Abstract: An experimental analysis of Niobium based superconducting tunnel junctions is presented, evaluating their performance as photon counting detectors. Several mechanism are found to be responsible for the degradation of the energy resolution. In particular, the high magnetic fields necessary to suppress the Josephson current in square junctions are shown to smear the energy bandgap. It is experimentally verified that in junctions with special geometries the Josephson current can be sufficiently suppressed by much lower fields. Several loss and contamination mechanisms are also discussed. Experimental results on new developments, such as quasiparticle trapping blocks, source collimation and substrate buffering, are presented, with a view to demonstrating significant improvements in energy resolution.!3
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