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

Abstract

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

$$\upbeta $$

β

-decay endpoint region with a sensitivity on

$$m_\nu $$

m ν

of 0.2

$$\hbox {eV}/\hbox {c}^2$$

eV /

c 2

(90% CL). For this purpose, the

$$\upbeta $$

β

-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

$$\upbeta $$

β

-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%

$$\hbox {T}_2$$

T 2

gas mixture at 30 K, as used in the first KATRIN neutrino-mass analyses, as well as a

$$\hbox {D}_2$$

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

$$\sigma (m_\nu ^2)< {{10}^{-2}}{\hbox {eV}^{2}}$$

σ

(

m ν 2

)

<

10

- 2

eV 2

[1] in the KATRIN neutrino-mass measurement to a subdominant level.