Probing the absolute neutrino mass scale with the 163Ho: The HOLMES project

The HOLMES project aims to directly measure the electron neutrino mass using the electron capture decay (EC) of 163Ho down to the eV scale. It will perform a precise measurement of the end-point of the 163Ho calorimetric energy spectrum to search for the deformation caused by a finite electron neutrino mass. The choice of 163Ho as source is driven by the very low Q-value of the EC reaction (around 2.8 keV), which allows for a high sensitivity while keeping the overall activities to reasonable value (O(102)Hz/detector), thus reducing the pile-up probability. A large array made of thousands of Transition Edge Sensor based micro-calorimeters will be used for a calorimetric measurement of the EC 163Ho spectrum. The calorimetric approach, with the source embedded inside the detector, eliminates systematic uncertainties arising from the use of an external beta-source, and minimizes the effect of the atomic de-excitation process uncertainties. The commissioning of the first implanted sub-Array is scheduled for the end of 2017. It will provide useful data about the EC decay of 163Ho together with a first limit on neutrino mass. In this paper the current status of the main tasks will be summarized: The TES array design and engineering, the isotope preparation and embedding, and the development of a high speed multiplexed SQUID read-out system for the data acquisition.