Final Report Summary - HOLMES (The Electron Capture Decay of 163Ho to Measure the Electron Neutrino Mass with sub-eV sensitivity) To the aim of the high sensitivity neutrino mass measurement, HOLMES leverages the low temperature microcalorimeters development for high energy resolution X-ray spectroscopy, in particular, the Transition Edge Sensors (TES) based microcalorimeters used for high resolution soft X-ray whose technology presents many added values: (1) fast and full calorimetric energy measurements can be achieved in metallic absorbers, (2) small footprint kilo-pixel arrays can be fabricated with well established photolithographic technologies, and (3) powerful SQUID multiplexing schemes for large detector arrays can be adopted.The baseline program of the HOLMES experiment is therefore to deploy an array of about 1000 TES based microcalorimeters each with about 300 Bq of Ho-163 fully embedded in the absorber, and with the goal of energy and time resolutions of the order of 1 electronvolt and 1 microsecond, respectively. The HOLMES project is characterized by 5 key tasks: (1) Ho-163 isotope production, (2) Ho-163 source embedding, (3) Single detector optimization and array engineering, (4) multiplexed detector read-out, and (5) data handling. These tasks indeed configure as a highly interdisciplinary effort to investigate the fundamental properties of a sub-nuclear particle (neutrino) measuring the atomic radiation emitted as consequence of a nuclear process (EC). It also calls for the exploitation of a wide spectrum of advanced techniques, including radiochemistry, rare-earth chemistry, superconducting electronics, microwave electronics, low-temperature techniques, thin film technology, device micro-fabrication, and digital signal processing. We were successful in achieving several crucial results for all these tasks: the many outcomes of the HOLMES project now allow us to schedule the challenging neutrino mass measurement for the near future, still in time to be extremely useful. The important HOLMES achievements include (1) the production of about 7mg of highly purified and ready-to-use Ho-163 isotope by means of neutron irradiation at the ILL nuclear reactor (Grenoble, France) followed by an innovative rare-earth radiochemical purification developed in collaboration with PSI (Villigen, Switzerland), (2) the optimization of new efficient processes to thermo-reduce the holmium sample and fabricate a synthered target out of an alloy with metallic holmium - necessary for the ion source of the ion implanter -, (3) the setting up of a prototype of high current ion implanter to mass separate the Ho-163 and embed it in the detectors, (4) the successful development of a vacuum system for high rate and uniformity gold deposition on the detectors during and after ion implantation - critical to both achieve the planned high implantation dose and to fully encapsulate the Ho-163 for a true calorimetric measurement -, (5) the successful design and testing of novel very fast and high energy-resolving TES microcalorimeters suitable for the project in collaboration with NIST (Boulder, CO, USA), (7) the design of small sub-arrays of 32 microcalorimeters and the demonstration of the full sub-array fabrication process - except the Ho-163 ion implantation -, (8) the successful implementation of an innovative low temperature detector readout system exploiting the microwave multiplexing of rf-SQUIDs - mandatory for the high-bandwidth readout of the large number of fast HOLMES detectors -, (9) the development and successful deployment of a room temperature acquisition system which includes a custom designed microwave up- and down- conversion analog circuit and a FPGA based Software Defined Radio - crucial to readout the front end rf-SQUID and process the multiplexed detector signals, and (10) the development of all the software codes for data acquisition and analysis. Along with the above, we completed the set-up and characterization of a powerful and large cryogen-free dilution refrigerator dedicated to the HOLMES measurements at a temperature of about 0.05K. The refrigerator is instrumented with fully characterized low-noise microwave electronics and coaxial lines for the read out of the detectors and dedicated measurements confirmed that the environment provides a low level of background radiation sufficient for the neutrino mass measurement.In addition to the many technical achievements, the outstanding outcome of the HOLMES project is the opening of a novel promising approach to directly measure the neutrino mass which has stimulated other similar experimental efforts worldwide accompanied by great expectations in the neutrino physics community.The successful development of the HOLMES detectors with their advanced microwave readout spurred similar developments also for other fields of application such as astrophysics, cosmology, astronomy, material science, nuclear physics and safeguard, and quantum technologies.
