On the development of superconducting tunnel junctions for use in astronomy

Superconducting Tunnel Junctions (STJ) have now been under development for a number of years for a wide range of astronomical applications. Devices ba sed on niobium-aluminium or tantalum-aluminium have been shown to be effici ent photon counting energy dispersive spectrometers from the near-infra-red to X-ray region of the spectrum. The basic performance characteristics of tantalum-based devices, in terms of detection efficiency, spectral resoluti on and signal linearity with photon energy, are provided for each wave band of astronomical interest, namely: the near-infra-red (NIR [1-5 mu m]), opt ical [400-1000 nm], ultraviolet (UV [100-400 nm]), extreme ultraviolet (EUV [10-100 nm]), soft X-ray (SXR [100-2000 eV]) and medium X-ray (MXR [2-10 k eV]) regions of the spectrum. Although design issues which allow the optimi zation of the performance for a specific waveband are of importance, we sho w that current generations of tantalum-based STJs have already very good pe rformance over the whole spectral range. A particular improvement in the re solving power and a description of the role played by various mechanisms in degrading the energy resolution from the theoretical tunnel limited value is given based on specific measurements at relevant photon wavelengths. Add itional astronomical characteristics for future applications, such as the t ime resolution and imaging capability are also presented, based on experime ntal data and the performance of the first small format 6 x 6 pixel STJ arr ay described. Finally an assessment is provided on the possibility for furt her improvements, particularly in the area of higher spectral resolution, t hrough the use of lower band gap superconductors. (C) 1999 Published by Els evier Science B.V. All rights reserved.