FIGURE 2. ATIC-1 and ATIC-2 spectra at balloon altitude, showing good agreement with each other.

From the following article:

An excess of cosmic ray electrons at energies of 300–800 GeV

J. Chang, J. H. Adams, H. S. Ahn, G. L. Bashindzhagyan, M. Christl, O. Ganel, T. G. Guzik, J. Isbert, K. C. Kim, E. N. Kuznetsov, M. I. Panasyuk, A. D. Panov, W. K. H. Schmidt, E. S. Seo, N. V. Sokolskaya, J. W. Watts, J. P. Wefel, J. Wu & V. I. Zatsepin

Nature 456, 362-365(20 November 2008)



The measured primary electron flux (scaled by E3) at flight altitude is shown for ATIC-1 (open squares) and ATIC-2 (filled circles). The errors are one standard deviation. Both balloon flights were from McMurdo, Antarctica, and circumnavigated that continent. ATIC-1 was a test flight in 2000–01 and the usable data correspond to an exposure of 0.61 m2 sr days. ATIC-2 was a science flight in 2002–03 with an exposure of 2.47 m2 sr days. To eliminate edge effects, we restrict the incident zenith angle to be less than approx37° (cos theta greater than or equal to 0.8), use only the central 80% of the SiM and eliminate events in the outer crystals in each BGO layer. Within these limits, the electron detection efficiency above 60 GeV is 84% essentially independent of energy. The effective acceptance was determined as a function of particle energy considering the trigger efficiency, trajectory reconstruction efficiency and the geometrical restrictions. The effective acceptance of the instrument increases from 0.075 m2 sr at 20 GeV to 0.15 m2 sr for E > 60 GeV. Above 100 GeV, a total of 1,724 electron events were observed, with the highest energy event at 2.3 TeV. The total background is also shown in the figure as the open triangles and is a combination of unresolved protons, unidentified italic gamma-rays and atmospheric secondary electrons produced in the material (approx4.5 g cm-2) above the instrument. ATIC becomes background limited for electrons only above several teraelectronvolts.

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