Precision X-ray transition radiation detection

Abstract

X-ray transition radiation detectors (TRDs) have traditionally been used exclusively as threshold devices in particle physics experiments. This is the case primarily because the large fluctuations in the signals from singly charged particles preclude accurate measurements of particle energy. However, for heavy nuclei, where the charge of the measured particle is larger, the possibility of making these measurements improves dramatically. This is due to a Z 2 increase in the transition radiation yield and the resultant 1/ Z reduction in the relative signal distribution widths. By exploiting these changes, a class of transition radiation detector can be built which provides excellent energy resolution for heavier nuclei ( Z≳3) over large ranges of particle energy. Such an instrument is an attractive option for the measurement of the energy spectra of cosmic ray nuclei at high energies. The flux of these particles is very small, so detectors with large exposure factors are required in order to make statistically significant measurements. Due to the purely electromagnetic nature of transition radiation, TRDs feature high area/mass ratios, and can provide the large geometric factors needed to make these measurements. Furthermore, unlike many cosmic ray detectors, TRDs can be fully calibrated at accelerator sites prior to deployment. In this paper, we discuss those TRDs designed to make high-resolution measurements of particle energies. We refer to this class of detector as precision TRDs, in contrast to the threshold devices found in accelerator experiments. We discuss some of the properties of these instruments, including the relevant design principles, and the physics which determines their performance. Additionally, we will look at a likely astrophysical application for precision TRDs, and discuss a possible design appropriate for this mission.