Accelerator-Based PIXE and STIM Analysis of Candidate
Solar Sail Materials
W.A Hollerman*, T.L. Stanaland*, D. Edwards†, P. Boudreaux*, L. Elberson*,
J. Fontenot*, E. Gates*, R. Greco*, M. McBride*, and A. Woodward*
* Department of Physics, University of Louisiana at Lafayette, Lafayette, Louisiana 70504 USA
† Space Environmental Effects Group, NASA Marshall Space Flight Center, MSFC, Alabama 35812 USA
Abstract: Solar sailing is a unique form of propulsion where a spacecraft gains momentum from incident photons.
A totally reflective sail experiences a pressure of 9.1 µPa at a distance of 1 AU from the Sun. Since sails are not
limited by reaction mass, they provide continual acceleration, reduced only by the lifetime of the lightweight film in
the space environment and the distance to the Sun. Practical solar sails can expand the number of possible missions,
enabling new concepts that are difficult by conventional means. One of the current challenges is to develop strong,
lightweight, and radiation resistant sail materials. This paper will discuss initial results from a Particle Induced XRay Emission (PIXE) and Scanning Transmission Ion Microscopy (STIM) analysis of candidate solar sail materials.
BACKGROUND
(R = 1) at a distance of 1 AU from the Sun
experiences a light pressure of 9.1 µPa [3].
It is often wrongly stated that particle pressure
from the “solar wind” powers sail spacecraft. Solar
wind is composed of low-density protons and
electrons moving at high velocity. The pressure due
to the solar wind (pw) is on the order of
The concept of using photon pressure for
propulsion has been considered since Tsiolkovsky in
1921 [1-3]. The term "solar sailing" was coined in
the late 1950s and was popularized by Arthur C.
Clarke in the short story Sunjammer in May 1964 [4].
The National Aeronautics and Space Administration
(NASA) used sailing techniques to extend the
operational life of the Mariner 10 spacecraft in the
mid-70s. A problem in the control system was
causing Mariner 10 to go off course. By controlling
the attitude of Mariner 10 and the angle of the solar
power panels relative to the Sun, ground controllers
were able to correct the problem without using
precious fuel [5].
Once thought to be difficult or impossible, solar
sailing has come out of science fiction and into the
realm of possibility. Any spacecraft using this
method would need to deploy a thin sail that could be
as large as many kilometers in extent.
The
availability of strong, lightweight, and radiation
resistant materials will determine the future of solar
sailing.
The total light pressure (pt) in pascals (Pa) for a
solar sail can be approximated using
pt = 4.56 x 10
−6
(1 + R) ,
2
rAU
pw ~ mp ρw v2,
(2)
where mp is the mass of the proton, ρw is the particle
density, and v is the velocity [3]. Near the Earth, a
solar wind density of 6 x 106 m-3 at a velocity of
4 x 105 m/s results in a particle pressure of about
1 nPa, which is more than three orders of magnitude
smaller than the equivalent photon pressure [3].
SAIL MATERIALS
Mylar was selected for this research due to its
relevance to the application, availability, and
manufacturability [6]. The base material was 2.5 µm
thick and coated on the front (sun facing side) with
50 nm of aluminum to maximize reflectivity. The
back, or anti-sun facing side, was also coated with
50 nm of aluminum to minimize emissivity. The
sample was taken from stock that contained Kevlar
threads on the back surface to serve as a rip-stop
mechanism [6]. These threads were positioned
25 mm (1 inch) apart. The sample was glued taut to a
clean metal washer. A piece of Mylar was glued taut
to a small stainless steel sample frame for subsequent
PIXE and STIM analysis.
(1)
where R is the surface reflectivity (0 ≤ R ≤ 1) and rAU
is equal to the distance to the sun in astronomical
units (AU) [3]. A perfectly reflective solar sail
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
452
EXPERIMENT
Aluminized Colorless Polyamide 1 (CP1) and
Kapton are two additional candidate solar sail
materials. CP1 is a moisture resistant polymer that
can be stored for long periods of time without
significant
property
degradation [7]. Kapton
maintains it integrity at extreme temperatures [6].
FIGURE 1 shows the measured reflectivity (R) for
the Mylar, CP1, and Kapton sail materials.
Additional research on these materials will be
completed in the future.
1.00
A 1.7 MV 5SDH-2 tandem Pelletron accelerator
is used as a source for the LAC nuclear microprobe
system [8]. It can be configured to complete a variety
of analysis techniques, such as particle induced x-ray
emission (PIXE) and scanning transmission ion
microscopy (STIM). With PIXE, characteristic xrays are created by particle irradiation [9]. The
energy of the emitted x-rays depends on elements
present in the target. PIXE is two orders of
magnitude more sensitive than similar electron-based
methods [9]. With STIM, an energetic particle beam
passes through a thin sample, losing energy by
several processes: the Rutherford scattering process
and electronic stopping. Electronic stopping is the
dominant contribution. Information on the density
and thickness of the sample can be obtained by
measuring the energy loss due to Rutherford forward
scattering and the electronic stopping of individual
projectiles. A secondary electron emitted during in
the interaction of the beam with the sample also
provides a surface image that is analogous to visible
light photography.
The PIXE and STIM measurements completed in
this research utilized 2 MeV protons with beam
currents of less than 100 pA. The resulting x-ray
spectra was obtained using a Princeton Gamma-Tech
(PGT) Si(Li) detector with an active area of 30 mm2
and energy resolution of 140 eV, measured at
5.9 keV. The x-ray detector was positioned about
2.5 cm from the target at an angle of 135° relative to
the beam direction.
A Perkin Elmer ULTRA ion-implanted silicon
charged-particle detector was used for the STIM
analysis. This detector has a maximum FWHM
resolution of 12 keV, measured using the 5.386 MeV
alpha particle from 241Am. The detector is positioned
about 5 cm behind the sample at an angle of
approximately 20° relative to the incident proton
direction. The detector is positioned behind an
aperture with an area of less than 2 mm2.
0.95
ANALYSIS AND DISCUSSION
(a) Aluminized Mylar
1.05
Reflectivity (R)
1.00
0.95
0.90
0.85
0
500
1000
1500
2000
2500
3000
2500
3000
Wavelength (nm)
(b) Aluminized CP1
1.05
1.00
Reflectivity (R)
0.95
0.90
0.85
0.80
0.75
0.70
0.65
0.60
0
500
1000
1500
2000
Wavelength (nm)
(c) Aluminized Kapton
Reflectivity (R)
1.05
FIGURE 2 shows 1,000 x 1,000 µm microprobe
PIXE scans for the Mylar solar sail material. These
scans show Kα x-rays of aluminum, sulfur,
phosphorus, and silicon. Scan regions that are black
correspond to high levels of x-ray yield. Areas that
are white correspond to no x-ray yield. Shades of
gray correspond to intermediate yield values.
FIGURE 2 clearly shows the outline of a bundle of
Kevlar threads that is glued to the back of the
aluminized Mylar sail material. Aluminum Kα x-rays
0.90
0.85
0.80
0
500
1000
1500
2000
2500
3000
Wavelength (nm)
FIGURE 1. Reflectivity versus wavelength for
aluminized (a) Mylar, (b) CP1, and (c) Kapton.
453
250 µm
Aluminum Kα
Sulfur Kα
Phosphorus Kα
Silicon Kα
FIGURE 2. Four 1,000 x 1,000 µm microprobe PIXE scans of a section of the Mylar sail sample.
phosphorus and silicon. Due to the small quantities
of elements in the sail material, this PIXE analysis
took more an hour to complete. PIXE is not used to
detect carbon since it has a small x-ray energy that
will not penetrate the window on the Si(Li) detector.
Elements with atomic numbers larger than about
eight can be analyzed using PIXE [8].
FIGURE 3 shows four 1,000 x 1,000 µm STIM
scans for the portion of the Mylar sail material
discussed above. Regions in each scan that are black
correspond to large numbers of detected scattered
protons. Conversely, areas that are white in the scan
correspond to no detected particles. Shades of gray
correspond to intermediate quantities of detected
protons. The numbers on top of each image
correspond to the STIM scan number as displayed by
the data acquisition computer. Particles that lose
small amounts of energy pass through the plain
aluminized Mylar as shown in slice #9. Conversely,
protons that lose large amounts of energy through the
Mylar plus the entire extent of the Kevlar fiber
bundle as shown in slice #1.
originate in the coatings on both sides of the Mylar
film. Scan regions directly above the Kevlar threads
have less aluminum yield because they attenuate xrays created on the back surface of the sail material.
FIGURE 2 shows a small aluminum x-ray yield
coming from the film on top of the thread bundle.
The larger x-ray yield away from the Kevlar fibers is
produced in both the front and rear surfaces of the
Mylar film. Remember the aluminum layers on both
sides of the Mylar are only 50 nm thick. The Kevlar
fiber bundle was found to be approximately 500 µm
in diameter.
The sulfur Kα x-ray was also observed during the
PIXE analysis. This finding was puzzling until it was
realized that something had to be used to attach the
fiber bundle to the aluminized Mylar. FIGURE 2
shows the presence of sulfur in the same scan region
as the Kevlar fiber bundle. This result indicates that
a glue containing sulfur was used to attach the Kevlar
fiber bundles to the aluminized Mylar sail material.
The PIXE analysis also shows the presence of
small amounts of phosphorus and silicon in the
region containing the Kevlar fibers. More than
likely, the glue also contains small amounts of
#9
#6
#4
#1
250 µm
Thinnest Slice
Thickest Slice
Smallest Energy Loss
Largest Energy Loss
FIGURE 3. Four 1,000 x 1,000 µm microprobe STIM scans of a section of the Mylar sail sample.
454
Propulsion, L.K. Korneev, Editor, pp. 303-321, Israel
Program for Scientific Translations, Jerusalem, Israel
(1964).
Notice in slices #4 and #6 that two areas of equal
thickness are observed in the fiber region. This can
best be explained if the Kevlar bundle is cylindrical
in shape. Protons scattered through the entire bundle
diameter will generate a scan similar to slice #1. As
the effective scatter length gets smaller, STIM should
generate slices like #4 and #6. When protons scatter
through only the aluminized Mylar, the resulting
STIM image should be similar to slice #9. Therefore,
the Kevlar fiber bundle is approximately cylindrical
in shape and is about 500 µm in diameter. These
results are entirely consistent with the PIXE
measurements shown in FIGURE 2. Additional
PIXE and STIM measurements will be completed on
CP1 and Kapton in the future.
[3] C.R. McInnes, Solar Sailing: Technology, Dynamics
and Mission Applications, pp. 1-50, Praxis Publishing,
Chichester, United Kingdom (1999).
[4] A.C. Clarke, The Wind From the Sun, A.C. Clarke,
Editor, "Project Solar Sail", pp.9-31, Penguin Books,
New York, NY (1990).
[5] D.L. Turcotte, Space Propulsion, pp. 108-125,
Blaisdell Publishing Company, New York, NY
(1965).
[6] Mylar and Kevlar are registered trademarks of E. I.
duPont de Nemours and Company.
[7] CP1 is a registered trademark of SRS Technologies.
CONCLUSIONS
[8] G.A. Glass, W.A. Hollerman, S.F. Hynes, J. Fournet,
and A.M. Bailey, Proceedings of the Sixteenth
International Conference on the Application of
Accelerators in Research and Industry, Edited by J.L.
Duggan and I.L. Morgan, American Institute of
Physics, 522-525 (2001).
Once thought to be difficult or impossible, solar
sailing has come into the realm of possibility. The
availability of strong, lightweight, and radiation
resistant materials will determine the future of solar
sailing. A preliminary analysis on a candidate Mylar
sail material indicated that small quantities of
elements could be characterized using the LAC
nuclear microprobe. The PIXE analysis found sulfur,
phosphorus, and silicon from the glue that attached
the Kevlar rip-stop fibers to the aluminized Mylar.
The presence of these elements was unknown before
this research. The corresponding STIM analysis
found that the Kevlar fiber bundle is roughly
cylindrical in shape and is about 500 µm in diameter.
Additional
microprobe
PIXE
and
STIM
measurements will be completed on CP1 and Kapton
sail samples in the future. Elemental analysis of the
glue was conducted to better quantify the expected
performance of the adhesive in future testing to be
done by NASA. This future testing will test the
endurance of the material and the properties of the
glue will affect the performance.
[9] S.A.E. Johansson and T.B. Johansson, Nuclear
Instruments and Methods in Physics Research 137,
473 (1989a).
ACKNOWLEDGEMENTS
The Louisiana Education Quality Support Fund
(LEQSF) using LEQSF (2000-03)-39 provided most
of the support for this research.
REFERENCES
[1] A.A. Blagonravov, Editor, K.E. Tsiolkovsky Selected
Works, Translation by G. Yankovsky, pp. 140-163,
Mir Publishers, Moscow, USSR (1968).
[2] F.A. Tsander, The Use of Light Pressure for Flight in
Interplanetary Space, Problems of Flight by Jet
455