GRAINE 2015, a balloon-borne emulsion $\gamma$-ray telescope experiment in Australia

Abstract

We are developing the GRAINE project, which consists of a 10 MeV–100GeV cosmic |$\gamma$|-ray observation with a precise (0.08|$^\circ$| at 1–2GeV) and polarization-sensitive (50% or below the minimum detectable polarization for the Vela pulsar above 100 MeV) large-aperture-area (⁠|$\sim$|10m|$^2$|⁠) emulsion telescope, by repetitive long duration balloon flights. In 2015, we performed a Japan–Australia JAXA collaborative balloon experiment at the Alice Springs balloon-launching station. The telescope had a 3780cm|$^2$| aperture and was aloft for a total of 14.4h. The experiment aims to detect the well-known bright Vela pulsar |$\gamma$|-ray source and thereby demonstrate the overall performance of the emulsion |$\gamma$|-ray telescope. The design, improvements, and preparations as well as the 2015 balloon experiment are described in this paper. Analysis of the data from this experiment is ongoing.