Impact of the absorber and absorber/trap interface quality on the resolving power of STJ X-ray spectrometers

We fabricated X-ray detection devices by instrumenting 2 Al/AlOx/Al/Nb tunneling junctions at each end of strips of epitaxial Ta. The Ta strip acts as photon absorber and is a few hundreds of microns long, the Al layers act as quasiparticle traps. The quality of the epitaxial Ta thin film as well as the absorber/trap interface was varied. Devices kept at 550 mK and biased around 300 μV were irradiated by 6 keV photons. The total charge collected by both junctions varies with the energy released by absorbed photons, depends on various quasiparticle loss processes, and is a function of the bias voltage of the tunneling devices. Our arrangement enables to distinguish between energy resolution degrading mechanisms occuring in the Ta absorber and those present in the vicinity of the tunneling junctions.