Investigation of superconductors as particle absorbers of metallic magnetic calorimeters

The choice of an absorber material for low temperature calorimeters is a crucial one. Generally, the absorber should possess high stopping power for incoming energetic particles or photons and have low heat capacity. Currently mostly gold and bismuth are used for this purpose. The very small specific heat of superconductors far below their Tc makes them very promising candidates as absorber material. However, these materials show different thermalization behaviour, of which the underlying thermalization processes are so far not understood. In superconducting rhenium this thermalization is of particular interest with regard to the development of detectors for neutrino mass measurements. We investigated three different superconducting materials (Re, Al and Al:Mn) as absorbers of metallic magnetic calorimeters (MMCs). MMCs are low temperature energy dispersive detectors composed of an energy absorber well thermally connected to a paramagnetic temperature sensor which resides in a small magnetic field. The change of magnetization following the absorption of energy is measured as a change of flux in a low noise high bandwidth dc-SQUID. We present the observed dependence of the energy thermalization on temperature and geometry for all three materials and discuss the importance of the diffusive transport of heat as well as of quasiparticle recombination.