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 ATIC - Advanced Thin Ionization Calorimeter

    ATIC is an investigation directed to resolving fundamental questions about the shape of the elemental differential energy spectra from the low energy region around 10¹⁰ eV through the highest practical energies, about 10¹⁴ eV. This ATIC investigation takes advantage of the existing NASA long-duration balloon flight capability in Antarctica and/or the Northern Hemisphere (e.g. Fairbanks). It offers the best opportunity to measure the proton and helium spectra over more than four decades in energy with a single instrument. Concurrently, it will also measure the spectra of heavy nuclei up to iron, with individual element resolution and superior energy resolution. The totally active BGO calorimeter, 22 radiation length thick, will measure the electromagnetic energy ensuing from nuclear interactions in a one interaction length thick carbon target. Trajectory information will be obtained from the location of the cascade axis in the BGO calorimeter and in the segmented scintillator layers of the upstream carbon target. The highly segmented charge module comprised of scintillator strips, a silicon matrix, and a Cherenkov array will minimize the effect of backscattered particles on primary charge measurements.

The science to be addressed by ATIC, i.e., the energy spectra of H and He in the ultrahigh energy regime have emerged as one of the outstanding questions in high energy particle astrophysics with implications for particle acceleration in Supernova remnants or other exotic objects, transport of particles through the interstellar medium and the nature of the cosmic ray sources. The importance of this science has been given high priority in two recent National Academy of Science reports, the Cosmic Ray Report by the Board on Physics and Astronomy; the Space Physics strategy report done by the Space Studies Board joint Committees on Solar and Space Physics (CSSP) and Solar Terrestrial Physics (CSTR), as well as by the 1994 Snowmass Summer Study on Particle and Nuclear Astrophysics and Cosmology in the Next Millennium.