PSB1-0046-02 1

BESS-Polar experiment

T. Yoshida1, A. Yamamoto1, J. Mitchell2, K. Abe3, H. Fuke1,3, S. Haino1,3, T. Hams2, N. Ikeda4, A. Itazaki4, K. Izumi1,3, M. H. Lee5, T. Maeno4, Y. Makida1, S. Matsuda3, H. Matsumoto3, A. Moiseev2, J. Nishimura3,

M. Nozaki4, H. Omiya1, J. F. Ormes2, M. Sasaki2, E. S. Seo5, Y. Shikaze4, A. Stephens2, R. Streitmatter2, J. Suzuki1, Y. Takasugi4, K. Tanaka1, K. Tanizaki4, T. Yamagami6, Y. Yamamoto3, K. Yamato4, and K. Yoshimura1

1 High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan 2 Goddard Space Flight Center / NASA, Greenbelt, MD 20771, USA

3 The University of Tokyo, Bunkyo, Tokyo 113-0033, Japan 4 Kobe University, Kobe, Hyogo 657-8501, Japan

5 University of Maryland, College Park, MD 20742, USA 6 Institute of Space and Astronautical Science (ISAS), Sagamihara, Kanagawa 229-8510, Japan

ABSTRACT

For investigating elementary particle phenomena in the early Universe, the BESS-Polar experiment was proposed for highly sensitive studies of the low-energy antiprotons and extensive searches for antinuclei in the cosmic radiations. A new superconducting spectrometer is being developed for long-duration balloon flights. In order to extend detectable energy range of the antiprotons down to 100 MeV, materials along the trajectory of the incident particle is minimized. The spectrometer will be completed in 2003, and the first long-duration flight is planned in 2004.

INTRODUCTION

Observations of anomalous excess of antiproton flux (Bogolomov et al. 1979, Golden et al. 1979, Buffington et al. 1981), compared to the predictions by the secondary production models, had suggested existence of novel processes of the antiproton production. To search for low-energy antiprotons of cosmic origin, BESS (Balloon-borne Experiment with a Superconducting Spectrometer) experiment was proposed in 1987 (Orito 1987).

Since 1993, seven successful flights were carried out at Lynn Lake, Canada. More than two thousand antiprotons were definitely identified in a kinetic energy range between 150 MeV to 4.2 GeV by 2000 (Yoshimura et al. 1995, Moiseev et al. 1997, Matsunaga et al. 1998, Orito et al. 2000, Maeno et al. 2001, Asaoka et al. 2002). As shown in Figure 1, a characteristic peak of the antiproton spectrum around 2 GeV was clearly measured. The results show that the cosmic antiprotons are dominantly produced by collisions of high-energy primary cosmic-rays with

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interstellar matters. And propagation models in the galaxy are basically consistent. However the spectrum below 1 GeV in the solar minimum period seems to be softer than that predicted by the secondary production models. Of course we have still not had enough statistics and propagation models have large uncertainties, but we cannot rule out possible existence of some exotic processes of the antiproton production such as evaporation of primordial black holes (Turner 1982, Maki et al. 1996) or annihilation of supersymmetric dark matters (Stecker 1985, Bergström et al. 1999) in the Universe.

The flux of the primary antiprotons, if they exist in the cosmic radiations, should be strongly affected by the solar modulation (Mitsui et al. 1996), because models of the primary antiproton production predicted flat spectra in low energy region. Thus it is very important to estimate the solar modulation effects on the low-energy antiprotons. Charge dependent solar modulation effects were pointed out (Bieber et al. 1999), i.e., negative charged particles might have different modulation effects from positive ones, and these effects used to be studied by the comparison between the modulation effects to protons and to electrons. The recent BESS result confirmed the charge dependence using high statistical samples of the protons and the antiprotons (Asaoka et al. 2002).

Searches for cosmic antimatters have been continued to investigate matter/antimatter asymmetry in the Universe. Under several assumptions, we can place very strong constraints on the distance to the antimatter domains by the γ-ray observation. But direct search for antimatters in the cosmic radiation is still meaningful. In the BESS flights no antihelium candidate was found (Ormes et al. 1997, Saeki et al. 1998, Sasaki et al. 2001). Sasaki et al. (2001) placed an upper limit on a ratio of antihelium to the helium nuclei of 6.8 × 10-7 (95% C.L.) in a rigidity range between 1 and 14 GV, which is the most stringent upper limit ever achieved (Figure 2). Since too large uncertainties exist in the models of the cosmic-ray propagation between galaxies, we cannot rule out existence of the antimatter domains in the Universe from this null result. But this result is the most direct evidence that our galaxy and its nearby consist of matters.

Based on these results from the BESS experiments, we prepare a new balloon-borne experiment to carry out further study of the low energy antiprotons (Yamamoto et al. 2001, Yamamoto et al. 2002a). Primary purpose of this “BESS-Polar” experiment is an intensive search for antiprotons of cosmic origin, which should be a probe of the early Universe. Since the flux of the primary antiprotons will be strongly suppressed by the solar wind, we need to have a high statistical observation at next solar minimum period. The high precision measurement enables to search for cosmic antiprotons with very high sensitivity, and the precise antiproton spectrum will provide a basic data to check propagation models and to study solar modulation processes.

BESS-POLAR SPECTROMETER

As shown in Figure 3, a concept of the BESS-Polar spectrometer is basically same as the one of the current BESS spectrometer (Ajima et al. 2000, Shikaze et al. 2000, Asaoka et al. 1998). A thin superconducting solenoid provides a strong magnetic field of 0.8 Tesla, and a tracking system consisting of a jet-type drift chamber (JET) and two cell-type drift chambers (Inner DC) measures a curvature of the trajectory of the incident particle. At the top and the bottom of the spectrometer, time-of-flight (TOF) plastic scintillator paddles will be mounted to measure

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Fig. 2. Upper limit on the ratio of antiheliums to helium nuclei.

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the velocity and the energy deposit of the incident particle. A silica-aerogel Čerenkov counter will also be installed as a redundant particle identifier.

We have to reduce the weight of the payload to meet requirements from the long duration flights at Antarctica and also reduce material thickness in the payload to measure the antiprotons at the lowest possible energy. A pressure vessel outside of the detector is eliminated to reduce the weight and the materials, so photomultipliers and high voltage supplies should work in vacuum. The silica-aerogel Čerenkov counter will be placed beneath the magnet to reduce the materials of the upper half of the spectrometer, since the counter is utilized for the antiproton identification in a high energy region. And an additional trigger scintillation counter system (Middle-TOF) is installed inside the magnet bore to keep high trigger efficiencies for the very low energy particles. Comparisons between the BESS and the BESS-Polar spectrometer are given in Table 1.

Furthermore, in order to study very low energy antiprotons by long duration flights, we have to overcome several technical challenges. To measure antiprotons down to 100 MeV, we need to minimize material thickness along the trajectory of the incident particles. We have developed an ultra-thin superconducting solenoid. It is also inevitably required to develop a new power supply system for the long duration flights.

Ultra-Thin Superconducting Solenoid The key technology to realize an ultra-thin superconducting solenoid is development of the high strength

superconductor (Yamamoto et al. 1999). Recently new aluminum stabilizer was developed with micro alloying of 0.5 % nickel followed by a cold-work hardening. Figure 4 shows a scanning electron microscope (SEM) image of the cross section of the aluminum stabilizer (Wada et al. 2000). In an aluminum stabilizer of the superconductor, pure aluminum domain acts as a conductor and the aluminum-nickel composite part acts as reinforcing. When the current BESS solenoid was fabricated, yield strength of the superconductor is around 100 MPa, but now, as shown

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Fig. 3. Cross-sectional view of the BESS-Polar spectrometer.

Table 1. Comparison between the BESS and the BESS-Polar spectrometer

BESS BESS-Polar Acceptance (m2str) 0.3 0.3 Magnetic field (T) 1.0 0.8 Superconducting coil diameter (m) 1.0 0.9 Cryogen life (days) 5.5 20 JET/IDC diameter (m) 0.83 0.76 Weight (kg) 2,400 1,500 Power source Primary batteries Solar cells Minimum thickness for trigger generation (g/cm2) 18 4.5

Detectable antiproton energy (GeV) 0.18~4.2 0.1~4.2 MDR (GV) 200 150

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in Figure 5, overall yield strength can be reached up to 240 MPa adopting high strength aluminum stabilizer.

Equivalent stress of the two-layer solenoid (Figure 6) calculated as 174 MPa at 1.2 Tesla is well below the yield strength of the high strength superconductor, so we could eliminate a thick structural cylinder of the solenoid, which used to keep the mechanical strength of the solenoid. Thus the material of the solenoid was reduced down to 1.0 g/cm2 and total thickness of the magnet including a cryostat will be about 2.0 g/cm2 (Yamamoto et al., 2002b).

Superconducting solenoidal coil itself has already been wound, and was tested up to 1.05 Tesla successfully. The coil is being assembled into the cryostat. In the new cryostat, a large reservoir tank for 400 ℓ liquid helium will be also assembled, which enables 20 day operation of the magnet.

Solar-Cell Power Supply System To supply electric power to the electronics onboard the payload, new power supply system using solar cells

are also developed. The power consumption of the BESS-Polar payload is estimated to be around 600 W, so taking into account efficiencies of the DC-DC converters, losses at cables and so on, solar cells should produce about 900 W continuously. The weight of the power supply system is also limited to be less than 300 kg in order to make total weight of the payload less than 1,500 kg.

We have designed an omni-directional solar cell array structure shown in Figure 7 to achieve highest reliability on the power production. Each side is designed to have same area so that no attitude control is required. And for now we do not plan to use re-chargeable battery system to realize the simplest system. Table 2 summarizes the specification of the solar cell module Sharp NT3436BD (Sharp Corp. 1991), which will be used because of its high efficiency, fabricating with a bypass diode, and light weight.

But the solar cell array structure becomes quite large to produce 900 W electric power. In order to confirm mechanical strength of the designed structure of the solar cell array, and also justify the simulation results on the

Fig 4. SEM image of high strength aluminum-nickel alloy.

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temperature and power generation during the flight, we had a technical flight at the Sanriku Balloon Center in Japan in May 2002.

Total weight of the payload (Figure 8) was about 380 kg including the house keeping system, which monitored temperatures of the panels, output voltages and currents, and solar inputs.

During the flight, solar cell array structure was monitored by a video camera installed at the lower end of the parachute. The appropriate performance was found on the launching and the termination. House keeping data were transmitted to the ground by the telemetry system and were analyzed to check the consistencies between the data and the simulation results. In Figure 9, dots indicate the temperature on the panel structure measured during the flight. Two envelop lines show the simulated maximum and minimum temperature on entire structure. In Figure 10, dots and circles indicate measured and simulated output voltage of the solar cell array, respectively. Both figures show good agreements between measured and simulated results. Thus we concluded that we could well-reproduce the results by the simulation and our basic design of the solar cell power supply system was justified.

Particle Detectors Particle detectors are also being developed.

Construction of the drift chambers for the tracking of the incident particles has already been completed. These chambers had installed into the conventional BESS payload, and had a scientific flight in the summer of 2002. The expected spatial resolution below 200 µm was confirmed by the data taken during the flight. Since the flow of the drift gas should be kept during the BESS-Polar flights to maintain its purity without a complex gas mixing system, they have to be operated

Table 2. Specification of the Sharp Solar cell module NT3436BD. Condition is as the irradiance of 1,000 W/m2 and the module temperature of 25 ºC

Cell Multi-crystal silicon solar cells, 97.5 × 75 mm No. of cells and connection 36 in series Dimensions 703 × 399 × 1.5 mm Weight 660 g Electro-Optical Characteristics Typical Open circuit voltage 22.1 V Maximum power voltage 17.8 V Short circuit current 2.70 A Maximum power current 2.53 A Maximum power 45.0 W Encapsulated solar cell efficiency 17.1 % Module efficiency 16.0 %

Fig. 8. Dummy structure for the Sanriku technical flight.

Fig. 7. Conceptual design of solar cell array structure for the BESS-Polar experiment.

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under the pure CO2 gas instead of the mixture of CO2 and argon utilized in the current BESS experiment. An operation test under the pure CO2 condition was successfully done and the gas flow control system is under development.

The time-of-flight scintillation counter system was designed with 10 and 12 scintillator paddles installed at the top and the bottom of the spectrometer, respectively. Each paddle has a dimension of approximately 100W × 950L × 10t mm. To reduce material along the trajectory of the incident particle, thinner scintillation paddles are adopted compared to the BESS detector. On both ends of the paddle, Hamamatsu R6504 2.5-inch fine-mesh photomultipliers will be mounted. A time-of-flight resolution of 100 ps is expected.

Silica aerogel blocks as a Čerenkov radiator are under production by Matsushita Electric Works. The refractive index of the block is 1.020, so the antiprotons up to 4 GeV will be identified from the enormous backgrounds of electrons. Čerenkov light will be collected by the R6504 photomultipliers.

Production of the photomultipliers for the time-of-flight counters and the aerogel Čerenkov counter was completed. High voltage supply modules are also in production.

A new Middle-TOF counter system consists of 64 rods of the plastic scintillator installed inside the bore of the solenoid. Each rod has a cross section of 5 × 10 mm and a length of 1 m. The light emitted will be transferred to newly-developed 2.5-inch fine-mesh 8-anode photomultipliers (Hamamatsu R6504MODX-M8) through bundles of the clear plastic fibers. The Middle-TOF counters will generate trigger signals for the low energy particles which cannot penetrate lower half of the magnet. The time-of-flight information will also be provided. A time resolution around 500 ps is to be obtained, which is enough to identify antiprotons below 150 MeV from the backgrounds.

Low-power electronics for the photomultipliers are now developed based on the ACE electronics (Yamato et al., 2002). On each front-end module, an onboard data signal processor (DSP) will be installed for the parallel processing in order to reduce dead-time of the data acquisition system. A low power flash ADC system is also being developed for the readout of the drift chambers. Signals from drift chambers over 500 channels will be digitized with a 40 MHz sampling speed. Data the front-ends will be transferred to the event building system via USB 2.0 serial connections.

The event building system consists of an online event reduction subsystem and a storage subsystem. Though we intend to carry an onboard large storage system up to several terabytes to be realized by a hard disk drive complex, strong online event reduction will be required to reduce the size of the event data. New online event reduction system is now being developed based on the Hitachi SH4 CPU and the Linux operation system.

A house keeping system is developed on the platform of Echelon LonWorks. The LonWorks network consists of modules of Neuron integrated circuits distributed over the payload. They communicate each other by a message-

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based protocol called as LonTalks. Each Neuron integrated circuit will control one of the command interpretations, telemetry coding, environment monitoring, and so on.

COMPARISON WITH OTHER SPACE EXPERIMENTS

Two space-based experiments are being developed to investigate similar physics objectives. One is the Alpha Magnet Spectrometer (AMS) and the other is a Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA). Table 3 summarizes a comparison between BESS-Polar, PAMELA and AMS. PAMELA is a compact detector for a polar-orbital satellite. The polar-orbital satellite passes the area of very low rigidity cut-off, so that it can cover down to 80 MeV antiprotons (Adriani et al., 1995). AMS has a large exposure factor and a strong particle identification capability (Ahlen et al., 1994). It can provide very precise cosmic-ray measurements above several hundred MeV. PAMELA will be launched in 2003 by a Russian rocket for three year observation and AMS will be launched by the space shuttle in October 2005 and will stay at the International Space Station for three years.

Figure 11 shows relative sensitivities for the antiprotons of three experiments as a function of the kinetic energy. Sensitivity means a product of the exposure factor, observation time and the efficiency to stay at the cut-off rigidity region where antiprotons of specific kinetic energy can be observed. A long duration ballooning at the high latitude is ideal for the low energy antiproton measurements since the LDB flights can stay at the lowest cutoff rigidity region.

Between 300 MeV to 4 GeV, BESS-Polar and AMS have an overlapped energy range and PAMELA will cover overall energy range. So in near future we will have a very precise spectrum of the cosmic-ray antiprotons by these three independent and complementary experiments.

FLIGHT PLAN

All components of the BESS-Polar spectrometer are expected to be ready by March, 2003. Assembly of the spectrometer will then take three months in KEK, Japan, and will be completed by June, 2003. We expect to have a technical flight in Fort Sumner, New Mexico, USA, in the autumn, 2003.

Table 3. Comparison between PAMELA, BESS-Polar and AMS

Project PAMELA BESS-Polar AMS02 Flight Vehicle Satellite LDB ISS Flight Duration 3 years 10~20 days 3~5 years Altitude 300~600 km 37 km (5 g/cm2) 320~390 km Orbit 70.4º >70ºS Lat. 51.7º Acceptance 0.0021 m2str 0.3 m2str 0.3 m2str MDR(GV) 740 150 ~1000 Particle identification TOF/TRD/CAL TOF/ACC TOF/TRD/RICH/CAL Number of Helium 4 × 107 (1~2) × 107 2 × 109 Launch 2003 2004 October 2005

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Fig. 11. Relative sensitivities for low energy antiprotons.

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Though we expect to have the first long-duration flight, it is not clear for us to have an Antarctic balloon flight in 2004. We are considering an alternative option to have a long-duration flight in the northern hemisphere. In case of a flight from Fairbanks, Alaska, additional primary batteries should be carried to provide sufficient power during the nights. Larger solar-cell array might be required due to the flight in the lower latitude compared to the Antarctica.

In the 2006/2007 Antarctic summer, when the solar activity will become minimum, we eager to have the second long-duration flight at Antarctica in order to search for the existence of the antiprotons from cosmic origin.

SUMMARY

The BESS has been carried out to investigate elementary particle phenomena in the early Universe. In the seven successful flights from 1993 to 2000, two thousand low-energy antiprotons were definitely identified in a kinetic energy range between 150 MeV and 4.2 GeV. A characteristic peak of the antiproton spectrum around 2 GeV was clearly measured, and the result explains that the cosmic antiprotons are dominantly produced by collisions of high-energy primary cosmic-rays with interstellar matters. The spectrum below 1 GeV, however, seems to be softer than that predicted by the secondary production models. We still cannot rule out possible existence of some novel processes of the antiproton production such as evaporation of primordial black holes or annihilation of supersymmetric dark matters in the Universe. For antinuclei searches, an upper limit of 6.8 × 10-7 has been placed on the ratio of the antihelium nuclei to the helium nuclei.

For extended studies of the low-energy antiprotons and extensive searches for antinuclei in the cosmic radiations, the BESS-Polar experiment was proposed. In order to extend detectable energy range of the antiprotons down to 100 MeV at the top of atmosphere, a new superconducting spectrometer is now being developed. A recent development of high-strength aluminum-stabilized superconductor enables to realize an ultra-thin superconducting solenoid with a transverse material thickness of 2 g/cm2. With further efforts to minimize materials along the trajectory of the incident particles, such as by adopting thinner scintillation counter for a time-of-flight measurement and by avoiding thick walls of the pressure vessel, the detector material in an upper-half wall becomes 4.5 g/cm2, equivalent to a half of the current BESS detector. The new scintillation counter hodoscope will be placed inside the bore of the solenoid to issue triggers of the low energy particles which cannot pass through the lower-half of the detector.

The BESS-Polar detector was designed to meet requirements from long duration balloon flights in Antarctica. The total weight of the payload will be about 1,500 kg. A dedicated solar battery system providing 600 W electric power is also developed for the front-end electronics and the data acquisition system.

A technical flight of the solar-cell array structure was carried out at Sanriku Balloon Center in Japan in May 2002. The BESS-Polar spectrometer will be available by June 2003. The first BESS-Polar flight in Antarctica is planned in December 2004, and the second one is expected to be carried out in the solar minimum around 2006/2007 Antarctic summer.

ACKNOWLEDGMENTS

The authors would like to thank Dr. W. V. Jones of the NASA Headquarters for his continuous encouragement for this US-Japan cooperative project. Sincere thanks are expressed to the NASA Balloon Project Office at GSFC/WFF and National Scientific Balloon Facility (NSBF) for their experienced support. They would thank ISAS and KEK for their continuous support and encouragement. This experiment is supported by Grant-in-Aid for Specially Promoted Research (13001004) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) in Japan and by NASA grants RTOP 188-05-10-01 for NASA/GSFC and NAG5-5347 for the University of Maryland in the US. Development of the thin superconducting solenoid has been carried out as a part of “Ground Research Announcement for Space Utilization” promoted by Japan Space Forum.

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E-mail address of T. Yoshida tetsuya.yoshida@kek.jp Manuscript received ; revised , accepted