CREST Program Overview
Electrons of energy beyond about 2 TeV have never been detected in the flux of cosmic rays at Earth, despite indirect evidence of their presence in at least one supernova remnant (SN 1006, from observation of non-thermal x-rays). The detection of high energy electrons at Earth would be extremely significant, yielding information about the spatial distribution of nearby cosmic ray sources. Electrons lose energy rapidly as they propagate through the Galaxy, due to synchrotron losses in the galactic magnetic fields and inverse Compton interactions with the cosmic microwave background radiation. Consequently, a 1 TeV electron observed at the Earth would have to originate within a distance of less than 1 kpc. If high-energy cosmic-ray electrons originate in supernova shock acceleration processes, as is the current paradigm for cosmic rays in general, there would be few known supernova remnants within the requisite distance from which these particles could originate. The spectral shape of high energy electrons should, therefore, be strongly effected by the number of nearby sources, and their distribution of distances. Conversely, if no such features in the high energy electron spectrum are observed it will call into question our understanding of cosmic ray sources and propagation.
Current and past electron detectors, typically flown by high altitude balloons, have been limited in their ability to study high energy electrons by their short exposure times and small apertures. To date, no measurements have been made at energies greater than 2 TeV. We propose a radically different approach to the detection of electrons of energies between 2 and 50 TeV (and beyond). With an ultra-long duration (100 day) balloon flight of an array of BGO crystal detectors (the Cosmic Ray Electron Synchrotron Telescope, or CREST instrument), we will detect the synchrotron photons generated at x-ray energies by these electrons in the Earth’s magnetic field. Several such photons will appear in characteristic linear arrangements in the crystal array, yielding a clear signature with little background. This technique results in a substantial increase in the acceptance and sensitivity of the apparatus. Simulation studies indicate that with a 100 day flight of a 2 × 2 m2 crystal array, as many as 250 such electrons will be detected with energies greater than 2 TeV, with an expected background due to random alignment of non-related signals at the level of only 1 event.
We propose to carry out this program of measurements in three phases. First, we will fly a small prototype instrument on a conventional balloon from North America to validate the techniques and measure all backgrounds. Next we will carry out an LDB flight of the full instrument, providing a ‘pathfinder’ measurement of high energy electron spectra. The culmination of the CREST program will be a full 100 day mission in Antarctica with the full sized instrument.