Detector development for a neutrino mass determination using the 163Ho electron capture spectrum

The absolute scale of the neutrino mass eigenstates is one of the puzzles in modern particle physics and can be directly investigated using electroweak decays. In the context of the ECHO collaboration we are developing metallic magnetic calorimeters (MMCs) to be used with an internal 163Ho source to measure its electron capture (EC) spectrum. MMCs are calorimetric particle detectors with paramagnetic temperature sensor operated below 100 mK. The sensor converts the temperature rise of the detector, due to the absorption of an energetic particle, to a change of magnetization which is detected by a SQUID magnetometer. MMCs fulfill the requirements for cryogenic neutrino mass investigations, namely an energy resolution Δ EFWHM below 2 eV and pulse formation times of τ < 1 μs, as recently obtained with micro-fabricated MMCs for soft X-ray detection. We present the calorimetric measurement of the EC spectrum of 163Ho obtained with our first detector prototype. We discuss the development of a 64 pixel array readout.