Properties of superconducting rhenium as an absorber for magnetic calorimeters

A still puzzling problem in the development of low temperature micro-calorimeters for the measurement of the (187)rhenium beta-spectrum is the understanding of the thermalization of energetic electrons in the superconducting rhenium absorber. We studied metallic magnetic calorimeters (MMC) with single crystal rhenium absorbers and paramagnetic Au:Er temperature sensors. The energy released into the detector leads to a change of magnetization of the paramagnetic sensor located in a weak magnetic field. A SQUID with meander shaped inductance is used to read out this change. This setup allows the study of several properties of the superconducting absorber. The transition to the superconducting state is studied by measuring the magnetic flux expelled by the rhenium sample. The resistivity of rhenium above T-c can be estimated from the measurement of the spectral power density of the Johnson noise. Furthermore the quasiparticle lifetime can be investigated through the analysis of the shape of detector signals caused by intrinsic beta-decays and the absorption of X-rays. We present the data obtained in these experiments and discuss the physical quantities which can be derived from these.