Flying Balloons in Antarctica for Cosmic-Ray Research

By Eun-Suk Seo

Our Galaxy is filled with a relativistic gas of high-energy protons, electrons, and heavy nuclei. These energetic particles from extraterrestrial sources are a direct sample of matter from outside our solar system. The interstellar energy density of these so-called cosmic rays is ~1 eV cm ^-3 - comparable to the energy density of the galactic magnetic field or the thermal energy of the interstellar medium. Cosmic-ray energy spectra extend to a maximum energy above 10²⁰ eV, well above the energy that man-made accelerators can generate on Earth. (A single atomic nucleus at this energy has more energy than the fastest baseball ever thrown!) High-energy particles are an important and distinguishing feature of radio galaxies, quasars, and active galactic nuclei. The direct measurement by space and balloon experiments of their charge distribution, mass composition, and energy spectra provide information on the cosmic-ray source regions within our Galaxy, on their injection and acceleration processes, and their transport through interstellar space. Observations from radio, gamma-ray, and X-ray astronomy define the distribution of energetic particles throughout our Milky Way Galaxy and establish their presence in extragalactic sources. The interpretation of these observations is significantly aided by the detailed study of cosmic rays near Earth, while our understanding of the sources and distribution of galactic cosmic rays is strongly dependent on the data from these allied fields.

Professor Seo's research employs satellite and balloon-borne instruments to make direct measurements of these particles from space over more than six orders of magnitude (factor of a million) of the cosmic-ray energy spectrum. The names and overlapping energy ranges of the complementary projects she is pursuing to obtain precise data from ~10⁸to ~10¹⁵ eV are summarized in Fig. 1.

Fig. 1 All particle flux as a function of total energy per particle. ACCESS: Advanced Cosmic-ray Composition Experiment for the Space Station.AMS: Alpha Magnetic Spectrometer ATIC: Advanced Thin Ionization Calorimeter BESS: Balloon Experiment with a Superconducting Spectrometer CREAM: Cosmic Ray Energetics and Mass

These research projects address three basic themes: (1) searches for exotic matter such as antimatter and dark matter; (2) precise measurements of galactic cosmic rays in the energy range where they are most abundant (~10⁸ to ~10¹² eV) to understand their origin, acceleration, and propagation; and (3) precise measurements with large aperture instruments at higher energies (~10¹² to ~10¹⁵ eV) where the fluxes are extremely low, in order to explore the limit of supernova shock wave acceleration.

The BESS balloon-borne instrument has many of the same goals as AMS, a large space experiment almost ready for launch to the International Space Station. Equal amounts of matter and antimatter were produced at the beginning of the universe as described by the Big Bang scenario, and yet we now seem to see only matter around us. Since cosmic rays are a direct sample of matter from outside the solar system, they can probe the distant universe for the existence of antimatter. Antiparticles are exactly the same as particles except for the opposite charge sign, so they bend in opposite directions in a magnetic field. Both BESS and AMS employ state-of-the-art superconducting magnet technology to identify antiparticles as well as particles. BESS has been flown annually since 1993 to search for antimatter. AMS-01 was flown on a Space Shuttle in 1998, and AMS-02 is scheduled for launch to the International Space Station in 2008.

Professor Seo's ATIC and CREAM instruments are basically smaller scale prototypes of the ACCESS instrument, with the same science goals, which were given high priority in National Academy of Science reports. Whereas ACCESS would be a flagship space mission, she is planning a series of balloon missions with the ATIC/CREAM instruments, each lasting 10 - 100 days, to achieve many, if not most, of the ACCESS objectives. High energy particles are very rare, so large collecting power is required to obtain meaningful data. The instrument has to be large enough to collect a sufficient number of particles with good charge and energy resolutions, and yet light enough to be flown. Professor Seo received a 1997 PECASE award (Presidential Early Career Award for Scientists and Engineers) in recognition of her innovative approach for making high quality measurements of cosmic rays over an energy range that had not previously been possible. Precise measurements with these balloon-borne experiments provide unprecedented improvements in spectral data on the rare, high-energy cosmic rays, so they check the long-standing, still unproven theory of cosmic-ray acceleration in supernova (exploding star) remnants, one of the most powerful accelerators in the universe.

It should be noted that Professor Seo's precision measurements fill the gap between the space and ground based research activities of other groups on campus. They are particularly complementary to Professor Jordan Goodman's ground based observations. The ATIC, BESS, and CREAM balloon-borne instruments are based on particle detectors like those used at accelerators, but the payloads are like large space experiments. They are for the most part built in-house by students and young scientists, many of them currently working in the on-campus laboratory Professor Seo developed to pursue her scientific interests. Her current research group includes about 20 research scientists, engineers, technicians, graduate students, and undergraduate students. The BESS and CREAM payloads are both currently in Antarctica waiting for their balloon flights. Visit the home page of the cosmic-ray physics group http://cosmicray.umd.edu/homepage and follow the link to the CREAM project or go to the CREAM home page http://cosmicray.umd.edu/cream/cream.html to follow the development. Once each payload is launched, real time tracking of its balloon trajectory will be available along with some basic science instrument housekeeping data.

Fig. 2 A picture showing the crates containing the CREAM equipment after arrival at Williams Field, McMurdo, Antarctica, the buildings where the BESS and CREAM payloads are prepared for flight, and Mt. Erebus, an active volcano, in the background.

Fig. 3 A picture of the ATIC launch in Antarctica in December 2002.