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The ECHo experiment aims at determining the effective electron neutrino mass by analyzing the endpoint of the 163Ho electron capture spectrum. High energy resolution detectors with a well-tailored detector response are the essential ingredient for the success of the ECHo experiment. Metallic magnetic calorimeter arrays enclosing 163Ho have been chosen for the ECHo experiment. The first MMC array, ECHo-1k, showed excellent performances with an average energy resolution of 5.5 eV FWHM @ 5.9 keV. Based on the results obtained with the ECHo-1k array, optimization studies have paved the way towards a new detector design for the next experimental phase, ECHo-100k. The ECHo-100k chip features an optimized single pixel design to improve the detector performance as well as an upgraded on-chip thermalization layout. The newly fabricated ECHo-100k detectors have been fully characterized at room temperature, at 4 K and at millikelvin temperature. The obtained results show that the ECHo-100k array achieved the expected performance with an average energy resolution of 3.5 eV FWHM @ 5.9 keV, fulfilling the requirements for the ECHo-100k experimental phase. [ABSTRACT FROM AUTHOR]

We describe a prototype γ-ray detector based on a metallic magnetic calorimeter with an energy resolution of 46eV at 60 keV and a reproducible response function that follows a simple second-order polynomial. The simple detector calibration allows adding high-resolution spectra from different pixels and different cool-downs without loss in energy resolution to determine γ-ray centroids with high accuracy. As an example of an application in nuclear safeguards enabled by such a γ-ray detector, we discuss the non-destructive assay of 242Pu in a mixed-isotope Pu sample. [ABSTRACT FROM AUTHOR]