Beam tests of the balloon-borne ATIC experiment

The Advanced Thin Ionization Calorimeter (ATIC) balloon-borne experiment is designed to perform cosmic-ray elemental spectra measurements from 50 GeV to 100 TeV for nuclei from hydrogen to iron. These measurements are expected to provide information about some of the most fundamental questions in astroparticle physics today. ATIC's design centers on an 18 radiation length (X0) deep bismuth germanate (BGO) calorimeter, preceded by a 0.75λint graphite target. In September 1999, the ATIC detector was exposed to high-energy beams at CERN's SPS accelerator within the framework of the development program for the Advanced Cosmic-ray Composition Experiment for the Space Station (ACCESS). In December 2000-January 2001 and again in December 2002-January 2003, ATIC flew on the first two of a series of long-duration balloon (LDB) flights from McMurdo Station, Antarctica. We present here results from the 1999 beam tests, including energy resolutions for electrons and protons at several beam energies from 100 to 375 GeV as well as signal linearity and collection efficiency estimates. We show how these results compare with expectations based on simulations and their expected impacts on mission performance.