  The KArlsruhe TRItium Neutrino experiment KATRIN aims at improving the upper
limit of the mass of the electron antineutrino to about 0.2 eV (90% c.l.) by
investigating the beta-decay of tritium gas molecules. The experiment is
currently under construction to start first data taking in 2012. One source of
systematic uncertainties in the KATRIN experiment is the formation of ion
clusters when tritium decays and decay products interact with residual tritium
molecules. It is essential to monitor the abundances of these clusters since
they have different final state energies than tritium ions. For this purpose, a
prototype of a cylindrical Penning trap has been constructed and tested at the
Max-Planck-Institute for Nuclear Physics in Heidelberg, which will be installed
in the KATRIN beam line. This system employs the technique of Fourier-Transform
Ion-Cyclotron-Resonance in order to measure the abundances of the different
stored ion species.
