An ULDB Mission to Study High Energy Cosmic Rays

Scientific ballooning technology will soon allow flights of about 100 days at altitudes in excess of 110,000 feet. Utilizing these Ultra Long Duration Balloon (ULDB) flights, the Cosmic Ray Energetics And Mass (CREAM) project will measure the energy spectra and elemental abundances of H to Fe over the energy range 1 to 1000 TeV. The goal is to observe spectral features and/or abundance changes that might be related to a supernova acceleration limit. The CREAM instrument will consist of a sampling tungsten calorimeter preceded by a carbon target with scintillator layers for trigger and track-reconstruction purposes, a transition radiation detector (TRD) for heavy nuclei, and a segmented scintillator-based charge detector. In this paper, we focus on an overview of the project, while an accompanying paper at this conference (Beatty et al., 1999) will discuss some technical aspects. A key feature of this instrument is the measurement of the energy of a subset of nuclei by complementary techniques, which can be used to intercalibrate the energy scales of the TRD and calorimeter. 1 The CREAM Mission: Cosmic-ray elemental composition has been measured by space-based experiments that determine the incident particle charge as well as its energy. However, the composition above 1 TeV is not very well known due to limited exposures for such experiments. Indirect measurements from ground-based experiments have traced the all-particle spectrum from about 10 eV to > 10 eV. These measurements have shown that the energy spectrum above 10 eV is somewhat steeper than the spectrum below 10eV. An explanation for this change in spectral shape, the spectral “knee,” is one of the major current goals in cosmic-ray astrophysics. Acceleration of cosmic rays in supernovae remnants is expected to be limited at energies near Z x 10 eV, where Z is the particle charge (Lagage & Cesarsky, 1983). This implies that the composition would begin to change beyond energies of about 10 eV. A search for changes of the elemental composition in the decade above 10 eV could reveal this supernova acceleration limit. As shown in Fig. 1, CREAM consists of a particle charge detector at the top, followed by a TRD, a target for nuclear interactions, and a calorimeter to measure the interaction products, thereby giving particle energy estimates. A large exposure can be accumulated in a series of ULDB flights of identical instruments. The different flights could be carried out at essentially any latitude, including the polar regions, in either the Northern or Southern hemisphere. The overall CREAM objective is to accumulate at least 500 particles each for protons, helium, CNO, Ne-Si, and Fe Figure 1: CREAM Baseline Configuration. Calorimeter Charge Detector Target 1 Target 2 TRD1