Experiment

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 2.8 keV, 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 300 Bq 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 1.5 eV 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 HOLMES experiment is located in the Laboratories of INFN Genova and INFN Milano Bicocca / University of Milano-Bicocca.

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Isotope production

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. The total inventory required for HOLMES is about 150 MBq 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.

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Isotope embedding

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 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 allows to separate 163Ho from other trace contaminants not removed by chemical methods at PSI, such as the radioactive isotope 166mHo.

The implanter is integrated with a compact sputtering system to deposit the final gold layer which fully encapsulate the 163Ho source.

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.

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Detectors

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 TES microcalorimeters 4×16 sub-arrays.

 

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Detector read-out

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.

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DAQ

The μMUX is suitable for a fully digital approach based on the Software Defined Radio (SDR) technique. HOLMES realizes an SDR leveraging the Reconfigurable Open Architecture Computing Hardware (ROACH2) board with a Xilinx Virtex6 FPGA. We completed the software and hardware set-up of one ROACH2 system complete of ADC (550MS/s, 12bit, 2 channels), DAC (for baseband frequency comb generation), IF (for comb up- and down-conversion) boards, and SFP+ GbE interfaces. The ROACH2 has a 10GbE link to a server connected to a 40TB RAID via fibrechannel. With this configuration the μMUX multiplexing factor is presently set to 32, for a signal sampling frequency of 0.5MS/s. We are presently testing a preliminary version of the firmware for the multiplexing of 4 channels. An expanded version for 32 channels is already is ready for deployment. To read-out the full 1000 pixel array a total of 32 ROACH2 systems will be required.

 

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Cryogenics

 The new cryogen-free dilution refrigerator hosted in the Cryogenic Laboratory at INFN Milano-Bicocca has been instrumented with microwave cryogenic coax cables and a HEMT amplifier to perform homodyne measurements with TES detectors read-out with microwave rf-SQUID multiplexer prototypes from NIST.

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Bibliography

  1. A. de Rújula and M. Lusignoli, "Calorimetric measurements of 163-Holmium decay as tools to determine the electron neutrino mass", Phys. Lett. B., vol. 118, issue 4-9, pp. 429–434 (1982)  DOI;
  2. M. Galeazzi et al., "The Electron Capture Decay of 163-Ho to Measure the Electron Neutrino Mass with sub-eV Accuracy", arXiv:1202.4763
  3. A. Nucciotti, "Statistical sensitivity of 163-Ho electron capture neutrino mass experiments",  Eur. Phys. J. C 74, 3161 (2014). arXiv:1405.5060
  4. B. Alpert et al., "HOLMES - The Electron Capture Decay of 163Ho to Measure the Electron Neutrino Mass with sub-eV sensitivity", Eur. Phys. J. C 75, 112 (2015). arXiv:1412.5060

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