  At Argonne National Laboratory, we are developing hard X-ray (2 to 20 keV)
Transition Edge Sensor (TES) arrays for beamline science. The significantly
improved energy resolution provided by superconducting detectors compared to
semiconductor-based energy-dispersive detectors, but with better collection
efficiency than wavelength-dispersive instruments, will enable greatly improved
X-ray emission and absorption spectroscopic measurements. A prototype
instrument with 24 microwave-frequency multiplexed pixels is now in testing at
the Advanced Photon Source (APS) 1-BM beamline. Initial measurements show an
energy resolution ten times better (150 eV compared to < 15 eV) than the
silicon-drift detectors currently available to APS beamline users, and in
particular demonstrate the ability to resolve closely-spaced emission lines in
samples containing multiple transition metal elements, such as integrated
circuits. Comparing fluorescence spectra of integrated circuits measured with
our TESs at the beamline to those measured with silicon detectors, we find
emission lines and elements largely hidden (e.g. Hf alongside Cu) from a
semiconductor-based detector but well resolved by a TES. This directly shows
the strengths of TES-based instruments in fluorescence mapping.
