KARL -- A Monte Carlo model for atomic and molecular processes in the tritium atmosphere of the KATRIN experiment

A new parallelized simulation code is presented, which uses a Monte Carlo method to determine particle spectra in the KATRIN source. Reaction chains are generated from the decay of tritium within the source. The code includes all relevant processes: elastic scattering, ionization, excitation (electric, vibrational, rotational), recombination and various clustering processes. The main emphasis of the code is the calculation of particle spectra and particle densities and currents at specific points within the source. It features a new technique to determine these quantities. It also calculates target fields for the interaction of particles with each other as it is needed for recombination processes. The code has been designed for the KATRIN experiment but is easily adapt-able for other tritium based experiments like Project 8. Geometry and background tritium gas flow can be given as user input. The code is parallelized using MPI and writes output using HDF5. Input to the simulation is read from a JSON description.