More than 2000 proportional tubes have been produced at the University of Chicago. The tubes are 2 m long with a diameter of 2 cm. The walls consist of aluminized mylar, about 125 micro m thick. Printed circuit boards inside the manifolds provide high voltage for the proportional tubes and allow a capacitive read-out of the signal wires. Eight layers of tubes serve to measure ionization losses of high energy particles.
Transition radiation is measured in eight further layers, installed below radiator material fibers.
The preamplifiers are installed inside aluminum housings directly at the manifolds in order to reduce electronic noise. The crate is installed together with the hard drives for flight data recording, a main electronics crate as well as the high voltage supplies for the proportional tubes and the photomultiplier tubes inside a pressurized sphere. All other electronic components like the valves for the gas system and the main amplifiers and ADC's for the proportional tubes and the photomultiplier tubes are mounted on the instrument frame without a pressurized shell. The whole detector is mounted inside a 2.5 x 2.5 x 3 m 3 aluminum structure without a surrounding pressurized shell. The outer skin of the instrument is a 10 cm foam layer covered with aluminized mylar acting as thermal insulation .