  The next generation of ultra-low-noise cryogenic detectors for space science
applications require continued exploration of materials characteristics at low
temperatures. The low noise and good energy sensitivity of current Transition
Edge Sensors (TESs) permits measurements of thermal parameters of mesoscopic
systems with unprecedented precision. We describe a radiometric technique for
differential measurements of materials characteristics at low temperatures
(below about 3K). The technique relies on the very broadband thermal radiation
that couples between impedance-matched resistors that terminate a Nb
superconducting microstrip and the power exchanged is measured using a TES. The
capability of the TES to deliver fast, time-resolved thermometry further
expands the parameter space: for example to investigate time-dependent heat
capacity. Thermal properties of isolated structures can be measured in
geometries that eliminate the need for complicating additional components such
as the electrical wires of the thermometer itself. Differential measurements
allow easy monitoring of temperature drifts in the cryogenic environment. The
technique is rapid to use and easily calibrated. Preliminary results will be
discussed.
