Metallic magnetic calorimeters for γ‐ray spectrometry

Metallic magnetic calorimeters allow a high energy resolution even using absorbers with large heat capacities. For this reason, they are well suited for applications such as ionizing radiation metrology requiring gamma‐ray spectrometry with high detection efficiencies. We are developing detectors for gamma‐ray spectrometry with gold absorbers (1.0 mm diameter, 0.33 mm thickness) that have an intrinsic detection efficiency of about 60% at 100 keV. They have a theoretical energy resolution of 23 eV at 20 mK. The paramagnetic Au:Er sensor is coupled to a two‐stage SQUID setup via a meander shaped pickup coil made of a niobium thin‐film. For a temperature of 13 mK, one of our detectors showed an energy resolution of 58 eV at 81 keV. The baseline energy resolutions were 46 eV and 68 eV respectively at 13 mK and 20 mK, which is more than calculated; the measured signal sizes were smaller than expected. This problem can be overcome by sputter‐depositing the sensor (enriched in $166Er)$ on the meander pickup coil; both the niobium and the Au:Er thin films are of high quality. The thermodynamic properties of a sputter‐deposited sensor were characterized below 100 mK for different bias currents up to 140 mA in the meander shaped pick‐up coil. We obtained a good agreement between the measured and calculated magnetizations, heat capacities and signal sizes. Hence, using these sensors for the next gamma‐ray magnetic calorimeter, we expect a significant improvement of the energy resolution in the near future.