  This study investigates the effects of space environmental radiation on the
performance of InGaAs Quadrant Photodiodes (QPDs) and assesses their
suitability for the Laser Interferometer Space Antenna (LISA) mission. QPDs of
1.0, 1.5 and 2.0 mm have been irradiated with 20 and 60 MeV protons, 0.5 and 1
MeV electrons, and Co$^{60}$ gamma. An exposure corresponding to a displacement
damage equivalent fluence of $1.0 \times 10^{+12}$ p/cm$^2$ for 20 and 60 MeV
protons and a total ionizing dose of 237 krad were applied, surpassing the
anticipated radiation levels for the LISA mission by a factor of approximately
five. Experiments were conducted to measure changes in QPD dark current,
capacitance, and responsivity. The QPDs are integrated with a low-noise
DC-coupled transimpedance amplifier to form the Photoreceiver (QPR). QPR noise
and performance in an interferometric system like LISA were also measured.
Although radiation impacted their dark current and responsivity, almost all
QPDs met LISA's validation criteria and did not demonstrate any critical
failure. These findings prove that the tested QPDs are promising candidates for
LISA and other space-based missions.
