Development of a balloon-style pressure vessel gondola for balloon-borne emulsion gamma-ray telescopes

A gamma-ray telescope using nuclear emulsions allows observations of stellar objects in the 0.01-100 GeV energy region with high angular resolution, polarization sensitivity, and large aperture area. The Gamma-Ray Astro-Imager with Nuclear Emulsion (GRAINE) project is designed to enable high-precision observation of cosmic gamma rays through the use of balloon-borne emulsion telescopes. We have developed a balloon-style pressure vessel gondola for the GRAINE 2018 balloon-borne experiment. It enables us to maintain robust and vacuum-packed emulsion chambers even at balloon altitudes. The greater part of the vessel is made of membrane materials consisting of ultra-thin polyurethane-coated polyester fabric. It is lightweight and portable, and can be used to expand the mounting area of the detector, unlike mostly metal vessels. Moreover, the absorption of signal gamma rays and background gamma-ray production due to interactions between cosmic rays and the vessel can be minimized. The pressure vessel gondola, equipped with a 0.38-m(2) emulsion telescope and other equipment, was launched on April 26, 2018. During the whole observation (17 h 21 m), the internal pressure was kept above 230 hPa. The GRAINE project will start scientific observation using enlarged telescopes and the pressure vessel gondola is planned to include an expansion of the mounting area of the detector to similar to 10 m(2) while following the design on the GRAINE 2018 model.