Transition Radiation Detectors for Cosmic Rays Near the Knee

Precise observations of the energy spectra and relative abundances of cosmic-ray nuclei require instruments that exhibit individual charge resolution and a calibrated energy response. If energies up to ∼ 10 15 eV are to be covered, the low intensity of the heavier nuclei (Z 3) also mandates detector areas of several square meters. X-ray transition radiation detectors (TRDs) appear to provide the only practical means of fulfilling all of these requirements for balloon or space-borne instruments. However, for measurements up to the cosmic-ray “knee”, care must be taken that the energy response of the TRD does not saturate for Lorentz factors less than ∼10 5. We have designed detectors to meet this goal, and have successfully tested prototypes at an accelerator beam at CERN. We shall present and discuss the results of these measurements. 1

Proceedings of ICRC 2001: 2247 c

Precision transition radiation detectors (TRDs) can be an effective tool for determining the energy spectra of cosmic ray nuclei up to the energies of the knee. They offer unique measurement capabilities and embody design principles which are somewhat different from those of the threshold TRDs used in accelerator experiments. We will discuss some of the characteristics of these instruments, including the relevant design principles and the properties which determine their performance

Proceedings of ICRC 2001: 1612 c

The first balloon flight of the TRACER instrument in 1999 led to a new measurement of the energy spectra of cosmic ray nuclei from Z=8 to Z=26 at energies from a few GeV/nucleon to several TeV/nucleon. We will present and discuss the results, compare them with other recent measurements and examine the implications for current cosmic ray propagation and acceleration models. Finally, we will comment on the prospects of planned flights of the TRACER instrument on long duration balloons, and on the adaptation of the measurement technique to anticipated space missions

The Response of the TRACER Detector: Design, Calibrations and Measurements

TRACER (“Transition Radiation Array for Cosmic Energetic Radiation”) is currently the largest detector system for direct measurements of cosmic-ray nuclei on balloons. The instrument combines arrays of single-wire proportional tubes for measurements of specific ionization and transition radiation with large-area plastic scintillators and acrylic Cherenkov counters. We shall describe the response functions of the individual detector elements, and the correlations between them which make possible an unambiguous identification of heavy cosmic-ray nuclei (8 £ Z £ 26) by charge Z and energy E or Lorentz factor ¤ = E/mc ¥ , covering an energy range of four decades. 1

Measurements with TRACER: Discussion of results and future prospects

The individual energy spectra and relative abundances measured with TRACER, are compared with previous measurements in space and balloons, and with interpretations of air shower data. From the individual spectral slopes, we discuss constraints on models of cosmic-ray propagation through the galaxy. We also discuss the extrapolated high-energy abundances of the elements at the cosmic ray sources. The TRACER instrument is currently being refurbished for a second long duration balloon flight. The dynamic range of the measurement will then be extended to include the lighter cosmic ray nuclei, down to boron (Z = 5). 1

Antarctic Balloon Flight and Data Analysis of TRACER

The TRACER cosmic-ray detector was successfully flown from McMurdo, Antarctica in December 2003. The instrument has a geometric factor of 5 m ¢ ster and provided measurements of cosmic ray nuclei from oxygen to iron (Z=8 to Z=26). The analysis of the data begins with the reconstruction of the trajectory of each nucleus through the instrument. Subsequently, the elemental charge Z and the particle energy are measured from 0.5 to 10,000 GeV/amu. This process uses known response functions and fluctuations in response of the individual detector elements, and the procedures are verified with extensive computer simulations. The analysis is able to cleanly select the very rare events at the highest energies without contamination due to low energy background which is more abundant by about a factor of £ 10 ¤. 1

Proceedings of ICRC 2001: 1608 c

A new large-area detector system was constructed at the University of Chicago for direct measurements of heavy cosmic ray nuclei (oxygen to iron) up to about 10 TeV/nucleon. TRACER ("Transition Radiation Array for Cosmic Energetic Radiation") uses plastic scintillators to measure charge and a proportional tube array to measure energy via specific ionization and transition radiation. While TRACER is designed for circumglobal long-duration balloon flights, an initial 28-hour flight was conducted in Autumn 1999 from Ft. Sumner, New Mexico. We will discuss the performance of the detector and present first data from the balloon flight