Thicknesses 0.98 0.91 0.84 0.77 0.7 0.63 0.56 0.49 0.42 0.35 0.28 0.21 0.14 0.07 0 Tantalum Kevlar 0 0.1 0.2 0.3 0.4 0.5 Thickness (cm) Figure 6: The above graph shows Tantalum versus the Kevlar composite in the transmission of a Cadmium 109 ion into each of the materials. 1.2 The Dose Transmission of Cs 137 into Tantalum and MFCM at Varying Thicknesses Dose Transmission 1 0.8 0.6 Tantalum 0.4 Kevlar 0.2 0 0 10 20 30 Thickness g/cm 2 40 50 Figure 7: The above graph shows Tantalum versus the Kevlar composite in the transmission of a Cesium 137 ion into each of the materials. 6.0
Task ahead
Experimental validation of the simulation observations – On going.
7.0
References
[1] The Space Environment And Its Effects On Space Systems, Piscane, V., 2008,
American Institute of Aeronautics and Astronautics, Inc., Reston, VA, Chap. 9.
[2] Ziegler, J. –no date- PARTICLE INTERACTIONS WITH MATTER. Retrieved
November 11, 2011 from http://www.srim.org/

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