From ECHo-1k to ECHo-100k: Optimisation of the High- Resolution Metallic Magnetic Calorimeters with Embedded ¹⁶³Ho

The ECHo collaboration aims to determine 𝑚(𝜈𝑒) by analysing the 163Ho electron capture spectrum. Arrays of tens to hundreds of Metallic Magnetic Calorimeters (MMCs) implanted with 163Ho have been chosen because of their excellent energy resolution in the range of a few eV, their fast response time below 1 𝜇s and their good linearity.
The MMC array enclosing 163Ho fabricated for the ECHo-1k phase has been fully characterised in terms of detector response, energy resolution and 163Ho activity. Based on these results a new 64-pixel-array design has been conceived for ECHo-100k, featuring an optimised single pixel geometry and allowing for a 163Ho activity of 10 Bq per pixel.
First wafers, each with 40 ECHo-100k chips, have been fabricated and characterised. The obtained results show that the ECHo-100k array achieved the expected performance, especially an average energy resolution of 3.5 eV, fulfilling the requirements for the ECHo-100k phase.