SQUID based readout techniques for the ECHo experiment
Sebastian Kempf (KIP Heidelberg)

Neutrinos, and in particular their tiny but non-vanishing masses, are presently considered to be a very promising experimental playground to study physics beyond the Standard Model. Precision studies of the kinematics of weak interactions such as the 3H β-decay and the 163Ho electron capture (EC) represent the only model independent approach to determine the absolute scale of neutrino masses. The electron capture in 163Ho experiment, ECHo, aims to investigate the electron neutrino mass with sub-eV/c2 sensitivity by performing a calorimetric measurement of the electron capture spectrum of the nuclide 163Ho. The maximum energy available for this decay is about 2.8 keV and constrains the type of detectors that can be used. It strongly suggests to employ metallic magnetic calorimeters (MMCs) since they provide an excellent energy resolution, a very fast signal rise time, an almost ideal linear detector response as well as a quantum efficiency close to 100%. A metallic magnetic calorimeter consists of an absorber for the particles to be detected that is in tight thermal contact with a paramagnetic temperature sensor situated in a weak magnetic field to create a temperature dependent sensor magnetization. The temperature change upon the absorption of an energetic particle, e.g. an electron created within an EC event of 163Ho, results in a change of sensor magnetization which can be precisely measured using a superconducting quantum interference device (SQUIDs). To achieve the target sensitivity on the electron neutrino mass, ECHo will operate large MMC arrays with more than 104 detectors. However, this large number of detectors sets constraints on the SQUID readout technique to be employed. In particular, it prohibits the use of single-channel readout schemes due to limitations on available cooling power, system complexity and costs. Therefore, sophisticated microwave SQUID based multiplexing techniques for the readout of MMCs are presently developed.
Within this talk, we will give a short introduction into the ECHo experiment as well as the fundamentals of metallic magnetic calorimeters. We then extensively discuss the basics and realization of SQUID based readout techniques presently used in the ECHo experiment as well as recent developments paving the way for performing a 163Ho based experiment with sub-eV/c2 sensitivity.