  A comprehensive summary of the measurements made to characterize test mass
charging due to the space environment during the LISA Pathfinder mission is
presented. Measurements of the residual charge of the test mass after release
by the grabbing and positioning mechanism, show that the initial charge of the
test masses was negative after all releases, leaving the test mass with a
potential in the range $-12$ mV to $-512$ mV. Variations in the neutral test
mass charging rate between $21.7$ e s$^{-1}$ and $30.7$ e s$^{-1}$ were
observed over the course of the 17-month science operations produced by cosmic
ray flux changes including a Forbush decrease associated with a small solar
energetic particle event. A dependence of the cosmic ray charging rate on the
test mass potential between $-30.2$ e s$^{-1}$ V$^{-1}$ and $-40.3$ e s$^{-1}$
V$^{-1}$ was observed and this is attributed to a contribution to charging from
low-energy electrons emitted from the gold surfaces of the gravitational
reference sensor. Data from the on-board particle detector show a reliable
correlation with the charging rate and with other environmental monitors of the
cosmic ray flux. This correlation is exploited to extrapolate test mass
charging rates to a 20-year period giving useful insight into the expected
range of charging rate that may be observed in the LISA mission.
