Low-energy electron emission at the separation of gold-platinum surfaces induced by galactic cosmic rays on board LISA Pathfinder

Galactic cosmic rays and solar energetic particles will induce spurious Coulomb forces on free-falling, gold-platinum test-masses on board the future interferometers for low-frequency gravitational wave detection in space. The European Space Agency mission LISA Pathfinder (LPF) was aimed to test the performance of instruments that will be placed on board LISA (Laser Interferometric Space Antenna), expected to be launched in 2034. Free-falling test-mass charging was measured on board LPF in 2016–2017. Simulations of this charging process were carried out before the mission launch with GEANT4 and Fluka Monte Carlo tools. A very good agreement was found between net charging measurements and simulations, while the shot noise, associated with the charging process, resulted 3–4 times larger than expected. New dedicated simulations aiming to study in detail the low-energy electron emission from metal electrodes surrounding the test-masses reveal that the mismatch between measurements and former simulations may be due to the lack of propagation of electrons with energies smaller than the average ionization potential in the Monte Carlo tools mentioned above. Preliminary results are presented and discussed here.