Large-format distributed read-out imaging devices for X-ray imaging spectroscopy

We present an experimental study of the performance of Distributed Read-Out Imaging Devices (DROIDs), based on two Ta/Al-based superconducting tunnel junctions (STJs) placed on either side of a Ta absorber strip. We focus our discussion on the prospects of building large-format photon-counting imaging spectrometers for applications at optical, UV and soft X-ray energies. Tunnel-limited spectroscopic resolutions have already been demonstrated for optical photons. With a 20x100 μm2 absorber we have measured an intrinsic energy resolution of 2 eV FWHM for 500 eV photons. This demonstrates that at soft X-ray energies resolutions close to the tunnel limit are also feasible for this type of detectors. A detailed analysis of pulse-shapes with numerical models allows us to assess the main parameters that determine the performance of these detectors. Extrapolation of these models indicates that it is possible to extend the length of the absorber to 1.5 mm, without a serious degradation of the detector's performance