An appreciation of quasiparticle trapping and the proximity effect

Experimental results on X-ray detection with NbAlAlOxAl superconducting tunnel junctions are presented. For the first time, we can report that the spectra from devices without a proximity effect at the NbAl interface are remarkably similar to those that do have a proximity effect. The similarity is due to the response of the Al junction being dominated by the detection of high energy phonons produced by the trapping, or recombination processes occurring in the Nb absorber after a particle interaction. These results indicate that the unequivocal demonstration of quasiparticle trapping requires a thorough understanding of the phonon response in these types of detector.