Abstract. In order to achieve a high resolution in the spectroscopy of low energy X-rays, detectors based on superconducting tunnel junctions as sensors are presently investigated. The knowledge of the processes affecting the signal generation in such sensors is essential for the interpretation of the detector response. Starting from a diffusion model including decay and tunneling of excess quasiparticles in the metal layers of a superconducting tunnel junction detector, the detector response is determined as a function of absorption position and of rate constants. Model predictions agree very well with experimental data. The advantages of a detector employing quasiparticle trapping are pointed out and the parameters determining the signal gain are deduced. The linearity of the detector signal is much more affected by pair recombination of the quasiparticles during their tunneling rather than during their diffusive propagation into the tunneling region. I IntroductionThe availability of a new generation of detectors has almost always initiated important discoveries. Presently so called 'cryodetectors' operating in the temperature range of mK to some K are being developed [l-51. Cryodetectors exploit effects such as superconductivity or low specific heat and give rise to a number of advantages over presently available detectors in nuclear physics and astrophysics.Our detectors (for radiation or particles) consist of an absorber and of a sensor. Energy being deposited in the absorber is converted into excitations such as phonons or quasiparticles. Detectors may be designated for a wide range of energy and for radiation of different types, depending on the ability of the absorber to completely absorb the incident radiation and to transfer excitations to the sensor. The sensor registers the excitations and generates a signal which is related to the initially absorbed amount of energy. We discuss monolithic sensors as well as complex sensors based on superconducting tunnel junctions. In some detector applications, sensor and absorber may be identical.The main advantages of detectors based on superconductors as compared to detectors based on semiconductors are an extraordinary high energy resolution and low energy threshold. In case of a superconductor the number of ultimately produced charge carriers is enhanced by a factor of -1000, because the energy gap in a superconductor

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