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

The Alice UV spectrograph aboard NASAs New Horizons mission is sensitive to MeV electrons that penetrate the instruments thin aluminum housing and interact with its microchannel plate detector. We have searched for penetrating electrons at heliocentric distance of 2-45 AU, finding no evidence of discrete events outside of the Jovian magnetosphere. However, we do find a gradual long-term increase in the Alice instruments global dark count rate at a rate of ~1.5% per year, which may be caused by a heightened gamma-ray background from aging of the spacecrafts radioisotope thermoelectric generator fuel. If this hypothesis is correct, then the Alice instruments global dark count rate should flatten and then decrease over the next 5-10 years.