Effects of Quasiparticle Recombination and Photoelectron Escape in Al-Superconducting Tunnel Junction Detectors

A slight deviation from linearity has been detected for single aluminum superconducting tunnel junction detectors (100×100×0.52=0.35 μm3) in the energy range between E=1.74 keV and E=6.49 keV. The nonlinearity can be explained by the escape of recombination phonons into the substrate. Varying the size of the junction shows that the recombination of the quasiparticles takes place after their diffusive propagation across the junction, hot spot effects are not observed. Energy resolution obtained with aluminum tunnel junction detectors so far is not affected by recombination effects. Pulse shape analysis allows to deconvolve the double-peak structure of the pulse height spectra and to identify background events that are caused by the range of photo- and Auger-electrons. The range of photoelectrons (Ekin=4.34 keV) is determined to be 0.2 μm in aluminum. In the pulse height spectrum, the escape of photoelectrons produces a flat background.