First overnight balloon flight of the GRAINE 2023 emulsion gamma-ray telescope enabled by a large-scale pressure-vessel gondola

The Gamma-Ray Astro Imager with Nuclear Emulsion (GRAINE) project uses balloon-borne nuclear-emulsion telescopes for precision observations of sub-GeV–GeV cosmic gamma rays with high angular resolution. GRAINE 2023 aimed to fly a 2.5-m² telescope on its first overnight balloon flight, including observation periods for the Vela pulsar and the Galactic center region.

To operate under low-pressure, low-temperature stratospheric conditions, the balloon-style pressure-vessel concept was scaled up to a lightweight gondola with an internal length of 4.9 m. This required developing a new aluminum-alloy ring structure and a lightweight membranous-shell material, SHL-300MDL. The aperture was increased by a factor of 6.6 over GRAINE 2018 while keeping the gondola mass to 179 kg. Ground tests of the completed assembly demonstrated a differential pressure above 100 hPa at room temperature and at a mean temperature of −66.0°C.

The payload launched from Alice Springs, Australia, in April 2023 and achieved a total flight duration of approximately 27 h, including 24.3 h of level flight. Although the upper membranous shell reached about −60°C at night, the vessel internal pressure stayed above the required 100 hPa throughout level flight.

These results show the gondola can accommodate a 2.5-m² emulsion gamma-ray telescope and maintain pressure through an overnight stratospheric flight. Scientific analyses of astrophysical and atmospheric gamma rays, including dedicated analysis of the Galactic center region, are ongoing using the recovered emulsion data, providing a technical basis for repeated observations with future large-area GRAINE telescopes.