https://doi.org/10.1140/epjc/s10052-019-7513-x
Regular Article - Experimental Physics
High-resolution and low-background 
Ho spectrum: interpretation of the resonance tails
1
Kirchhoff-Institute for Physics, Heidelberg University, Heidelberg, Germany
2
Institute of Nuclear Chemistry, Johannes Gutenberg University, Mainz, Germany
3
GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany
4
Helmholtz Institute Mainz, Mainz, Germany
5
Institute of Physics, Johannes Gutenberg University, Mainz, Germany
6
Institute for Theoretical Physics, Heidelberg University, Heidelberg, Germany
7
Physics Institute, University of Tübingen, Tübingen, Germany
8
Max-Planck-Institut für Kernphysik, Heidelberg, Germany
9
Petersburg Nuclear Physics Institute, Gatchina, Russia
10
Laboratoire Souterrain de Modane, Modane, France
11
Physics Department, CERN, Geneva, Switzerland
12
Institut Laue-Langevin, Grenoble, France
13
Institute of Nuclear and Particle Physics, Dresden University, Dresden, Germany
14
Present address: TRIUMF, Wesbrook Mall, Vancouver, Canada
15
Present address: Institut für Technische Informatik, Heidelberg University, Heidelberg, Germany
* e-mail: Clemens.Velte@kip.uni-heidelberg.de
Received:
8
September
2019
Accepted:
26
November
2019
Published online:
21
December
2019
The determination of the effective electron neutrino mass via kinematic analysis of beta and electron capture spectra is considered to be model-independent since it relies on energy and momentum conservation. At the same time the precise description of the expected spectrum goes beyond the simple phase space term. In particular for electron capture processes, many-body electron-electron interactions lead to additional structures besides the main resonances in calorimetrically measured spectra. A precise description of the Ho spectrum is fundamental for understanding the impact of low intensity structures at the endpoint region where a finite neutrino mass affects the shape most strongly. We present a low-background and high-energy resolution measurement of the Ho spectrum obtained in the framework of the ECHo experiment. We study the line shape of the main resonances and multiplets with intensities spanning three orders of magnitude. We discuss the need to introduce an asymmetric line shape contribution due to Auger–Meitner decay of states above the auto-ionisation threshold. With this we determine an enhancement of count rate at the endpoint region of about a factor of 2, which in turn leads to an equal reduction in the required exposure of the experiment to achieve a given sensitivity on the effective electron neutrino mass.
© The Author(s), 2019
