Jörg R. Hörandel Homepage Joerg R. Hoerandel
Set-up of the TRACER instrument

TRACER is a new instrument for direct measurements of heavy cosmic ray nuclei (oxygen to iron) in the energy range from 1013 to several 1014 eV.

TRACER measures the nuclear charge, the energy, and the trajectory through the instrument. For charge measurements, two layers of plastic scintillators (2 x 2 m2), and one Cerenkov counter (2 x 2 m2) are used. A major challenge is the energy measurement. TRACER uses a transition radiation detector system which determines the Lorentz factor. This approach permits the construction of large-area detectors without requiring an exorbitant detector mass.

TRACER consists of eight double layers of proportional tubes which are oriented alternately in two orthogonal directions in order to determine the particle trajectory and thus to permit corrections in the data pathlength variations. The ionization losses of the particles are measured with the four upper double layers forming a proportional tube array. Four radiators of plastic fiber material, each followed by a double layer of proportional tubes, form a transition radiation detector to measure the particle energy. Two scintillators are placed on top and bottom of the detector stack, respectively acting as instrument trigger. In addition through measurement of the specific ionization, the charge is determined. A Cherenkov counter made of acrylic plastic at the bottom of the detector is used to reject non-relativistic particles.
The plastic scintillators are 1 cm thick, and are read out via wavelength shifter bars with 24 photomultiplier tubes. The Cerenkov signal are measured by 24 photo multiplier tubes.
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.
The proportional tubes are mounted in 16 sub assemblies ("manifolds"), each containing a double layer of 99 tubes.
Printed circuit boards inside the manifolds provide high voltage for the proportional tubes and allow a capacitive read-out of the signal wires.
16 manifolds with 99 tubes each are installed in TRACER forming an 2x2 m2 array of proportional tubes with 16 layers and 1584 proportional tubes in total.
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 wire signals of the proportional tubes are read with a low power integrated circuit (AMPLEX chip). The preamplifiers are installed inside aluminum housings directly at the manifolds in order to reduce electronic noise.
Heart of the TRACER electronics is a Intel 486 CPU inside a VME-crate. 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 ADCs 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.

Crush pads mounted below the instrument frame and around the four top corners provide a smooth landing.

For the measurements TRACER is flown on a helium filled balloon with a volume of 40000000 ft3 in an altitude of about 40 km (120 000 ft).

Jörg R. Hörandel