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Abstract. The payload of the Italian/Dutch spacecraft BeppoSAX includes a set of four X-ray concentrators each of geometric area 124 cm 2 together with imaging gas scintillation proportional counter detectors located at the focal planes. One of these detectors, the Low-Energy Concentrator Spectrometer (LECS), is sensitive to X-rays in the energy range 0.1 − 10 keV, while the other three cover a narrower energy range of 1.3 − 10 keV. In order to achieve the extended low-energy response of the LECS a novel type of gas scintillation proportional counter has been developed which dispenses with the separate drift and scintillation regions of conventional instruments. The design and performance of the instrument together with its calibration and data analysis system are described here.

We have developed a theory of quasiparticle and phonon energy downconversion in nonequilibrium superconductors following the absorption of an energetic photon. This stage of energy downconversion cascade is important for the production of quasiparticles and is shown to split into two phases. The first is controlled by the evolution of the phonon distribution while the second is dominated by quasiparticle downconversion. The relative durations of the two phases and hence the rates of quasiparticle generation depend on material parameters, and most common superconductors could be classified into three different groups. For typical superconductors used for x-ray detection the downconversion cascade was shown to be fast compared to various time scales in the tunneling regime.

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