X-ray measurements are critical for the understanding of cycles of matter and energy in the Universe, for understanding the nature of dark matter and dark energy, and for probing gravity in the extreme limit of matter accretion onto a black hole. We propose a program to mature the current x-ray microcalorimeter technology, while developing transformational technology that will enable megapixel arrays.

X-ray calorimeters based on superconducting transition-edge sensors achieve the highest energy resolution of any non-dispersive detector technology. The performance of single x-ray calorimeter pixels has reached that required for many possible future missions such as IXO, RAM, and Generation-X, but further optimization is still useful. In the last years, we have made progress in developing techniques to control and engineer the properties of the superconducting transition. We propose to continue this single-pixel optimization, and to improve both the practical and theoretical understanding of the correlation between alpha, beta, and noise to identify favorable regions of parameter space for different instruments.

A greater challenge is the development of mature TES x-ray calorimeter arrays with a very large number of pixels. Advances in the last several years have been significant. We have developed modestly large (256 pixel) x-ray calorimeter arrays with time-division SQUID multiplexing, and demonstrated Walsh code-division SQUID multiplexing, which has the potential to allow scaling to much larger arrays. Here we propose to extend this work, and also to introduce a new code-division SQUID multiplexing circuit with extremely compact, low-power elements. Using this approach, it is possible for the first time to fit all of the detector biasing and multiplexing elements underneath an x-ray absorber, allowing in-focal-plane multiplexing. This approach eliminates the requirement to bring leads from each pixel out of the focal plane, while reducing the power dissipati