University of Washington Undergraduate Research Symposium 2015
http://earthweb.ess.washington.edu/space-propulsion/
Passive Hybrid Thermoplastic Graded-Z Radiation
Shielding for a Europa CubeSat Mission Nicolas Gutierrez (nag4@uw.edu), Robert Winglee, Ian Johnson, Paul Sturmer, Paige Northway
Abstract
Data
Experiment
The Advanced Propulsion Laboratory at the University of Washington is conducting a concept
study for a 3U CubeSat to be launched on NASA’s Europa Clipper mission to perform magnetic
field measurements around the Jovian moon Europa. The primary concern with electronics
surviving in Jupiter’s orbit is withstanding the high energy and high density radiation fields that
exist around the planet, with beta particles being in highest flux. Typically, satellites on Jovian
missions are spot shielded with high proton density metals to protect sensitive electronics
from radiation induced damage. This study will analyze and test a number of existing graded-z
shield compositions and new hybrid thermoplastic graded-z shields. A graded-z shield is a type
of passive radiation protection in which layers of material are placed in a gradient from low to
high proton density, with a thin final layer of low proton density material. Preliminary
calculations show that substituting the aluminum portion of the graded-z shield with crystalline
polypropylene will both increase shielding capabilities and lower total shield mass. This new zshield designed at the Advanced Propulsion Lab will have a hybrid composition with both metal
and thermoplastic layers. The test itself will be conducted by exposing a custom built Arduino
based Geiger counter to a beta particle source and assessing five test articles including the new
hybrid shield as well as traditional shields. Due to the non-linearity and complexity of radiation
interactions with matter, different shields may be more effective at blocking different particle
energies.
A critical point of this study is supporting with data that a Graded-Z shield is more effective at
blocking beta radiation than a traditional slab shield and supporting the replacement of
aluminum in the Z-shield with polypropylene. The following graphs created from NIST data
shows the theoretical reasoning behind the replacement.
The experiment itself will be conducted by simulating a to some extent the radiation
field that the CubeSat would possibly encounter near Europa. Since this study focuses
on Beta radiation as the primary issue, an experiment was designed to test the
effectiveness of the shields against high-energy electrons. For this radiation sources of
Thallium-204 and Strontium-90 will be used which produce 0.7 MeV beta particles at
approximately 1 micro Currie activity and 2.2 MeV beta particles at 0.1 micro Currie
activity respectively. The experiment will quantify the performance in three areas:
beta radiation shielding, Bremsstrahlung production, and Bremsstrahlung effect
shielding. The most effective shield will have the lowest, weighted combined counts
per minute (cpm) of beta particles and Bremsstrahlung rays at a Geiger counter. In
order to differentiate between beta particles and Bremsstrahlung rays, two GeigerMuller tubes will be used. One that is sensitive to only high energy photons and one
that is sensitive to both beta particles and high energy photons. This will allow us to
know which type particle caused a “hit” to register on the counter.
Theoretical Background
In 2012, NASA began planning for a mission to Jupiter’s icy moon, Europa. The purpose of the
mission is to take a variety of specific measurements of the moon ranging from magnetic field
readings to topographical imaging. Advanced Propulsion Lab (APL) would provide a
supplementary CubeSat to accompany NASA’s Clipper spacecraft and assist it in measuring
Europa’s B (magnetic) fields. The CubeSat would take a measurement, store the data on an
onboard memory module and transmit the data to the Clipper for final transmission to Earth.
Compared to Earth’s Van Allen belts and intra-solar radiation levels, Jupiter’s radiation
environment is significantly stronger in relation to other parts of the solar system. Radiation
flux density exceeding 105 protons (cm2-s) of 10 MeV energies and 107 electrons (cm2-s)-1 of 1
MeV energies.
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•
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Polypropylene is 30.7% more effective than aluminum for beta at expected energies
Aluminum has greater radiative stopping power  more Bremsstrahlung
Polypropylene has greater collision stopping power
Polypropylene has lower radiative stopping power  less Bremsstrahlung
Beta Shielding
• Graded-Z shield vastly superior (60%)
Proton Shielding
• Graded-Z shield slightly lower
performance (5-10%)
Traditional Slab Shield
Graded-Z Shield
Anticipated Results
The ultimate goal of this study is to present and qualify an alternative to expensive
radiation hardened electronics for the Europa CubeSat. If the designed hybrid gradedZ shield can sufficiently protect the payload from radiation while also falling within
mass and volume limitations, then fewer radiation hardened components will have to
be used. This will save a large amounts of money due to the high price of radiation
hardened components, which can be anywhere between 100 to 10,000 times more
expensive than their commercial counterparts. This study hopes to show that for beta
radiation energy levels within one order of magnitude of 1 MeV, that a graded-Z
shield is 60% more mass effective at shielding when compered to a traditional slab
shield, and that the new hybrid graded-Z shield is between 10-15% more mass
effective than a regular graded-Z shield.
Sources and Acknowledgements
No Shielding
20mm Shielding
The Bremsstrahlung Effect
What makes shielding against beta particles exceptionally difficult is the Bremsstrahlung
Production Effect, in which, due to the conservation of energy, high-energy beta particles
colliding with atom nuclei produce incident high-energy photons. The incident photon
energy is a function of both the inbound beta particle energy level and the proton density
of the atom nucleus. To effectively shield against beta radiation, this effect must be taken
into consideration.
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Pappalardo, Robert, Brian Cooke, Barry Goldstein, Louise Prockter, Dave Senske, and Tom
Manger. "The Europa Clipper: OPAG Update." Lunar and Planetary Institute (n.d.): n. pag.
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Truscott, Pete, Ramon Nartallo, Fan LEi, Daniel Henderickx, Sebastian Bourdarie, and
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Dr. Michael McCarthy – For assistance with the Geiger-Muller tube and its support
electronics and with data interpretation.