  Test mass charging caused by cosmic rays will be a significant source of
acceleration noise for space-based gravitational wave detectors like LISA.
Operating between December 2015 and July 2017, the technology demonstration
mission LISA Pathfinder included a bespoke monitor to help characterise the
relationship between test mass charging and the local radiation environment.
The radiation monitor made in situ measurements of the cosmic ray flux while
also providing information about its energy spectrum. We describe the monitor
and present measurements which show a gradual 40% increase in count rate
coinciding with the declining phase of the solar cycle. Modulations of up to
10% were also observed with periods of 13 and 26 days that are associated with
co-rotating interaction regions and heliospheric current sheet crossings. These
variations in the flux above the monitor detection threshold (approximately 70
MeV) are shown to be coherent with measurements made by the IREM monitor
on-board the Earth orbiting INTEGRAL spacecraft. Finally we use the measured
deposited energy spectra, in combination with a GEANT4 model, to estimate the
galactic cosmic ray differential energy spectrum over the course of the
mission.
