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We have built a high-resolution soft x-ray spectrometer using a 3 × 3 array of superconducting Nb-AI-AIOx-AI-Nb tunnel junctions. The detector is cooled to ∼0.1 K by a two-stage adiabatic demagnetization refrigerator while being held at the end of a 40-cm-long detector snout that can be inserted into an experimental chamber. The 0.6 mm × 0.6 mm array has an energy resolution of 10-15 eV FWHM for x-ray energies below I kev and total count rate capabilities above 100 000 counts/s. We demonstrate the capabilities of the instrument in a study of anisotropy in c-axis oriented MgB[sub 2] films by near edge x-ray absorption spectroscopy. The absorption fine structure at the B K-edge differs for excitation with x-rays polarized along the c-axis and in the ab-plane of the MgB[sub 2] film. We discuss the implications of the anisotropy for the density of states in MgB[sub 2] at the Fermi surface. [ABSTRACT FROM AUTHOR]

We present recent results from our γ-ray transition-edge sensor (TES) microcalorimeters. We have demonstrated an energy resolution of 52 eV at 60 kev with devices composed of a high-purity Sn absorber and a Mo/Cu multilayer thin-film TES. In this paper, we present a detailed noise analysis of these devices and show that the major noise sources are device originated (thermal fluctuation and Johnson noise). Our performance analysis explicitly includes the noise contribution due to the composite geometry of these devices and electro-thermal feedback (ETF). [ABSTRACT FROM AUTHOR]