On the role of radiation monitors on board LISA Pathfinder and future space interferometers

LISA (Laser Interferometer Space Antenna) and its precursor mission LISA Pathfinder (LISA-PF) will carry particle monitors for noise diagnostics. It was proposed to build and place radiation detectors on board the ASTROD missions as well. We present here a study of the solar energetic particle (SEP) events that the LISA-PF radiation monitors are able to detect above the galactic cosmic-ray (GCR) background predicted at the time of the mission data taking in 2015. In order to optimize the correlation between radiation monitor measurements and gravitational sensor test-mass charging, the energy threshold for particles traversing both detectors should be approximately the same. In LISA-PF, the radiation monitor particle energy cut-off was conservatively set at 75 MeV per nucleon (MeV/n) for protons and ion normal incidence, while the minimum energy of the same particles reaching the test masses is 100 MeV/n. We find that SEP events detectable on LISA-PF are characterized by peak fluxes and fluences at energies >75 MeV/n larger than about 45%, on average, with respect to those at energies >100 MeV/n. We conclude that for an accurate correlation between radiation monitor count rates and test-mass charging, it is mandatory to benefit from absolute flux measurements of both galactic and high-energy solar particles provided by experiments carrying magnetic spectrometers in space at the time of LISA-PF (PAMELA, AMS). On the other hand, the role of the radiation detectors on board LISA-PF is crucial allowing for SEP event onset and dynamics monitoring.