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HOLMES Report Summary

Project ID: 340321
Funded under: FP7-IDEAS-ERC
Country: Italy

Mid-Term Report Summary - HOLMES (The Electron Capture Decay of 163Ho to Measure the Electron Neutrino Mass with sub-eV sensitivity)

The HOLMES experiment aims at directly measuring the electron neutrino mass using the electron capture (EC) decay of 163Ho. HOLMES performs a calorimetric measurement of the energy released in the decay of 163Ho. This allows to measure all the atomic de-excitation energy, except the fraction carried away by the neutrino. With a transition energy of about only 2.8keV, 163Ho is a promising isotope. The direct measurement exploits only energy and momentum conservation and it is therefore completely model-independent. At the same time, the calorimetric measurement eliminates systematic uncertainties arising from the use of external beta sources, as in experiments with beta spectrometers, and minimizes the effect of the atomic de-excitation process uncertainties.
The baseline of the HOLMES experiment is to use TES microcalorimeters with about 300Bq of 163Ho fully embedded in their absorbers. HOLMES will deploy an array of about 1000 low temperature microcalorimeters with implanted 163Ho nuclei. HOLMES may reach a statistical sensitivity of about 1.5eV and it will be an important step forward in the direct neutrino mass measurement with a calorimetric approach as an alternative to spectrometry. It will also establish the potential of this approach to extend the sensitivity down to 0.1eV.
The 163Ho isotope necessary for HOLMES is produced by neutron irradiation of Er2O3 enriched in 162Er at the ILL (Grenoble, France) high neutron flux nuclear reactor. After preliminary tests, a new irradiation is now starting at ILL with the aim to produce as much as 150MBq of 163Ho, which could account for the total need for the experiment. The holmium produced after irradiation is chemically separated at PSI in a hot-cell by means of a specially developed efficient process.
To perform a calorimetric measurement of the EC spectrum of 163Ho, the isotopes must be embedded in the absorber of the low temperature microcalorimeters. The system for embedding the isotope is being set-up in the INFN-Genova laboratory: it is made of an ion implanter and a holmium evaporation chamber that will produce the metallic target for the ion implanter source. The ion implanter features a Penning sputter ion source and a magnetic mass selection sector, to achieve an optimal mass separation for 163Ho. This will allow to separate 163Ho from other trace contaminants not removed by chemical methods at PSI, such as the radioactive isotope 166mHo. The implanter will be integrated with a compact sputtering system to deposit the final gold layer which fully encapsulate the 163Ho source. The target chamber is being set-up in Milano-Bicocca and it will be initially used to tune the detector gold absorber fabrication process without any implanted holmium. It will be finally integrated with implanter. The metallic cathode for the ion source will be made out of metallic holmium pellets containing 163Ho which are produced in the holmium evaporation chamber by thermo-reduction and distillation at about 1600°C of the Ho2O3 extracted at PSI from the irradiated Er2O3. The evaporation system is set-up in Genova and tests are in progress with natural Ho2O3 to tune and characterize the reduction process.
The detectors used for the HOLMES experiment are TES microcalorimeters with gold absorbers. The devices are provided by NIST (Boulder, Co, USA) with a 1μm gold layer. A thin (few 100 Angstrom) layer of Au:163Ho is deposited by means of simultaneous ion implantation and ion beam sputtering in the target chamber, then the gold absorber is completed in-situ by sputtering the second 1μm gold layer. The R&D on the single TES pixel without 163Ho has been completed closely matching the HOLMES specifications. We have now finalized the design of the 4×16 sub-arrays. First sub-arrays will be fabricated at NIST shortly and will be used for testing the two-step fabrication process, the ion implantation, and to assess the thermal effect of high holmium concentrations in the absorbers.
The 1000 TES signals are read-out with a microwave SQUID multiplexing approach which is crucial to preserve the performances of the individual detectors, especially in terms of available signal bandwidth, i.e. time resolution. The microwave homodyne measurements to characterize the TES devices with multiplexed read-out are carried out in the cryogen-free dilution refrigerator hosted in the Cryogenic Laboratory at INFN Milano-Bicocca. The TES prototypes show an energy resolution of about 5eV and the 10μs rise time which is the target for the HOLMES signal read-out. With the new MUX chips which are being produced we will further improve the energy resolution. The HOLMES digital data acquisition uses an FPGA-based board (ROACH2) to implement a Software Defined Radio (SDR). We completed the software and hardware set-up of one ROACH2 system which can read-out 32 detectors.
In the second half of the HOLMES project, we will fabricate the first TES microcalorimeters with the 163Ho isotope embedded in their absorbers. After a first phase dedicated to the full process optimization, we will carry out a one month measurement with a 4×16 sub-array. This measurement is scheduled for the end of 2017 and it will provide precious data about the EC decay of 163Ho. It will also be the first crucial step towards the full set-up for the high statistics HOLMES measurement which will gradually start during 2018.

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