Test-mass charging simulations for the LISA Pathfinder mission

We present detailed Monte Carlo simulations of test-mass charging caused by energetic cosmic rays in the LISA Pathfinder mission, using the GEANT4 software toolkit. This effect can lead to a variety of disturbances, threatening the goal of the mission-to achieve a low-frequency acceleration noise within an order of magnitude of that required for LISA. We calculate the test-mass charging rates and charging shot noise for different solar conditions, including solar energetic particle eruptions. Further simulations show that the inclusion of additional masses within the LTP inertial sensor, to balance the gravitational accelerations felt by the test masses, does not significantly alter the overall charging rates. The specifications of a particle monitor for LISA Pathfinder, which can detect variations in the energetic particle flux and enable correlation with test-mass charging, are also described