Study of naturally occurring radionuclides in the ECHo set-up

Abstract

The determination of the effective electron neutrino mass by analyzing the end point region of the ^{163}Ho electron capture (EC) spectrum relies on the precise description of the expected ^{163}Ho events and background events. In the ECHo experiment, arrays of metallic magnetic calorimeters, implanted with ^{163}Ho, are operated to measure the ^{163}Ho EC spectrum. In an energy range of 10 eV below Q_{mathrm {EC}}, the maximum available energy for the EC decay of about 2.8 keV, a ^{163}Ho event rate of the order of 10^{-4} day^{-1} pixel^{-1} is expected for an activity of 1 Bq of ^{163}Ho 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 Q_{mathrm {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}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}Ho EC spectrum for an exposure time of half a year.