Low-energy response of superconducting tunnel junction x-ray spectrometers

Thin film structures incorporating metallic superconducting layers and tunnel junctions can be configured as low-energy X-ray spectrometers. We present results obtained when low-energy X-rays are absorbed in niobium films coupled to aluminum layers that serve as quasiparticle traps in an Nb/Al/Al{sub 2}O{sub 3}/Al/Nb tunnel junction X-ray detector. The linearity of the pulse height as a function of energy is discussed along with the energy dependence of the observed resolution and its relation to the broadening mechanisms. A resolution of 14 eV at 1 keV has been measured with our detector cooled to 0.3 K