Cryogenic micro-calorimeters for low energy beta spectroscopy

We are using calorimeters with completely encapsulated sources to study low energy beta spectra. Our devices work as true calorimeters in the sense that they totally absorb and sum all the various quanta released in a decay into a single temperature pulse with amplitude proportional to the total energy deposited. We use a neutron transmutation doped (NTD) germanium thermistor as the temperature sensor and superconducting NbTi leads, which form a weak thermal link to the cold stage (80 mK). We have been using superconducting tin as our absorber material, and we discuss various techniques for source and absorber preparation. Annealing the absorber leads to shorter pulses with larger amplitude, and significantly improves detector performance. The production of a device to study the beta decay of¹⁰⁷Pd presented special difficulties due to the low specific activity of this isotope (halflife 6.5×10⁶ y), and the low enrichment (15%) that was available. This meant it was necessary to incorporate a large amount of palladium into our tin absorber. We found we could avoid a corresponding increase in heat capacity by forming a superconducting PdSn alloy.