"We propose to continue our successful research program in developing x-ray microcalorimeter arrays for astrophysics. This development will directly benefit not only the International X-ray Observatory (IXO), but also other possible mission concepts. We will investigate various array and pixel optimizations such as would be needed for large arrays for surveys, or arrays of fast pixels optimized for neutron star burst spectroscopy.

The main emphasis of our research will be the further development of arrays of superconducting transition-edge sensors (TES) for imaging x-ray spectroscopy. We have developed a TES pixel that achieves better than 2.5-eV resolution at 6 keV, and arrays of such pixels that are sufficiently uniform in characteristics as to permit common biasing without significant compromises in the operation of any pixel. We are also making arrays of position sensitive TES pixels that show promise for use on IXO. We propose to advance both the single-pixel and position-sensitive arrays so that we can produce arrays suitable for subsystem-level read-out demonstrations of the IXO X-ray Microcalorimeter Spectrometer focal plane.

Additionally, we propose to re-evaluate out successful pixel design which, while certainly suitable for use in developing the architecture of arrays and addressing detector systems issues, is not necessarily the final optimization. The performance of a TES depends on the functional form of the current, temperature, and magnetic-field dependence of the resistive transition, and also on the noise at each point on this transition surface. The parameters that describe this transition surface occupy a large phase space, and we have only probed a small portion of it. We propose to fabricate a series of test devices to explore other parts of the phase space and to learn how to engineer the superconducting transition. Recently, our understanding of the physical effects governing the observed resistive transitions has improved, th