On the factors governing the energy resolution of Ta-based superconducting tunnel junctions

The spectral response of Ta-based superconducting tunnels junctions to monochromatized synchrotron radiation in the photon energy range between 15 eV and 1.7 keV has been investigated with a view to establishing the physical processes contributing to the energy resolution. The resolution over the whole spectral range is only a factor of two or three above the expected intrinsic tunnel limited resolution. Between 15 and 60 eV peak widths of ∼1 eV have been achieved. Spanning over two orders of magnitude in photon energy the high resolution has allowed us to isolate the various intrinsic components contribution to the spectral response and in essence confirm the validity of models describing the photoabsorption and tunnel processes in a superconducting tunnel junction.