The Karlsruhe Tritium Neutrino (KATRIN) experiment is targeted to measure the absolute electron neutrino mass with a sensitivity of 0.2$\,$eV/c$^2$ at 90% confidence level.
In order to determine the neutrino mass from a spectroscopic measurement of the tritium $\beta$-spectrum near the endpoint, molecular tritium is circulated in a windowless source and the kinetic energy of the decay electrons is analysed with a MAC-E-type spectrometer.
One of the major systematics of the experiment is the energy loss of the signal electrons due to inelastic scattering with the source gas.
This energy loss causes a distortion of the experimental response function which must be considered in the data analysis.
Hence, a precise description of the energy-loss function is required, which can be obtained from response function measurements using a monoenergetic and angular-selective photoelectron source.
During the commissioning of the experiment in 2018 and the first tritium measurement campaign in 2019, measurements were carried out for both molecular deuterium and tritium using integral and differential measurement techniques.
The data was analysed by performing a combined fit to the measurement data comprising a new semi-empirical energy-loss function model.
The achieved precision of the measurement result is unprecedented and
meets the stringent requirements of KATRIN in order to reach the sensitivity goal.