Investigations of Environmental Robustness of Semiconductor Detectors

for the GAPS Dark Matter Search

Jacqueline Yang and Nate Yang

1

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

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.

2

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 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.

3

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

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

4

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].

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).

5

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

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

6

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 Need for Detector Protection, 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.

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

7

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

Demonstration of Detector Operation 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

8

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 Detector Robustness 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

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

9

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

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

10

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

048

1216

Tape Test with Box Car Red

# of Successes# of Failures

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.

11

Cooling Test

0 Days 1 Day 2 Days 5 Days 6 Days 7 Days 8 Days0

5

10

15

20

25

Cooling Test with Box Car Red

# of Suc-cesses# of Failures

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.

12

Figure 5. Silicon detector with polyimide passivation after 11 days of curing and 7 cooling cycles. Surface damage is clearly visible.

1 2 3 4 5 60

0.51

1.52

2.53

3.54

4.55

Leakage Current(v)

Polymide | BCRNo Passivation

Polyimide 8 days cure

Box Car Red 2 days cure

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

13

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

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

14

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.

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

15

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

conditions expected during deployment, such as a water test that matches Japan’s high level

of humidity.

16

References

Yao, W-M. et al. 2006. Journal of Physics G: Nuclear and Particle Physics, 33(1), 233-237.

Hailey, C.J. et al. 2004. Development of the gaseous antiparticle spectrometer for space-based antimatter detection. Nuclear Instruments and Methods in Physics Research Section B, 214, 122-125.

Hailey, C.J. et al. 2006. Accelerator Testing of the General Antiparticle Spectrometer, a Novel Approach to Indirect Dark Matter Detection. Journal of Cosmology and Astroparticle Physics, 2006(1), 1-22.

Aramaki, T. et al. 2010. Antideuterons as an indirect dark matter signature: Si(Li) detector development and a GAPS balloon mission. Advances in Space Research, 46(11), 1349-1353.

Hailey, C.J. et al. (In press). Antideuteron based dark matter search with GAPS: Current progress and future prospects. Advances in Space Research. Retrieved from http://coyote.astro.columbia.edu/~koglin/

Jantunen, M., & Audet, S.A. 1994. Surface passivated Si(Li) detectors for an X-ray detector array. Nuclear Instruments and Methods in Physics Research Section A, 353(1-3), 89-92.

17