Simulating the charging of isolated free-falling masses from TeV to eV energies: detailed comparison with LISA Pathfinder results

A model is presented that explains the charging rate of LISA Pathfinder test masses by interplanetary cosmic ray environment. The incorporates particle-tracking from TeV to eV energies using a combination GEANT4 and custom low-energy particle generation tracking code. electrostatic environment mass simulated allowing for comparison test-mass charging-rate dependence on local electric fields with observations made in orbit. able reproduce observed behavior good accuracy gold surface properties compatible literature values. results confirm significant fraction net current caused population ($\sim$eV) electrons produced electron- ion-induced kinetic emission surrounding metal surfaces. Assuming work function 4.2 eV, unbalanced flow these unbiased contributes $\sim$10% overall rate. Their contribution charging-current shot noise disproportionately higher it adds $\sim$40% predicted noise. However, even this increased still only 40% measured in-orbit, remains an unsolved issue.