To achieve its scientific objectives, the ATIC experiment must be capable of measuring the incident cosmic ray charge and energy over an energy range of 50 GeV to >100 TeV. A schematic of the instrument, with pressure vessel removed, is shown in Figure 1 with a picture of the instrument as of August, 2000 in Figure 2.  This 
Figure 1:  Schematic of the ATIC instrument
instrument is based on the technique of ionization calorimetry, the most practical method of energy determination for cosmic ray nuclei from H to Fe over the target energy range. The fully active ATIC calorimeter is composed of 10 layers of Bismuth Germanate (BGO) scintillating crystals and is located on the bottom of the instrument. In Figure 1 and 2 the calorimeter is on the bottom. Above the calorimeter is the target section consisting of three plastic scintillator strip hodoscopes to define the instrument aperture and provide redundant charge and trajectory measurements, as well as layers of inert carbon (between hodoscopes) to provide a volume for the incident particles to interact.  On the top of the detector stack is the highly segmented

 silicon matrix detector that provides an accurate measure of the incident particle charge even in the presence of shower particles backscattered from the calorimeter.  Surrounding the detector stack, electronics bays hold the flight computers, readout electronics, power system boards and other instrument electronics. Finally, on each of the four corners three struts transfer the loads of the experiment through the pressure vessel ring to an external structure (see Figure 2) that attaches to the balloon. All components of the experiment were designed with Antarctica long duration balloon flight weight, power and recovery constraints in mind. The total weight of ATIC is about 1,500 kg (3,300 lbs), the total power consumed is less than 350 Watts (including power conversion efficiency), and the payload is designed to be quickly field stripped under Antarctic conditions. The geometrical factor of ATIC varies from 0.45 m² sr (calorimeter top) to 0.24 m²  sr (calorimeter bottom).

Figure 2: The ATIC instrument in the support frame