Assessment of low temperature X-ray detectors

X-ray detectors operating at temperatures below 1 K can provide high spectral resolution over a large bandpass with high efficiency. A number of different approaches to low-temperature X-ray detectors have been developed using a wide range of X-ray absorbing materials, sensor systems and amplifiers. These devices will provide new capabilities for basic research in many fields ranging from biophysics to astrophysics, as well as improved sensitivity for commercial materials analysis systems. We derive figures of merit for some applications of these detectors. We then discuss the operating principles of several different detector systems, as well as the factors limiting performance. A summary of the performance that has been achieved in some devices is presented, followed by a summary what might be achievable in the near future.