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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.

We present the first direct search for lepton flavour violating muon decay mediated by a new light particle X, $$\upmu ^+ \rightarrow \mathrm {e}^+\mathrm {X}, \mathrm {X} \rightarrow \upgamma \upgamma \ $$ μ + → e + X , X → γ γ . This search uses a dataset resulting from $$7.5\times 10^{14}$$ 7.5 × 10 14 stopped muons collected by the MEG experiment at the Paul Scherrer Institut in the period 2009–2013. No significant excess is found in the mass region 20–45 MeV/c$$^2$$ 2 for lifetimes below 40 ps, and we set the most stringent branching ratio upper limits in the mass region of 20–40 MeV/c$$^2$$ 2 , down to $${\mathcal {O}}(10^{-11})$$ O ( 10 - 11 ) at 90% confidence level.

HOLMES is an ERC project started in 2014 that will perform a model independent measurement of the neutrino mass with a sensitivity of the order of 1 eV. In order to reach its goal sensitivity, HOLMES will use 1000 low temperature microcalorimeters, each implanted with an activity of 300 Bq of 163Ho, performing thus a calorimetric measurement. This contribution presents the recent results achieved that lay the grounds for the low-activity phase of the HOLMES experiment, that will lead to its first limit on the neutrino mass.

One of the major challenges in nowadays particle physics and astrophysics is the determination of the absolute neutrino mass scale. A powerful tool to evaluate the effective neutrino mass is the calorimetric measurement of the energy released in a nuclear decay involving neutrino. In order to reach a sensitivity on the neutrino mass of the order of 1 eV, not only detectors characterized by high performances (i.e. energy and time resolution of eV at keV and 1 s, respectively) are needed but also many detectors working in parallel are required. Microwave frequency readout provide an effective technique to read out large arrays of low temperature detectors allowing to reach a multiplex factor of the order of thousands. This technique is the one used to read out the 1000 Transition Edge Sensors of HOLMES, an experiment that aims at measuring the electron neutrino mass by means of the electron capture (EC) decay of 163Ho with an expected sensitivity of the order of 1 eV. In this contribution we present the characterization of the microwave-multiplexed readout system, and the results obtained with the detectors specifically designed for HOLMES.

We present a novel technique, called DSVP (Discrimination through Singular Vectors Projections), to discriminate spurious events within a dataset. The purpose of this paper is to lay down a general procedure which can be tailored for a broad variety of applications. After describing the general concept, we apply the algorithm to the problem of identifying nearly coincident events in low temperature microcalorimeters in order to push the time resolution close to its intrinsic limit. In fact, from simulated datasets it was possible to achieve an effective time resolution even shorter than the sampling time of the system considered. The obtained results are contextualized in the framework of the HOLMES experiment, which aims at directly measuring the neutrino mass with the calorimetric approach, allowing to significally improve its statistical sensitivity.

The HOLMES experiment will perform a precise calorimetric measurement of the end point of the Electron Capture (EC) decay spectrum of 163Ho in order to extract information on neutrino mass with a sensitivity below 2 eV. In its final configuration, HOLMES will deploy 1000 detectors of low-temperature microcalorimeters with implanted 163Ho nuclei. The baseline sensors for HOLMES are Mo/Cu TESs (Transition Edge Sensors) on SiNx membrane with gold absorbers. Considering the large number of pixels and an event rate of about 300 Hz/pixel, a large multiplexing factor and a large bandwidth are needed. To fulfill this requirement, HOLMES will exploit recent advances in microwave multiplexing. In this contribution, we present the status of the activities in development, the performances of the developed microwave-multiplexed readout system, and the results obtained with the detectors specifically designed for HOLMES in terms of noise, time, and energy resolutions

HOLMES is an experiment aiming at performing a direct measurement of the neutrino mass from the electron capture (EC) spectrum of $$^{163}\hbox {Ho}$$. In order to reach a sensitivity of the order of $$\sim $$1 eV/c$$^2$$ on the neutrino mass, it is necessary to gather as many as $$10^{13}$$ events in the 3-year projected live time of HOLMES, keeping the pileup fraction as low as $$10^{-4}$$. This is not a trivial matter when it comes to low- temperature calorimeters, which usually have a rather slow time response. At the same time, a large number of detectors need to be operated simultaneously, and hence, in order to avoid an extremely large cryogenic facility, multiplexing is required. In this contribution, I will outline the current status and perspective of HOLMES, with special care devoted to the detectors and readout system, which have currently reached their target performance.

International audience; The absolute neutrino mass is still an unknown parameter in the modern landscape of particle physics. The HOLMES experiment aims at exploiting the calorimetric approach to directly measure the neutrino mass through the kinematic measurement of the decay products of the weak process decay of $^{163}$Ho. This low energy decaying isotope, in fact, undergoes electron capture emitting a neutrino and leaving the daughter atom, $^{163}$Dy$^*$, in an atomic excited state. This, in turn, relaxes by emitting electrons and, to a considerably lesser extent, photons. The high-energy portion of the calorimetric spectrum of this decay is affected by the non-vanishing neutrino mass value. Given the small fraction of events falling within the region of interest, to achieve a high experimental sensitivity on the neutrino mass, it is important to have a high activity combined with a very small undetected pileup contribution. To achieve these targets, the final configuration of HOLMES foresees the deployment of a large number of $^{163}$Ho ion-implanted TESs characterized by an ambitiously high activity of 300 Hz each. In this paper, we outline the status of the major tasks that will bring HOLMES to achieve a statistical sensitivity on the neutrino mass as low as 2 eV/c$^2$.

We present the first performance results obtained with microwave multiplexed Transition Edge Sensors prototypes specifically designed for the HOLMES experiment, a project aimed at directly measuring the electron neutrino mass through the calorimetric measurement of the $$^{163}$$ Ho electron capture spectrum. The detectors required for such an experiment feature a high energy resolution at the Q–value of the transition, around $$\sim $$ 2.8 keV, and a fast response time combined with the compatibility to be multiplexed in large arrays in order to collect a large statistics while keeping the pile-up contribution as small as possible. In addition, the design has to be suitable for future ion-implantation of $$^{163}$$ Ho. The results obtained in these tests allowed us to identify the optimal detector design among several prototypes. The chosen detector achieved an energy resolution of (4.5 ± 0.3) eV on the chlorine K $$_\alpha $$ line, at $$\sim $$ 2.6 keV, obtained with an exponential rise time of 14 $$\upmu $$ s. The achievements described in this paper pose a milestone for the HOLMES detectors, setting a baseline for the subsequent developments, aiming to the actual ion-implantation of the $$^{163}$$ Ho nuclei. In the first section the HOLMES experiment is outlined along with its physics goal, while in the second section the HOLMES detectors are described; the experimental set-up and the calibration source used for the measurements described in this paper are reported in Sects. 3 and 4, respectively; finally, the details of the data analysis and the results obtained are reported in Sect. 6.

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.

International audience; One of the most crucial challenges in today particle physics and cosmology is the determinationof the neutrino absolute mass scale. Currently, the only model independent method to set a limitneutrino mass is the study of the nuclear beta spectrum end-point. Performing a calorimetricmeasurement of the end point of the Electron Capture decay spectrum of 163 Ho, the HOLMESexperiment aims at pushing down the sensitivity on the smallest neutrino mass at the order of∼ eV. In its final configuration HOLMES will deploy an array of 1000 microcalorimeters basedon Transition Edge Sensors with gold absorbers in which the 163 Ho will be ion implanted with atarget activity of 300 Hz/det. In order to achieve a statistical sensitivity on the neutrino mass inthe eV range, there are stringent requirements on the detector performances: fast time resolution(∼ 1 μs) to solve pile-up events and an energy resolution of few eV at the Q-value (2.8 keV).Furthermore, the detectors must be multiplexable. The best technique to easily readout such anumber of detector with a common readout line is the microwave frequency domain readout.We outline the HOLMES project with its physics reach and technical challenges together withits status and perspectives. In detail, we report the status of HOLMES activities concerning the163 Ho isotope production, the TES and multiplexed array read-out and the isotope embeddingprocess.

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.

HOLMES is an experiment aimed at directly measuring the neutrino mass through the calorimetric measurement of the 163Ho electron capture decay. The final goal of the project consists in providing a sensitivity on the neutrino mass below 2 eV; in addition, it will establish the potential of this approach to achieve a sub-eV sensitivity. The detectors performances play a crucial role in achieving the desired sensitivity. Indeed, for such an experiment, the following characteristics are required: short response time ( ∼ 1 μ s ) to solve pile-up events, great energy resolution ( ∼ 1 eV @ 2.8 keV) and compatibility to be multiplexed in large detector arrays ( ≳ 1000). HOLMES will deploy 1000 Transition Edge Sensors which will be readout with the microwave multiplexing technique. In this contribution we outline the experimental setup used in the characterization phase of the detectors.

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

A powerful tool to determine the effective electron-neutrino mass is the calorimetric measurement of the energy released in a nuclear beta decay. Performing a precision measurement of the end point of the Electron Capture decay spectrum of 163Ho, HOLMES aims at pushing down the sensitivity on the neutrino mass below 1 eV. In its final configuration HOLMES will deploy an array of 1000 microcalorimeters based on Transition Edge Sensors with gold absorbers in which the 163Ho will be ion implanted. The best technique to easily read out such a number of detector with a common readout line is the microwave frequency domain multiplexing. Therefore, the TESs are coupled to multiplexed rf-SQUIDS operated in flux ramp modulation for linearization purposes. The rf-SQUIDS are then coupled to superconducting quarter wavelength resonators in the GHz range, from which the modulating signal is finally recovering using software defined radio techniques. In the last two years an extensive R&D activity has been carried out in order to maximize the multiplexing factor while preserving the performances of each detector which fulfil the HOLMES requirements (i.e. an energy resolution of few eV and a time-resolution of a few microseconds). We report here the progress made towards the characterization of the multiplexing system together with the results of the characterization of the HOLMES detectors.

The MEG experiment, designed to search for the μ+→e+γ decay, completed data-taking in 2013 reaching a sensitivity level of 5.3×10^−13 for the branching ratio. In order to increase the sensitivity reach of the experiment by an order of magnitude to the level of 6×10^−14, a total upgrade, involving substantial changes to the experiment, has been undertaken, known as MEG II. We present both the motivation for the upgrade and a detailed overview of the design of the experiment and of the expected detector performance

International audience; The assessment of the 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 to directly measure the neutrino mass by performing a calorimetric measurement of the energy released in the electron capture decay of the artificial isotope 163Ho. 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. HOLMES will deploy a large array of low temperature microcalorimeters implanted with 163Ho nuclei. The achievable neutrino mass statistical sensitivity is expected in the eV range, thereby making HOLMES an important step forward in the direct neutrino mass measurement with a calorimetric approach as an alternative to spectrometry. HOLMES will also establish the potential of this approach to achieve a sub-eV sensitivity. HOLMES is designed to collect about 3 × 1013 decays with an instrumental energy resolution around 1 eV FWHM and a time resolution around 1 µs. To achieve this in three years of measuring time, HOLMES is going to deploy 16 sub-arrays of TES microcalorimeters. Each sub-array has 64 pixels ion implanted with 163Ho nuclei to give a pixel activity of 300 Bq per pixel. The TES arrays are read out using microwave multiplexed rf-SQUIDs in combination with a Software Designed Radio data acquisition system. The commissioning of the first implanted sub-array is scheduled for 2018 and it will provide first high statistics data about the EC decay of 163Ho together with a preliminary limit on the neutrino mass. In this contribution we outline the HOLMES project with its physics reach and technical challenges, along with its status and perspectives. In particular we will present the status of the HOLMES activities concerning the 163Ho isotope production by neutron irradiation and purification, the TES pixel design and optimization, the multiplexed array read-out characterization, the cryogenic set-up installation, and the setting up of the mass separation and ion implantation system for the isotope embedding in the TES absorbers.

HOLMES is aiming at a direct measurement of neutrino mass by performing a calorimetric measurement of the energy released in the decay of 163Ho. In such approach, the 163Ho source, with the required activity, needs to be embedded in the detector. HOLMES will deploy a large array of transition-edge sensor microcalorimeters with implanted 163Ho ions. While good progress has been made in optimizing single pixel design and fabrication to achieve the target resolution, a major challenge is the fabrication of arrays of such microcalorimeters with the required amount of 163Ho ions embedded in the detectors absorber. We describe the multi-step microfabrication process implemented to produce the detector arrays for HOLMES. One crucial part of such process is the ability to perform co-deposition of gold during the 163Ho implantation process on the detectors absorber. We describe the UHV target chamber, with integrated gold deposition system, we have built to achieve this goal.

Measuring the neutrino mass is one of the most compelling issues in particle physics. HOLMES is an experiment for a direct measurement of the neutrino mass. HOLMES will perform a precise measurement of the end point of the electron capture decay spectrum of $$^{163}\hbox {Ho}$$ in order to extract information on the neutrino mass with a sensitivity as low as 1 eV. HOLMES, in its final configuration, will deploy a 1000-pixel array of low-temperature microcalorimeters: each calorimeter is made of an absorber, where the Ho atoms will be implanted, coupled to a transition-edge sensor (TES) thermometer. The detectors will be operated at the working temperature of $$100\,\hbox { mK}$$ provided by a dilution refrigerator. In order to read out the 1000-detector array of HOLMES, a multiplexing system is necessary: the choice is to couple the transition-edge sensors to a multiplexed rf-SQUID. In this contribution we outline the progress made towards the final configuration of HOLMES regarding both the performances of the TES detectors and the characteristics of the multiplexing system.

HOLMES is a new experiment to directly measure the neutrino mass with a sensitivity as low as 0.4 eV. HOLMES will perform a calorimetric measurement of the energy released in the electron capture decay of 163 Ho. HOLMES will deploy a large array of low temperature microcalorimeters with implanted 163 Ho nuclei. HOLMES baseline detector is an array of 1000 microcalorimeters each with an implanted 163 Ho activity of about 300 Bq, an energy resolution FWHM of about 1 eV at the spectrum end-point ( Q ≈ 2.5 keV), and a time resolution of about 1 μs. Matching these performances requires a careful optimization of all components, from the microcalorimeters to the signal processing algorithms. We outline here the project technical challenges and the present status of the development.

The main goal in the HOLMES experiment is the neutrino mass measurement using an array of 1000 micro-calorimeters with standard metallic absorber. A good isotope for such measurement is the 163 Ho, those isotopes embedded in the metallic absorber will be 10 11 –10 13 . Since 163 Ho is not available in nature, a dedicated process must be set up to produce the amount needed for this neutrino mass experiment. The process with the highest born-up cross-section is the neutron irradiation of Er 2 O 3 enriched in 162 Er: 162 Er(n,γ) 163 Er → 163 Ho+ν e , where the decay is an EC with half-life of about 75 min and the (n,γ) is about 20 barns for thermal neutron. After the neutron irradiation in the oxide powder there are several radioactive isotopes which are potentially disturbing because of the background that they cause below 5 keV. The chemical separation of holmium from the irradiation enriched Er 2 O 3 powder is therefore mandatory and will be performed by means of ion exchange chromatography. On the end of those processes the oxide powder enriched in 162 Er will have the 163 Ho isotope number required. The holmium chemical state influences the end point of the EC spectrum, in order to avoid such effect it is necessary to embed in the absorber only the metallic isotope. Reduction and distillation technique allowed us to obtain a pure metallic holmium, starting from natural oxide holmium. This technique will be applied on the irradiated oxide powder to obtain the metallic 163 Ho, ready to be embedded in the micro-calorimeter absorber.

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 neutrino mass determination is an open issue in particle physics. The study of the endpoint of beta decay is the best experimental way to provide a model-independent measurement. The HOLMES experiment aims to measure directly the neutrino mass with a calorimetric approach studying the 163Ho electron-capture decay. The very low Q-value (2.8 keV), the half-life (4570 y) and the proximity of the endpoint to M1 resonance make the 163Ho decay a very good choice. However, there are two critical steps to be considered for the realization of the experiment. The first step is embedding of the source isotope inside the cryogenic microcalorimeters so that the energy released in the decay process is entirely contained within the detectors, except for the fraction taken away by the neutrino. The second one is the rejection of 166Ho radioactive isotope that could produce false signal in the region of interest. Taking into account these two requirements, a dedicated implanter with a sputter ion source, an acceleration section (up to 50 keV) and a magnetic dipole (for ion selection and beam focusing) has been designed and developed. The implanter calibration and performance have been evaluated using 63Cu/65Cu and 197Au beams. Currently, different holmium compounds are being tested to find the candidate with the best efficiency in the sputter process. This work will show the status of the machine development and commissioning.