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The Alice UV spectrograph aboard NASA's New Horizons mission is sensitive toMeV electrons that penetrate the instrument's thin aluminum housing andinteract with its microchannel plate detector. We have searched for penetratingelectrons at heliocentric distance of 2-45 AU, finding no evidence of discreteevents outside of the Jovian magnetosphere. However, we do find a graduallong-term increase in the Alice instrument's global dark count rate at a rateof ~1.5% per year, which may be caused by a heightened gamma-ray backgroundfrom aging of the spacecraft's radioisotope thermoelectric generator fuel. Ifthis hypothesis is correct, then the Alice instrument's global dark count rateshould flatten and then decrease over the next 5-10 years.

The Search for MeV Electrons 2-45 AU from the Sun with the Alice Instrument Microchannel Plate Detector Aboard New Horizons