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Thermal Detectors as Single Photon X-Ray Spectrometers

Thermal detectors operating at cryogenic temperatures can be used as sensitive microcalorimeters for measuring small pulses of energy such as produced by the absorption of individual X-ray photons. This scheme has two major advantages for photon detection. First, in principle, all the X-ray energy can be degraded into phonons to which we are sensitive, thus avoiding the statistical noise associated with the partition of the energy into several channels. Second, the limiting noise of the energy measurement is set by thermodynamic fluctuations. At low temperatures, these fluctuations in practical devices will allow energy resolution on the order of a few electron volts (eV) FWHM. We present results of experiments performed to evaluate the characteristics of thermal detectors. We find that our measurements agree well with the the general theory of thermal detectors. Specifically, the magnitude of the equilibrium temperature fluctuations agrees well with that predicted by thermodynamics for the detectors tested. The completeness of energy thermalization, however, varies among the devices tested. We will discuss this as it affects detector energy resolution.