First tests on phonon threshold spectroscopy

Using the proximity effect of iridium and gold we fabricate sensitive phase transition thermometers. In spite of problems with other material combinations iridium/gold thermometers show an excellent long-term stability. Up to now we reached transition temperatures in the range from 20 to 100 mK on different absorber materials for example silicon, germanium and sapphire. A study of detectors consisting of a dielectric sapphire absorber, an Ir/Au thermometer and an additional superconducting film is presented. In this case some phonons produced by an interaction in the absorber can break up Cooper–Pairs in the superconducting film. Some energy is stored in the quasiparticle system and reemitted into the absorber with a delay. The recombination phonons are again detected with the thermometer. From the pulse shape of the temperature signal the lifetime of the quasiparticles can be extracted. The results for aluminum and tantalum are presented.