High-resolution superconducting X-ray spectrometers with aluminum trapping layers of different thicknesses

Superconducting tunnel junctions coupled to superconducting absorbers may be used as high-resolution, high-efficiency X-ray spectrometers. We have tested devices with niobium X-ray absorbing layers coupled to aluminum layers that serve as quasiparticle traps. We present a study of device performance as a function of thickness of the trapping layers. We measured the best energy resolution using a device with a high-quality barrier and 200 nm-thick trapping layers on both sides of the tunnel barrier. This energy resolution was 36 eV full width at half maximum at 6 keV, about 4 times better than that obtainable using semiconductor ionization detectors.>