Viktor I. Zatsepin

Advanced Thin Ionization Calorimeter (ATIC) balloon experiment: expected performance

An advanced thin ionization calorimeter (ATIC) will be used to investigate the charge composition and energy spectra of ultrahigh energy primary cosmic rays in a series of long- duration balloon flights. While obtaining new high priority scientific results, this balloon payload can also serve as a proof of concept for a BGO calorimeter-based instrument on the International Space Station. The ATIC technical details are presented in a companion paper at this conference. Here we discuss the expected performance of the instrument based on a GEANT code developed for simulating nuclear- electromagnetic cascades initiated by protons. For simulations of helium and heavy nuclei, a nucleus-nucleus interaction event generator LUCIAE was linked to the GEANT based program. Using these models, the design of the ATIC detector system has been optimized by simulating the instrument response to particles of different charges over the energy range to be covered. Results of these simulations are presented and discussed.

Upturn in the Heavy Nuclei to Iron Ratio in the ATIC Experiment above 100 GeV/Nucleon

It is argued that the upturn observed in heavy nuclei to iron ratios as measured in the ATIC experiment can be understood within a model of a closed galaxy with embedded local regions containing sources of cosmic rays. This model predicts a universal upturn near energies of 200–300 GeV/nucleon in the spectra of abundant primary nuclei. It also predicts the source spectral index to lie near 2.5.