Abstract The BESS-Polar experiment was proposed for further studies of the lowenergy antiprotons and extensive searches for antinuclei in the cosmic radiation. To extend detectable energy range of the antiprotons down to 100 MeV at the top of the atmosphere, and to satisfy the requirements of long duration balloon flights, new payload components have been developed with features of an ultrathin superconducting solenoidal magnet and a dedicated solar battery system for the front-end and the data acquisition electronics. A technical flight is scheduled in September 2003, and the first scientific balloon flight will be conducted in 2004

A A Moiseev

A Itazaki

A Yamamoto

E S Seo

H Fuke

H Matsumoto

H Omiya

J F Ormes

J Nishimura

J Suzuki

J W Mitchell

K Abe

K Izumi

K Matsumoto

K Takeuchi

K Tanaka

K Tanizaki

K Yamato

K Yoshimura

M H Lee

M Nozaki

M Sasaki

N Ikeda

R E Streitmatter

S Haino

S Matsuda

T Hams

T Kumazawa

T Yamagami

T Yoshida

Y Makida

Y Shikaze

Y Takasugi

Y Yamamoto

CiteSeerX

28th International Cosmic Ray Conference 2081
BESS-Polar Experiment
T.Yoshida,1 A.Yamamoto,1 J.W.Mitchell,2 K.Abe,3 H.Fuke,4 S.Haino,4
T.Hams,2 N. Ikeda,3 A. Itazaki,3 K. Izumi,4 T.Kumazawa,1 M.H.Lee,5
Y.Makida,1 S.Matsuda,4 H.Matsumoto,4 K.Matsumoto,1 A.A.Moiseev,2
J.Nishimura,4 M.Nozaki,3 H.Omiya,1 J.F.Ormes,2 M.Sasaki,2 E.S. Seo,5
Y. Shikaze,3 R.E. Streitmatter,2 J. Suzuki,1 Y.Takasugi,3 K.Takeuchi,3
K.Tanaka,1 K.Tanizaki,3 T.Yamagami,6 Y.Yamamoto,4 K.Yamato,3 and
K.Yoshimura1
(1) High Energy Accelerator Research Organization, Tsukuba, 305-0801, Japan
(2) Goddard Space Flight Center, NASA, Greenbelt MD20771, USA
(3) Kobe University, Kobe, 657-8501, Japan
(4) The University of Tokyo, Bunkyo, Tokyo, 113-0033, Japan
(5) University of Maryland, College Park, MD20742, USA
(6) The Institute of Space andAstronautical Science, Sagamihara, 229-8510, Japan
Abstract
The BESS-Polar experiment was proposed for further studies of the low-
energy antiprotons and extensive searches for antinuclei in the cosmic radiation.
To extend detectable energy range of the antiprotons down to 100 MeV at the
top of the atmosphere, and to satisfy the requirements of long duration balloon
flights, new payload components have been developed with features of an ultra-
thin superconducting solenoidal magnet and a dedicated solar battery system for
the front-end and the data acquisition electronics. A technical flight is scheduled
in September 2003, and the first scientific balloon flight will be conducted in 2004.
1. Introduction
BESS (Balloon-borne Experiment with a Superconducting Spectrometer)
has been carried out since 1993 to investigate elementary particle phenomena in
the early Universe. Results of the previous BESS flights [4] show that cosmic-
ray antiprotons are predominantly produced by collisions of high-energy primary
cosmic-rays with interstellar matter. But possible existence of some novel pro-
cesses of the antiproton production in the Universe, such as evaporation of primor-
dial black holes [6,9] or annihilation of supersymmetric dark matter [8], cannot
be ruled out because the spectrum below 1 GeV seems to be softer than the
prediction of the secondary production models.
A direct search for cosmic antimatter has also been continued by BESS, to
pp. 2081–2084 c©2003 by Universal Academy Press, Inc.
2082
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Fig. 1. Cross-sectional views of the BESS-Polar spectrometer
investigate matter/antimatter asymmetry in the Universe. Our previous searches
have placed the most stringent upper limit on the ratio of antihelium to the helium
nuclei of 6.8 × 10−7 (95% C.L.) in the rigidity range between 1 and 14 GV from
null the result [7].
Based on these results from the BESS experiments, we are preparing a
new long-duration balloon-borne experiment to carry out further studies of low
energy antiprotons [11]. This “BESS-Polar” experiment aims to probe the early
Universe by searching intensively for antiprotons of cosmic origin. A search for
antimatter in the cosmic radiation will also be performed with higher sensitivity.
2. Development of the BESS-Polar detector
The design concept of the BESS-Polar spectrometer (Fig. 1) is basically
the same as that of the current BESS spectrometer [3]. A tracking system sitting
in a thin superconducting solenoid measures the curvature of the trajectory of the
incident particles. Time-of-flight plastic scintillation paddles will be mounted at
the top and the bottom of the spectrometer, in order to measure the velocity and
the energy deposit of the incident particles. A silica-aerogel Čerenkov counter
will also be utilized as a redundant particle identifier.
We have to reduce the weight of the payload to meet the requirements of
long duration flights over Antarctica, and must also reduce material thickness in
the payload, in order to measure antiprotons at the lowest possible energy. To
reduce weight and material thickness there will be no pressure vessel outside the
detector. The Čerenkov counter will be installed beneath the magnet to reduce
the thickness of materials in the upper half of the spectrometer, since the counter
is used to identify antiprotons in the high energy region. To maintain high trigger
efficiencies even for very low energy particles which cannot penetrate the lower
half of the solenoid, an additional scintillation counter system (Middle TOF) is
placed inside the magnet bore.
In order to minimize material thickness along the trajectory of the incident
particle, an ultra-thin superconducting solenoidal magnet has been developed.
2083
The key technology necessary to realize such a superconducting solenoid is the
development of high strength superconductor [10]. Adopting a high strength
aluminum stabilizer, the solenoid’s material including a cryostat was reduced
down to a half of the superconducting solenoid of the current BESS spectrometer
[12]. The superconducting magnet has successfully tested up to 0.84 Tesla with a
flight-model cryostat. The magnetic field was stable against an acceleration load
over 4 G. The magnet will be available after several modifications of the cryostat
to improve mechanical strength and life-time of cryogen.
To supply electric power continuously up to 20 days to the electronics
onboard, a new power supply system using solar cells should be developed. An
omni-directional solar-cell array structure has been designed to realize maximum
reliability of power production, and tested by a technical flight performed at
Sanriku Balloon Center, ISAS in Japan. In the flight, mechanical strength of the
array structure was proofed and the partially-mounted solar-cells were worked as
designed. The flight model of the solar-cell power system is now in production.
The tracking system consisting of drift chambers worked successfully in
the BESS scientific flight in 2002 [5]. TOF counters are under production and
new front-end electronics based on the ACE electronics are also being developed.
The design of the data acquisition system is basically the same as those of the
BESS detector, and the flight models are presently under development.
3. Schedule
As described above, detector components of the BESS-Polar spectrometer
are presently under development. Core components, such as the superconducting
spectrometer, a part of the solar battery system, and the communication system
will be examined by a technical flight at Fort Sumner, New Mexico, in September
2003. General integration of the spectrometer will be carried out after the tech-
nical flight. The first scientific flight is planned to be conducted either at either
Fairbanks (Alaska), Karlsbörg (Sweden) or Antarctica in 2004. Then, during the
period of the coming solar minimum, we are eager to conduct a second scientific
long duration flight over Antarctica in the 2006/2007 Antarctic summer in order
to optimize the search for the existence of antiprotons of cosmic origin.
4. Summary and future prospects
For extensive searches for antiprotons and antinuclei of cosmic origin, the
BESS-Polar experiment has been proposed. A new ultra-thin superconducting
spectrometer is being developed. A technical flight of the core components of the
BESS-Polar spectrometer is scheduled in September 2003, and the first and the
second scientific flights by long duration ballooning are planned to take place in
2004 and in the 2006/2007 Antarctic summer, respectively.
2084
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Fig. 2. Relative sensitivities for low en-
ergy antiprotons of three experiments
as a function of kinetic energy
For now two other space-based
experiments, AMS [2] and PAMELA [1],
are developed to investigate similar ob-
jectives. The observation by a long du-
ration balloon is much shorter compared
to the space-based experiment. How-
ever, the long-duration balloon flight can
stay at high geomagnetic latitude. Thus
BESS-Polar is still ideal for the search
for low-energy antiprotons of cosmic ori-
gin (Fig. 2). PAMELA will cover the
overall energy range, and the energy
range of BESS-Polar and AMS will over-
lap. BESS-Polar will provide a very
precise measurement of the spectrum of
the cosmic-ray antiprotons together with
other two independent and complementary experiments.
Acknowledgement
The author would like to thank NASA HQ, WFF BPO and NSBF for
their continuous encouragement in this US-Japan cooperative project. We also
wish to thank ISAS and KEK for their continuous support and encouragement.
This experiment is supported by KAKENHI(13001004) from MEXT in Japan,
and by NASA grants RTOP 188-05-10-01 for NASA/GSFC, and NAG5-5347 for
the UMD. Development of the thin superconducting solenoid had been partially
supported by the Japan Space Forum.
References
1. Adriani O. et al. 1995, Proc. 24th Intl. Cosmic Ray Conf. 3, 591
2. Ahlen S. et al. 1994, Nucl. Instr. Methods A350, 351
3. Ajima Y. et al. 2000, Nucl. Instr. Methods A443, 71
4. Asaoka Y. et al. 2002, Phys. Rev. Lett. 88, 051101, and references therein
5. Haino S. et al. 2003, To be appeared in this proceedings
6. Maki K., Mitsui T., Orito S. 1996, Phys. Rev. Lett. 76, 3474
7. Sasaki M. et al. 2002, Nucl. Phys. (Proc. Suppl.) B113, 202
8. Stecker F.W., Rudaz S., Walsh T.F. 1985, Phys. Rev. Lett. 55, 2622
9. Turner M.S. 1982, Nature 297, 379
10. Yamamoto A. et al. 1999, Nucl. Phys. (Proc. Suppl.) B78, 565
11. Yamamoto A. et al. 2002, Adv. Space Res. 30(5), 1253
12. Yamamoto A. et al. 2002, IEEE Trans. Applied Superconductivity 12, 438


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