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X-ray Microcalorimeter

Microcalorimeters offer a promising technology for high throughput, high resolution X-ray spectroscopy. A microcalorimeter is, in essence, an ultra sensitive thermometer. It measures the energy of each incident X-ray photon by sensing a tiny increase in temperature of the absorber and then converts the temperature pulse to a measurable electrical pulse, typically by making use of the fact that its electrical resistance of a microcalorimeter is a strong function of temperature. It recovers and is ready for the next X-ray photon, when the heat generated leaks from the absorber to the heat bath of constant temperature; the more quickly the leakage occurs the faster the pulse decays. The accuracy to which a microcalorimeter measures the photon energy is fundamentally limited by thermodynamic fluctuations in the exchange of thermal energy between the absorber and the heat bath, which is proportional to the temperature squared and also to heat capacity. To achieve good spectral resolution, therefore, it is necessary to operate the microcalorimeter at cryogenic temperatures (typically < 100 mK).

I particpated in the development of semiconductor-based X-ray calorimeters and, subsequently, in the successful demonstration of operating such devices as excellent X-ray spectrometers in space in sounding rockets flights, as a graduate student at UW-Madison. Those devices are made of heavily dopped silicon. At sufficiently low temperatues, they conduct electricity via a "hopping" mechanism, with their electrical resistance being a strong function of temperature. They possess far superior energy resolution compared to the CCDs that have been used as the primary spectrometer in X-ray astronomy, and also have much higher quantum efficiency than state-of-the art gratings (e.g., those employed by the Chandra X-ray Observatory and XMM-Newton Observatory). Efforts have been made to employ microcalorimeters in a major satellite mission but, unfortunately, none has been successful for reasons unrelated to the devices themselves.

We are now developing superconductor-based microcalorimeters, commonly known as transition-edge sensor (TES). A conventional superconductor behaves like a normal electrical conductor at high temperatures. But, when the temperature drops below some characteristic value (which depends on material), its electrical resistance vanishes, i.e., it becomes a perfect conductor with zero electrical resistance. The transition between the normal and superconducting states can occur over a very narrow temperature range. Now imagine, a TES is operated at the edge of the transition. Its electrical resistance is extremely sensitive to temperature variation, which makes it superior to any semiconductor-based microcalorimeter, in principle. The TES technology has other advantages too. For instance, standard micro- and nano-fabrication techniques are used to produce TES devices, so it is easier to produce a large array of sensors with good uniformity. The focus of our work is to produce an array of TES detectors with fairly large pixels (~0.5 mm in size), optimized to deliver good energy resolution (2 eV @ 0.6 keV) at soft energies (0.1 to ~1 keV).