2. 2
Abstract
For many years, physicists have been searching for the elusive cosmological substance
known as the “dark matter.” The existence of dark matter is inferred from large gravitational
effects that cannot be attributed to known masses in space alone. However, this mysterious
matter neither emits nor scatters electromagnetic radiation (i.e. light), rendering conventional
optical and radio detection impossible. Recently, physicists initiated a series of experiments
known as the General Antiparticle Spectrometer (GAPS) to detect possible dark matter
candidates, antideuterons, in the upper levels of Earth’s atmosphere. These GAPS projects
will be carried out in scientific balloons that will be launched into the higher limits of Earth’s
atmosphere in a precarious pressure area called the Paschen region. To ensure accurate GAPS
results and prevent the damage of the detectors, it is essential to “passivate” the detectors by
applying a protective coating. The present project probed the different materials suitable for
effective and cost-friendly detector passivation. Two types of materials, polyimide and a paint
called Box Car Red, were subjected to three tests: drying, cooling, and leakage, each for
compatibility, robustness, and endurance, respectively. The results showed that Box Car Red
provides better passivation, with lower sensitivity to environmental effects, such as drying
time, and electronic effects, such as leakage current. There has been much speculation and
brainstorming on further testing possibilities to be implemented into the experiment, in order
3. 3
to better match the conditions expected during deployment, such as a water test that matches
Japan’s high level of humidity, one of the launching sites for an experimental run.
Executive Summary
Dark matter is a substance that many scientists propose to exist in space. It defies the
conventional notion of matter in that it does not reflect light; namely, dark matter cannot be
seen by naked eyes or light-based detection methods. Physicists believe that dark matter
exists because visible matter alone does not have sufficient mass to produce the gravity that
holds the universe together. Because the existence of dark matter cannot be shown via
conventional means, scientists have been designing innovative mechanisms to capture
particles that would in turn prove the presence of dark matter. One such approach, the
General Antiparticle Spectrometer (GAPS), involves launching balloons that carry special
dark matter detectors into Earth’s outer atmosphere hoping to catch bypass dark matter
particles. Since these balloons have to withstand harsh conditions during deployment and
floating process, protective measures needed to be developed to ensure the survival of the
device, mostly against weather damage for the journey to the targeted level in the atmosphere,
and the integrity of the experiments. This project probed the different materials suitable for
effective and cost-friendly detector protection. Two types of materials, polyimide and a paint
called Box Car Red, were tested for compatibility, robustness, and endurance. It turned out
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that Box Car Red provided better protection, with lower sensitivity to environmental effects
such as drying time and electronic effects that may increase the production of noise, allowing
for a wider margin of error, otherwise measured as leakage current.
Introduction
Dark Matter: An Unknown in the Universe
The existence of matter beyond the conventionally accepted, otherwise known as
“dark matter,” has been suggested through observing clusters in space that seem to be
affected by an unknown substance yet to be detected by scientists. Among its characteristics
is the lack of electric charge, which prevents any optical detection but still allows for its
presence to be hypothesized through the observance of its gravitational interactions with
other celestial objects.
Fritz Zwicky first postulated in 1934 the existence of dark matter by hypothesizing
that there was “missing mass” in some galactic clusters. He observed that the galactic clusters
seemed to have too little mass in comparison to their rotational velocities to generate enough
gravitational pull to keep the galaxies from disintegrating. Thus, this observation led
scientists to the belief that an unknown form of mass is providing the calculated gravitational
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force necessary to keep the galaxies intact. Another strong hypothesis was again introduced
by Zwicky; he observed that light was bent around positions in space where there was no
“luminous” mass [1]. What kind of matter could create such strange effects?
Previous studies of the early universe postulate that dark matter should be relatively
stable, cool, and non-relativistic. According to Drees and Gerbier (2005), dark matter must
meet certain requirements: be stable in the cosmological timeframe; interact very weakly with
radiation, i.e., light; and have substantial density in the early universe. These criteria narrow
down the possible candidates for dark matter particles. Current candidates include axions,
primordial black holes, and new kinds of weakly interacting massive particles, or WIMPs [1].
One favorite candidate for a new WIMP is the neutralino. But how can an experiment
hope to detect a particle that doesn’t interact with light? According to the Supersymmetric
Particle Theory, the neutralino is a Majorana particle and therefore is its own antiparticle.
When matter and antimatter annihilate, it produces other particles; one product of this
neutralino-neutralino annihilation is the antideuteron atom, which is composed of one
antiproton and one antineutron. This provides a mechanism for detecting dark matter:
detecting the antideuterons created in the neutralino-neutralino annihilation [2].
GAPS: An Instrument for Detecting Low-Energy Anti-Deuterium
The General Antiparticle Spectrometer (GAPS) is an experiment that detects low-
energy antideuterons in the upper levels of Earth’s atmosphere. It seeks low-energy
antideutrons because they are otherwise rare in the cosmic rays and those few antideuterons
present in the cosmic rays contain higher energy. Hence, when a search for antideuterons is
conducted at low energies, there is a high likelihood that those antideuterons detected
originated in neutralino-neutralino annihilations [3].
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Figure 1. The GAPS detector (left) is one of the flight detectors that have not yet been passivated. This
experiment is the first attempt at passivating a p+ Si(Li) detector. The Pressure Chamber (right) is where
the Paschen region tests took place (see Materials and Methods below).
An antideuteron entering the GAPS instrument is captured by an atom on the surface
of the detector that behaves like a surrogate electron. The antideuteron then drops to a lower
quantum level, forcing out all electrons in the atom along the way via a process known as
Auger ionization, and emitting X-rays. When the antideuteron reaches the lowest energy level
(n=1) of the atom, it annihilates with a proton or a neutron in the nucleus, and pions are
emitted. The X-rays subsequently strike another atom in the detector, expelling an electron
through a process known as the photoelectric effect. This initial photoelectron initiates an
avalanche of electron-hole pairs in the detector that can be read by electronics. By measuring
the voltage of the electron avalanche, a measurement of the incident X-ray energy is made.
This simultaneous detection of X-rays and pions serves as evidence for the presence of an
antideuteron, since pions appear only when antimatter annihilates in the nucleus. By way of
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this method, we detect the presence of antimatter. The energy of each X-ray provides an
independent measure of the mass of the captured particle, based on the Bohr formula. Thus,
pions plus proper X-ray energies indicate antideuterons.
Figure 2. The GAPS detector can differentiate between antiprotons (left) and anti -deuteron
(right). In this figure, blue horizontal bands are the detector layers. Green squares are points of
contact. Red squiggles are X-rays emitted. The detector differentiates between the two in three
different ways: One, the anti-deuteron plunges deeper into the detector. Two, a greater number
of pions are produced. Three, the X-ray photons are different energies.
Of course, the antiparticle that annihilates may not be an antideuteron and may very
well be an ordinary antiproton. In order to selectively identify antideuterons, the GAPS
instrument uses three selection methods: depth sensing, X-ray energy, and pion multiplicity.
A plastic scintillator mounted on the detector allows for the velocity of the incoming particle
to be inferred which, when merged with the depth of penetration, identifies the mass of the
particle. X-ray energies are different for antiprotons and antideuterons. More pions are
detected from antideuteron annihilation in the nucleus than from antiproton annihilation.
Thus, GAPS uses multiple methods to confirm the presence of antideuterons [4].
GAPS: A Needfor DetectorProtection, or ‘Passivation’
The GAPS project comprises a small prototype mission (pGAPS) and a larger
mission (bGAPS). The pGAPS project will be launched into the higher limits of Earth’s
atmosphere (above 100,000 feet) via a scientific balloon. The chamber containing the
detector is not sealed and allows for interactions between the detector and surrounding air. A
problem arises when the airflow also provides a leeway for any dust or moisture to
contaminate and hamper the detector performance. To ensure accurate GAPS results, it is
essential to protect the detectors without adding much weight to the apparatus. It is therefore
sensible to protect, or “passivate,” the detector surface with some kind of paint.
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This paper probes the different materials suitable for detector passivation that will be
environmentally robust. The coating material must be moisture-resistant, dielectric, withstand
reasonably harsh contact abuse, and not deform under extreme temperature changes [5].
There have been no previous efforts to simply and cheaply passivate the p+ side of this type
of silicon detector (more specifically, silicon detector containing lithium). Furthermore, the
most up-to-date method for passivating the p+ side of these detectors is too expensive to be
employed on the GAPS project, which requires thousands of these detectors. The GAPS
experiment reads out its electrical signals from the p+ (heavily doped p-side) side of the
detector. Previous passivation work, all done on the n+ side of these types of detectors more
than 30 years ago, used an organic material called polyimide and a model paint used by
railroad enthusiasts called Box Car Red. The problem is that commercially produced
polyimides contain trace contaminants and slightly different chemical compositions that can
affect detector performance. No commercial polyimide exists in exactly the same
composition as that used in the original research, conducted more than 30 years ago on n+
contacts. The present paper thus tests the effectiveness of polyimide and Box Car Red coated
on the p+ side of the GAPS silicon detectors. Such research has never been previously done.
In the following sections, we discuss our aims, methods and materials, results, and
conclusions. We will demonstrate the detector operation at the low pressures of outer space,
then describe the theory of the detector operations and the setup of our experiment. We will
demonstrate the environmental robustness of the passivation layers, describe the selection
criteria for quality passivation layers, and describe the observables we intend to test in our
experiment. Finally, we will test the Polyimide and Box Car Red layers on actual detectors
and ascertain the best performing material.
Materials and Methods
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Demonstration of DetectorOperation at Low Pressures of Outer Space
The detectors will operate under high-voltage at 1-20 torrs at the edges of our
atmosphere. Unfortunately, this pressure region is deemed to be very dangerous for the
detector because at low pressures, there are just enough air molecules to bridge electrical arcs
from the high voltage power source onto the detector surface or other surfaces carrying the
electrical signal; thus, the detector is rendered inoperable. Moreover, the electrical arc will
brighten up the detector module, the light of which will cause noise in the detector resolution.
Hence, it was crucial to test its performance in this pressure region (called the Paschen
region). To our surprise, the detector performed well and proved that it can withstand voltage
breakdowns. Operating in the Paschen region, it was able to withstand high voltages of 300 V,
somewhat above the required 250V operating voltage for the thinnest Silicon detectors used
on GAPS.
Demonstration of Increased DetectorRobustness by Application of Passivation Layers
Selection of Passivation Coatings For p+ Detectors
In order to determine the qualities that define the most effective of our passivation
layers, we conducted a search of published and unpublished literature. We discovered that,
according to previous literature, polyimide was successful with n+ silicon detectors as a
passivation material, thus giving us a valid reason to test the material on p+ silicon detectors
[6]. This is the first experiment to passivate p+ silicon detectors with polyimide. However,
during our tests, we later discovered that polyimide proved futile, and concluded that the n+
polyimide passivation was not valid for p+ silicon detectors.
Later, we found out from another scientist that previous experiments in the 1970’s to
1980’s used Box Car Red as a successful passivation material for the n+ side of a silicon
detector (Norman Madden, personal communication, 2011). We proceeded to test Box Car
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Red with the p+ side of the silicon detectors, since GAPS reads its electrical signals out from
this side of the detectors.
Metrics and Tests of Passivatation Performance
Several additional metrics (in addition to good environmental passivation) are
valuable to test in our simulated outer space environment.
Drying time is important because it gives us exact information as to when the coated
detectors will be fully functional. To test the drying time, silicon pieces were categorized into
different test groups and were exposed to different environments for different durations with
variable temperature. The drying environment was varied to be either room air or pure
nitrogen. We speculated that they would dry more quickly in the nitrogen environment, as
nitrogen has zero humidity level. Temperature was also thought to be a factor because higher
temperature would expedite the drying process; test groups were left at room temperature,
heated to 40 degrees Celsius, or heated to 60 degrees Celsius. These particular temperatures
were chosen because 60 degrees Celsius is the maximum temperature at which the silicon
detectors can be operated. To see if the materials dried, we applied a standard pull-tape test
by sticking scotch tape on the painted surface and pulling it off to see if any material stuck to
the tape. Pull-tape tests are commonly used to test the strength of adhesion between two
materials. Dried passivation material would not peel off or be damaged by the pull-tape test
and would maintain good contact integrity. A failed test, in which we would see bits of the
paint torn off, would indicate that whatever circumstance the silicon piece was subjected to
was insufficient to effectively evaporate the solvents. Incidentally, the Tape Test was first
developed for epoxy adhesion studies and was used by the GAPS project after witnessing its
role as a successful test method in a NASA NuSTAR project.
A second test that we performed was the cooling test. Here, we aimed to see if the
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passivation material could survive the rapid decrease in temperature during the ascent in the
GAPS mission, quickly dropping from 20 degrees Celsius to -35 degrees Celsius. This also
simulated the extreme seasonal conditions induced by thermal shock and summer shock. To
simulate this environment, the silicon pieces were rapidly cooled with a thermoelectric cooler
to -35 degrees Celsius and then immediately brought back to room temperature. This process
was repeated seven times for every test; the surface was checked under a microscope after
trial 1, 4, and 7 to see if the passivation material developed any cracks, pinholes, or scratches.
Samples showing defects were disqualified. Unfortunately, the opaque nature of Box Car Red
disallowed observation through a microscope. Instead, a visible check was done to the Box
Car Red surface.
In a third test, both polyimide and Box Car Red were inserted into the grooves
of ”test detectors,” similar to the actual flight detectors. The energy resolution and leakage
current of the passivated detectors were then checked. This enabled us to determine if either
passivation would alter or inhibit actual performance of the GAPS instrument. Again, the
detectors were dried under varied conditions. Energy resolution data was taken under variable
pressure, shaping time, and bias.
Results
The Tape Test
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Figure 3. The Success of Box Car Red in the Tape Test. Curing Environment had little effect on
the Box Car Red material, which passed nearly every tape-test.
As can be seen from the graph above the environment in which the curing had taken
place (e.g., pure nitrogen or normal air, various drying temperatures) had no significant effect
on the successes and failures of the tape test for the Box Car Red passivation material. Out of
sixty total trials, only four Box Car Red samples failed. Polyimide was not tested.
Cooling Test
0
5
10
15
20
60˚ C 40˚ C Nitrogen Gas Room
Temperature
Room
Temperature
After 20
Hours
Tape Test with Box Car Red
# of Successes
# of Failures
0
5
10
15
20
25
0 Days 1 Day 2 Days 5 Days 6 Days 7 Days 8 Days
Cooling Test with Box Car Red
# of Successes
# of Failures
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Figure 4. The Success of Box Car Red in the Cooling Test. Even after seven sequential freezing
cycles, Box Car Red showed very little damage.
The passivated silicon pieces were put under seven cooling cycles, and observed under
the microscope after the first, fourth, and seventh trials to check for irregularities. Of the
seventy-three tests of the polyimide material, 41 samples passed and 32 samples failed.
We repeated the same process for Box Car Red. However, Box Car Red is an opaque
substance, and we were unable to observe the deeper layers of silicon with the microscope.
Instead, we conducted a simple visible surface check. All Box Car Red samples passed this
simpler check.
Figure 5. Silicon detector with polyimide passivation after 11 days of curing and7 cooling cycles.
Surface damage is clearly visible.
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Figure 6. Leakage Current is affected by passivation. The two bars on the left are the leakage current of
the unpainted detector. The middle two bars show the leakage current of the detector (leakage current
being proportional to measured voltage) after curing with polyimide. The right two bars show the leakage
current of the detector after curing Box Car Red.
Leakage Current Performance
Surprisingly, leakage current is affected by the application of passivation. Both
polyimide and Box Car Red were applied to two separate detector quadrants with 600 V of
high voltage applied. As can be seen from Figure 6, the leakage current observed depended
on both the drying time and the passivation material used. Notice that measured voltage is
shown in this figure, but voltage is proportional to current, so the graph is a measure of
leakage current. The detector allowed a significantly more leakage current after the
application of polyimide (4.0 V vs. 0.5 V). Box Car Red, on the other hand, showed no
significant impact on detector leakage current (0.6 V vs. 0.5 V). This suggests that Box Car
Red may provide better passivation under the conditions in which leakage current is
important. Leakage current adds noise to the detector, and this affects how accurately X-ray
energies can be determined. If X-ray energies are mis-measured, antideuteron X-rays might
be mistaken for antiproton X-rays, compromising the experiment.
Energy Resolution Performance
0
1
2
3
4
5
1 2 3 4 5 6 7 8 9
L
e
a
k
a
g
e
C
u
r
r
e
n
t
(
v
)
Polymide | BCR
No Passivation
Polyimide
8 days cure
Box Car Red
2 days cure
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The most direct test for overall performance of the passivation is the energy
resolution. This ultimately determines how effective the detector will be at detecting low-
energy antideuteron signals, and inferring the existence of neutralinos. We compared the
performances of polyimide and Box Car Red passivated detectors, and determined that Box
Car Red provided better performance.
Conclusions and Discussion
Throughout this series of tests, we aimed to determine which material – polyimide or
Box Car Red – would provide more effective passivation for the GAPS instrument’s silicon
detector. The passivation would need to survive the rigors of exposure to near outer space
conditions, while positively affecting detector performance.
Box Car Red fared well in the Tape Test, succeeding 92% of the time (56 out of 60),
and across all curing environments. We can therefore conclude that environment has an
insignificant role in the adhesion factor of Box Car Red. Similar data for polyimide was not
obtained.
Polyimide failed the cooling test 44% of the time (32 out of 73), suggesting that
polyimide may not be able to withstand extreme temperature differences without cracking,
tearing, or developing bubbles or pinholes. Our examination of Box Car Red showed no such
defects; however, this visual test was conducted without a microscope. Therefore, our results
indicate Box Car Red may be a more effective passivation material. The Leakage Current
Test also showed that polyimide dramatically increased the leakage current (which is
proportional to measured voltage) observed on the detector (from 0.6 V to 1.2 V in trial 1 and
to 4.5 V in trial 2). Box Car Red showed no significant increase in leakage current. This
increase in leakage current would decrease the sensitivity of the detector to the scientific
objective, and therefore is not optimal.
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Most important, however, is that passivation did not decrease the energy resolution of
the detector. This energy resolution is the ultimate measure of the sensitivity of the instrument,
and a high-energy resolution similarly translates into effectiveness in detecting anti-deuterons.
Our tests show that Box Car Red gives an improved energy resolution when compared to
polyimide. This measurement presents the origin of exactly what and how the passivation
material affects the detector.
The results of our tests indicate conclusively that Box Car Red provides a better
passivation, with less sensitivity to environmental effects such as drying time, and electronic
effects such as leakage current. Box Car Red paint dried more quickly and withstood the tape
test better, while also proving to be an excellent passivation material (as the low leakage
current and high energy resolution show).
We do note that there is an inherent obstacle in using Box Car Red as a passivation
material: the more Box Car Red paint applied to the detector, the higher the leakage current
and the lower the energy resolution (not shown). The pronounced increase was troubling at
first, but we posit that it is due to the fact that the paint does not dry uniformly. We predict
that a dry outer layer first forms on the detector, trapping the rest of the wet Box Car Red
solvents underneath. These solvents hamper the detector. Future work with Box Car Red
would necessitate that the paint be dried slowly for some time to allow the solvents to escape
before a top layer of paint becomes impermeable. It may be possible to solve this problem by
using a heater to dry the material more quickly, although this was not tried in order to prevent
unexpected damage to the detector.
The authors feel the results of this experiment are quite concrete. During our
literature search, we noted that the GAPS passivation could potentially be improved even
further by testing Conathane, which is believed to be another excellent passivation material
for magnetic detectors. Finally, additional tests can be suggested to better match the
17. 17
conditions expected during deployment, such as a water test that matches Japan’s high level
of humidity.
References
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Hailey, C.J. et al. 2006. Accelerator Testing of the General Antiparticle Spectrometer, a
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Aramaki, T. et al. 2010. Antideuterons as an indirect dark matter signature: Si(Li) detector
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Jantunen, M., & Audet, S.A. 1994. Surface passivated Si(Li) detectors for an X-ray detector
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