Precision measurement of the electron energy-loss function in tritium and deuterium gas for the KATRIN experiment

Abstract

The KATRIN experiment is designed for a direct and model-independent determination of the effective electron anti-neutrino mass via a high-precision measurement of the tritium $\mathrm{\beta }$-decay endpoint region with a sensitivity on $m_{\nu }$ of 0.2 $\text{eV}/{\text{c}}^2$ (90% CL). For this purpose, the $\mathrm{\beta }$-electrons from a high-luminosity windowless gaseous tritium source traversing an electrostatic retarding spectrometer are counted to obtain an integral spectrum around the endpoint energy of 18.6 keV. A dominant systematic effect of the response of the experimental setup is the energy loss of $\mathrm{\beta }$-electrons from elastic and inelastic scattering off tritium molecules within the source. We determined the energy-loss function in-situ with a pulsed angular-selective and monoenergetic photoelectron source at various tritium-source densities. The data was recorded in integral and differential modes; the latter was achieved by using a novel time-of-flight technique. We developed a semi-empirical parametrization for the energy-loss function for the scattering of 18.6-keV electrons from hydrogen isotopologs. This model was fit to measurement data with a 95% ${\text{T}}_2$ gas mixture at 30 K, as used in the first KATRIN neutrino-mass analyses, as well as a ${\text{D}}_2$ gas mixture of 96% purity used in KATRIN commissioning runs. The achieved precision on the energy-loss function has abated the corresponding uncertainty of $\sigma (m_{\nu }^2)<{10}^{-2}{\text{eV}}^2$ [1] in the KATRIN neutrino-mass measurement to a subdominant level.