Field testing for Extreme Universe Space Observatory aboard a Super Pressure Balloon (EUSO-SPB): logistics and first results

Abstract

Extreme Universe Space Observatory aboard a Super Pressure Balloon (EUSO-SPB) is a prototype cosmic ray detector that will, for the first time, record cosmic rays from above. It is planned to fly for nearly 100 days at an altitude of \unit[33]{km}, looking downward onto Earth's atmosphere, and measure the ultraviolet light emitted by ultra-high energy cosmic ray extensive air showers. It is the primary scientific payload aboard the 2017 NASA super pressure balloon flight launched from Wanaka, NZ. \\ It was necessary to perform an end-to-end characterization of the instrument, as post test recovery is not guaranteed. A laser underflight study for EUSO-SPB is planned to occur to evaluate the assembled instrument's response to realistic optical tracks. In case of failure, it was necessary to conduct ground-to-ground testing, where laser test beams at "flight-like" distances through Earth's atmosphere would be provided.\\ The Black Rock Mesa (BRM) Telescope Array (TA) test site in Delta, Utah was chosen as the location for the instrument field testing. The assembled and working EUSO-SPB instrument was transported using a ground loading, weather-sealed, air-suspension trailer and a custom "vibration-proof" dolly. During transportation, the maximum acceleration experienced by the instrument was \unit[1.2]{g's}. Test beams were provided by a fixed energy, vertical laser at \unit[21]{km} and by a steerable, variable energy, laser at \unit[24]{km}. Hundreds of thousands of pulsed, UV laser shots were successfully recorded during the six nights of operation, with various energies, directions, and triggering methods. Energy sweeps were performed with a $45^{\circ}$ tilted away laser at \unit[24]{km} to determine the nominal energy threshold of EUSO-SPB in a balloon simulated geometry for two different lens configurations.\\ First results show that the 2 lens configuration of EUSO-SPB was approximately twice as sensitive as the 3 lens configuration to laser light, with $50\%$ energy thresholds of \unit[1]{mJ} and \unit[2]{mJ}, respectively. Cloud coverage and aerosols were ruled out as potential causes of the lens configuration energy threshold discrepancy. A preliminary conversion to equivalent cosmic ray energy was performed using custom laser and cosmic ray simulations. For EUSO-SPB in flight configuration, the $50\%$ energy threshold was estimated to be \unit[$3*10^{18}$]{eV} and \unit[$7*10^{18}$]{eV} for the 2 and 3 lens configurations, respectively, assuming aerosol content of $\alpha$=\unit[$5*10^{-5}$]{$\mathrm{m}^{-1}$} and $\mathrm{H}_{\mathrm{scale}}$=\unit[2]{km} during the laser data collection. The estimated event rate for EUSO-SPB was updated using the results of this work, and found to be 4.5 events per week.