Radiation qualification and optical performance of the InGaAs type imaging sensor for the vSWIR spectral range in the VEM instrument on the VERITAS mission

The Venus Emissivity Mapper (VEM) as part of NASAs Venus Emissivity, Radio science, InSAR, Topography, And Spectroscopy mission (VERITAS) is designed for mapping the surface of Venus within dedicated atmospheric spectral windows. The instrument will provide global coverage for detection of thermal emissions like volcanic activity, surface rock composition, water abundance and cloud formation as well as dynamics by observing 15 narrow filter bands in the near infrared to short wavelength infrared (NIR, SWIR) range of 862 nm to 1510 nm. An almost identical instrument will be part of ESAs EnVision mission to Venus, the VenSpec-M in the Venus Spectroscopy Suite (VenSpec). The utilized photodetector is an InGaAs type imaging sensor with integrated thermoelectric (TE) cooling. It comprises a 640x512 pixel array with 20 μm pixel pitch. Following the mission requirements we irradiated the detector with a set of ions of various stopping powers and range distributions from lower energy Argon (Ar) to higher energy Xenon (Xe). Therefore, exploiting the mentioned ions and proper tilt angles during irradiation, our data covers a Linear Energy Transfer (LET) range of 7 to 75 MeVcm²/mg which fulfills NASA/JPL led space qualification standards (up to 75 MeVcm²/mg) as well as ESA space qualification standards (up to 60 MeVcm²/mg) for heavy-ion irradiation. Our electrical setup consists of a dedicated over-current protection detecting high-current states occurring during irradiation steps and immediate power cycling to prevent physical damage of the device. From the event rates seen during the test we calculated the specific cross-sections and therefore can estimate the expected event rates at Venus during the mission. The detector showed saturated cross-sections below 1E-3 cm² at 10°C with acceptable event rates for the highest LETs and our applications.