Study of naturally occurring radionuclides in the ECHo set-up

The determination of the effective electron neutrino mass by analyzing the end point region of the 163Ho electron capture (EC) spectrum relies on the precise description of the expected 163Ho events and background events. In the ECHo experiment, arrays of metallic magnetic calorimeters, implanted with 163Ho, are operated to measure the 163Ho EC spectrum. In an energy range of 10 eV below QEC, the maximum available energy for the EC decay of about 2.8 keV, a 163Ho event rate of the order of 10−4 day−1 pixel−1 is expected for an activity of 1 Bq of 163Ho per pixel. This means, a control of the background level in the order of 10−5 day−1 pixel−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 QEC 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 163Ho. We conclude that typical concentration of radioactive nuclides found in the used materials should not endanger the analysis of the endpoint region of the 163Ho EC spectrum for an exposure time of half a year.