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HOLMES is an experiment to directly measure the neutrino mass with a calorimetric approach. The calorimetric technique eliminates several systematic uncertainties usually present in spectrometers where the external source and decays to excited states affect the measurement. 163 Ho is chosen as source for its very low Q value (2.8 keV), the proximity of the end-point to resonance M1 and its half life (4570 year). These features are optimal to reach simultaneously a reasonable activity to have sufficient statistics in the end-point and a small quantity of 163 Ho embedded in the detector not to alter significantly its heat capacity. 163 Ho will be produced via neutron irradiation of enriched 162 Er 2 O 3 at the Institute Laue–Langevin (Grenoble, France), and chemically separated at Paul Scherrer Institut (Villigen, Switzerland). It will arrive at INFN laboratory of Genova in oxide form (Ho 2 O 3 ) with traces of others Ho isotopes and contaminants not removable using chemical methods. In particular, the metastable 166 m Ho undergoes beta decay with a half life of about 1200 year, if present 166 m Ho induces background below 5 keV. The removal of these contaminants is critical for HOLMES so a dedicated system is being set up. The system is designed to achieve an optimal mass separation for 163 Ho and consists of two main components: an evaporation chamber and an ion implanter. In the evaporation chamber, holmium will be reduced in metallic form, using the reaction 2Y+Ho 2 O 3 → Y 2 O 3 +2Ho and used to produce a metallic target for the ion implanter source. The ion implanter consists of five main components: a Penning sputter ion source, an acceleration section, a magnetic/electrostatic mass analyser, a magnetic scanning stage and a focusing electrostatic triplet. In this contribution, we describe the procedures, under continuous refinement, for the holmium evaporation process, the ion-implanted metallic target production and the status of the ion implanter.

International audience; The measurement of neutrino masses is still one of the most compelling issues in modern particle physics. HOLMES is an experiment that aims to measure the effective νe mass using a calorimetric approach. It will measure the spectrum end point of the electron capture (EC) decay of $^{163}$Ho. The very low Q-value (2.8 keV) of the decay and its half life (4570 y) are optimal to reach simultaneously a reasonable activity to have sufficient statistics in the end-point, reducing the pile-up probability and have a small quantity of $^{163}$Ho embedded in the detector not to alter significantly its heat capacity. Holmium will be implanted into a micro calorimeter made by a metallic absorber coupled to transition edge sensor (TES). Each detector will be implanted with around 300 Bq of holmium and the goal of the experiment is implanting ≈500 detectors (8x64 array of detectors) to reach an accuracy of the order of eV. In this contribution, we show the HOLMES experiment with its physics reach and technical challenges, along with its status and perspectives.

The HOLMES experiment aims to directly measure the ν mass studying the 163Ho electron capture decay spectrum developing arrays of TES-based microcalorimeters implanted with O(300 Bq/detector) Ho atoms. The embedding of the source inside detectors is a crucial step of the experiment. Because the 163Ho production process (neutron irradiation of a 162Er sample) is not perfectly free from impurities, Ho source must be separated from a lot of contaminants. A chemical processing removes every species other than Ho, but it is not sufficient to remove all isotope-related background sources: Indeed, 166mHo beta decay can produce fake signal in the region of interest. For this reason, a dedicated implantation system was set up. It is designed to achieve the separation power better than 5σ at 163/166 a.m.u. allowing an efficient Ho ions implantation inside microcalorimeter absorbers. Its main components are a 50 kV sputter-based ion source, a magnetic dipole and a target chamber. A specially designed co-evaporation system was designed to “grow” the gold microcalorimeter absorber during the implantation process, increasing the maximum achievable activity which can be implanted. The machine performances were evaluated by means of calibration runs using 63Cu/65Cu and Mo beams. A special care was given to the study of the more effective way to populate source plasma with Ho ions obtained from different Ho compounds by sputtering process. In this work, the machine development and commissioning are described.

The HOLMES experiment aims to directly measure the ν mass studying the 163Ho electron capture decay spectrum, using arrays of TES-based micro-calorimeters implanted with O(102 Bq/detector) 163Ho atoms. The neutron irradiation of 162Er enriched samples, used for the production of 163Ho, also generates radioactive contaminants. Chemical processes have been developed to extract the Ho fraction with high efficiency. The radioactive Ho isotope 166mHo is also present in the final sample and could significantly contribute to background events in the final spectrum. For this reason a dedicated implantation/mass separator has been set up and commissioned. It is designed to achieve more than 5 σ separation 163/166 u simultaneously allowing an efficient 163Ho atoms embedding inside micro-calorimeter absorbers. Its main components are a 50 kV sputter-based ion source, a magnetic dipole and a target chamber. A specially designed co-evaporation system has been implemented to deposit gold on the detector absorbers during implantation to overcome the 163Ho source saturation problem. The system performances in terms of achievable beam current, profile and mass separation have been evaluated by means of calibration runs using Cu, Mo, Au and 165Ho beams. An intensive study was done to optimize the target containing 163Ho in order to achieve a large fraction of ionized Ho in the beam. In this work, the machine development and commissioning will be described.

The HOLMES project aims to directly measure the ν mass down to the eV scale using the electron capture decay (EC) of 163Ho. It will perform a precise calorimetric measurement of the end-point of the Ho energy spectrum looking for the deformation caused by a finite ν mass. The choice of 163Ho as source is driven by the very low Q-value of the EC reaction, which allows for high sensitivity with low activities (O(10 2 )Bq/detector), thus reducing the pile-up probability. A large array made by thousands of transition edge sensors based micro-calorimeters will be used. The calorimetric approach eliminates systematic uncertainties arising from the use of an external β source, and minimizes the effect of the atomic de-excitation process. The commissioning of the first implanted sub-array is scheduled for the end of 2018. It will provide useful data about the EC decay of 163Ho together with a first limit on ν mass.

The HOLMES experiment aims to directly measure the ν mass with a calorimetric approach. The choice of 163Ho isotope as source is driven by the very low decay Q-value ( ∼ 2.8 keV), which allows for high sensitivity with low activities (O(10 2 )Hz/detector), thus reducing the pile-up probability. 163 Ho will be produced by neutron irradiation of 162Er 2 O 3 then chemically separated; anyway, traces of others isotopes and contaminants will be still present. In particular 166 m Ho has a beta decay ( τ ∼ 1200y) which can induce background below 5 keV. The removal of the contaminants is critical so a dedicated system has been set up. It is designed to achieve an optimal mass separation @ 163 a.m.u. and consists of two main components: an evaporation chamber and an ion implanter. The first item is used to reduce Ho in metallic form providing a target for the ion implanter source. The implanter is made by the sputter source, an acceleration section, a magnetic dipole, a x–y scanning stage and a focusing electrostatic triplet. In this contribution we will describe the procedures for the Holmium “distillation” process and the status of the machine commissioning.

This paper describes the production and chemical separation of the 163Ho isotope that will be used in several nuclear physics experiments aiming at measuring the neutrino mass as well as the neutron cross section of the 163Ho isotope. For this purpose, several batches of enriched 162Er have been irradiated at the Institut Laue-Langevin high flux reactor to finally produce 6 mg or 100 MBq of the desired 163Ho isotope. A portion of the Er/Ho mixture is then subjected to a sophisticated chemical separation involving ion exchange chromatography to isolate the Ho product from the Er target material. Before irradiation, a thorough analysis of the impurity content was performed and its implication on the produced nuclide inventory will be discussed.

The assessment of neutrino absolute mass scale is still a crucial challenge in today particle physics and cosmology. Beta or electron capture spectrum end-point study is currently the only experimental method which can provide a model independent measurement of the absolute scale of neutrino mass. HOLMES is an experiment funded by the European Research Council to directly measure the neutrino mass. HOLMES will perform a calorimetric measurement of the energy released in the electron capture decay of the artificial isotope $^{163}$Ho. In a calorimetric measurement the energy released in the decay process is entirely contained into the detector, except for the fraction taken away by the neutrino. This approach eliminates both the issues related to the use of an external source and the systematic uncertainties arising from decays on excited final states. The most suitable detectors for this type of measurement are low temperature thermal detectors, where all the energy released into an absorber is converted into a temperature increase that can be measured by a sensitive thermometer directly coupled with the absorber. This measurement was originally proposed in 1982 by A. De Rujula and M. Lusignoli, but only in the last decade the technological progress in detectors development has allowed to design a sensitive experiment. HOLMES plans to deploy a large array of low temperature microcalorimeters with implanted $^{163}$Ho nuclei. In this contribution we outline the HOLMES project with its physics reach and technical challenges, along with its status and perspectives., This is a pre-print of an article published in Journal of Low Temperature Physics. The final authenticated version is available online at: https://doi.org/10.1007/s10909-018-2025-x