The Suppression of Phonon induced Noise in Niobium Superconducting Tunnel Junction X-ray Detectors

An investigation into the phonon contamination of X-ray sensitive superconducting tunnel junctions arising from the X-ray photoabsorption in various substrates has been conducted. Results are presented on the design of a superconducting tunnel junction (STJ) which substantially reduces or even eliminates phonon induced noise from the substrate. Such noise is the predominant feature in X-ray spectra from junctions due to the bulk of the photons being absorbed in the substrate rather than in the thin superconducting lm. The design involves the choice of a suitable buuer sandwich between the substrate and the STJ. Such a buuer would appear not only to attenuate the phonons created in the X-ray photoabsorption in the substrate but also to scatter the phonons inelastically, introducing a frequency down-conversion. Such a process ensures that few phonons of energy suucient to break Cooper pairs in the superconducting lm of the STJ enter the junction.