For above ground particle physics experiments, cosmic muons are common source of background, not only for direct detector hits, but also for secondary radiation created in neighboring materials. The ECHo experiment has been designed for the determination of the effective electron neutrino mass by the analysis of the endpoint region of the $$^{163}\text {Ho}$$ 163 Ho electron capture spectrum. The fraction of events occurring in the region of interest of 10 eV below the $$Q_{\mathrm {EC}}$$ Q EC value of about 2.8 keV is only of the order of $$10^{-9}$$ 10 - 9 . This means that the background in that region need to be studied, characterized and methods to suppress it need to be developed. We expect a major background contribution to be due to cosmic muons and radiation produced by muons traveling through material around the detectors. To determine the muon-related background in metallic magnetic calorimeters (MMCs) used in the ECHo experiment, we have performed an experiment in which a muon veto was installed around the cryostat used for the operation of the detectors. We analysed the acquired events to investigate the pulse shape of MMC events in coincidence with the muon veto and the rate of multiple coincidences among detector array pixels. With different methods used for identification of muon related events, we studied events generated by muons and secondary radiation depositing energy in the substrate close to the ECHo pixels. In addition, energy depositions of muons and secondary radiation in the detectors was studied via Monte Carlo simulation. At the present status of investigation, we conclude that muon related events will be a negligible background in the region of interest of the $$^{163}\text {Ho}$$ 163 Ho spectrum.

The determination of the effective electron neutrino mass by analyzing the end point region of the $$^{163}$$ 163 Ho electron capture (EC) spectrum relies on the precise description of the expected $$^{163}$$ 163 Ho events and background events. In the ECHo experiment, arrays of metallic magnetic calorimeters, implanted with $$^{163}$$ 163 Ho, are operated to measure the $$^{163}$$ 163 Ho EC spectrum. In an energy range of 10 eV below $$Q_{\mathrm {EC}}$$ Q EC , the maximum available energy for the EC decay of about 2.8 keV, a $$^{163}$$ 163 Ho event rate of the order of $$10^{-4}$$ 10 - 4  day$$^{-1}$$ - 1  pixel$$^{-1}$$ - 1 is expected for an activity of 1 Bq of $$^{163}$$ 163 Ho per pixel. This means, a control of the background level in the order of $$10^{-5}$$ 10 - 5  day$$^{-1}$$ - 1  pixel$$^{-1}$$ - 1 is extremely important. We discuss the results of a Monte Carlo study based on simulations, which use the GEANT4 framework to understand the impact of natural radioactive isotopes close to the active detector volume in the case of the ECHo-1k set-up, which is used for the first phase of the ECHo experiment. For this, the ECHo-1k set-up was modeled in GEANT4 using the proper geometry and materials, including the information of screening measurements of some materials used in the ECHo-1k set-up and reasonable assumptions. Based on the simulation and on assumptions, we derive the expected background around $$Q_{\mathrm {EC}}$$ Q EC and give upper limits of tolerable concentrations of natural radionuclides in the set-up materials. In addition, we compare our results to background spectra acquired in detector pixels with and without implanted $$^{163}$$ 163 Ho. We conclude that typical concentration of radioactive nuclides found in the used materials should not endanger the analysis of the endpoint region of the $$^{163}$$ 163 Ho EC spectrum for an exposure time of half a year.

The definition of the absolute neutrino mass scale is one of the main goals of the Particle Physics today. The study of the end-point regions of the βand electron capture (EC) spectrum offers a possibility to determine the effective electron (anti-)neutrino mass in a completely model independent way, as it only relies on the energy and momentum conservation. The ECHo (Electron Capture in 163 Ho) experiment has been designed in the attempt to measure the effective mass of the electron neutrino by performing high statistics and high energy resolution measurements of the 163 Ho electron capture spectrum. To achieve this goal, large arrays of low temperature metallic magnetic calorimeters (MMCs) implanted with with 163 Ho are used. Here we report on the structure and the status of the experiment.