Eur. Phys. J. Special Topics 226, 1623-1694 (2017)
https://doi.org/10.1140/epjst/e2017-70071-y

Regular Article

The electron capture in ¹⁶³Ho experiment – ECHo

¹ Kirchhoff Institute for Physics, Heidelberg University, Heidelberg, Germany
² Max-Planck Institute for Nuclear Physics, Heidelberg, Germany
³ Institute for Physics, Johannes Gutenberg-University, Mainz, Germany
⁴ ISOLDE, CERN, Geneve, Switzerland
⁵ Institute for Nuclear and Particle Physics, TU Dresden, Germany
⁶ Institute for Nuclear Chemistry, Johannes Gutenberg University, Mainz, Germany
⁷ Laboratory of Radiochemistry and Environmental Chemistry, Department Biology and Chemistry, Paul Scherrer Institute, Villigen PSI, Switzerland
⁸ Laboratory of Radiochemistry and Environmental Chemistry, Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, Bern, Switzerland
⁹ GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt, Germany
¹⁰ Helmholtz Institute Mainz, Mainz, Germany
¹¹ Institute for Theoretical Physics, University of Tübingen, Tübingen, Germany
¹² Physics Institute, University of Tübingen, Germany
¹³ Goethe-Universität, Frankfurt am Main, Germany
¹⁴ Institut Laue-Langevin, Grenoble, France
¹⁵ Chemical Sciences Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata, India
¹⁶ Department of Physics, Indian Institute of Technology Roorkee, Roorkee, India
¹⁷ Petersburg Nuclear Physics Institute, Gatchina, Russia
¹⁸ St.Petersburg State University, St. Petersburg, Russia
¹⁹ Institut für Physik, Humboldt-Universität zu Berlin, Berlin, Germany
²⁰ Karlsruhe Institute of Technology, Institute for Data Processing and Electronics, Karlsruhe, Germany
²¹ Department of Nuclear Physics and Biophysics, Comenius University, Bratislava, Slovakia
²² Institute of Nuclear Research of the H.A.S., Bem ter 18/C, Debrecen, Hungary

^a e-mail: Loredana.Gastaldo@kip.uni-heidelberg.de
^b Present address: Institut für Kernphysik, Universität Münster, Münster, Germany

Received: 17 March 2017
Published online: 20 June 2017

Abstract

Neutrinos, and in particular their tiny but non-vanishing masses, can be considered one of the doors towards physics beyond the Standard Model. Precision measurements of the kinematics of weak interactions, in particular of the ³H β-decay and the ¹⁶³Ho electron capture (EC), represent the only model independent approach to determine the absolute scale of neutrino masses. The electron capture in ¹⁶³Ho experiment, ECHo, is designed to reach sub-eV sensitivity on the electron neutrino mass by means of the analysis of the calorimetrically measured electron capture spectrum of the nuclide ¹⁶³Ho. The maximum energy available for this decay, about 2.8 keV, constrains the type of detectors that can be used. Arrays of low temperature metallic magnetic calorimeters (MMCs) are being developed to measure the ¹⁶³Ho EC spectrum with energy resolution below 3 eV FWHM and with a time resolution below 1 μs. To achieve the sub-eV sensitivity on the electron neutrino mass, together with the detector optimization, the availability of large ultra-pure ¹⁶³Ho samples, the identification and suppression of background sources as well as the precise parametrization of the ¹⁶³Ho EC spectrum are of utmost importance. The high-energy resolution ¹⁶³Ho spectra measured with the first MMC prototypes with ion-implanted ¹⁶³Ho set the basis for the ECHo experiment. We describe the conceptual design of ECHo and motivate the strategies we have adopted to carry on the present medium scale experiment, ECHo-1K. In this experiment, the use of 1 kBq ¹⁶³Ho will allow to reach a neutrino mass sensitivity below 10 eV/c². We then discuss how the results being achieved in ECHo-1k will guide the design of the next stage of the ECHo experiment, ECHo-1M, where a source of the order of 1 MBq ¹⁶³Ho embedded in large MMCs arrays will allow to reach sub-eV sensitivity on the electron neutrino mass.