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 adaptable 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. Program Title: KARL - KA trin WGTS elect R on and ion spectrum Monte Car L o CPC Library link to program files: https://doi.org/10.17632/5bj3vwc6rg.1 Licensing provisions: GNU Public License v3 Programming language: C++ External routines/libraries: C++ compiler (tested with g++ 8.2 and 9.4.0), MPI 1.1 (tested with OpenMPI 3.1), HDF5 with support for parallel I/O (tested with version 1.10.0), Blitz++ (tested with version 1.0.2), Jansson (tested with version 2.12 and 2.13) Nature of problem: In the KATRIN experiment (and other experiments alike that feature large vessels filled with tritium) electrons are created from beta decay. These electrons interact with the ambient gas to produce secondary electrons through ionization. Subsequent processes include excitation, secondary ionization and collisions. The resulting electron and ion differential energy spectrum at various positions is relevant for further plasma analysis, and the current of charged particles to the ends of the experiments is an observable. Solution method: Semi-classical Monte Carlo. Additional comments including restrictions and unusual features: The geometry of the experiment is currently limited to the KATRIN experiment, but this may easily be changed. The configuration is stored in JSON files.