The Gamma-Ray Astro-Imager with Nuclear Emulsion (GRAINE) project is aimed at the precise observation of astronomical gamma-ray sources in the energy range of 10 MeV–100 GeV using a balloon-borne telescope utilizing a nuclear emulsion, which can help realize precise imaging with a high angular resolution (1.0○ at 100 MeV), polarization sensitivity and a large aperture area (10 m2). In 2018, the third balloon experiment was carried out as a demonstration of the detection of the brightest known astronomical gamma-ray source, the Vela pulsar, with an aperture area of 0.38 m2. In this data, some gamma-rays were produced by the π0 → 2γ decay, which was caused by the hadronic interactions of cosmic rays in the detector. These could be used to calibrate the reconstructed angle, energy, and so on. In this study, we established a method of searching for hadronic interactions and concomitant gamma rays with high statistics and purity. Our analysis indicates that the performance of our detector for gamma rays are as expected in wide incidence-angle and energy ranges. We plan to commence scientific observations using the proposed system with the verified high angular resolution and largest aperture area in 2022 or later. Subject Index F10, H22
Since 2008, the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope (Fermi-LAT) has surveyed the sub-GeV/GeV gamma-ray sky and achieved high statistics measurements. However, observation at low galactic latitudes remains difficult owing to the lack of the angular resolution, and new issues following the operation of Fermi-LAT have arisen. We devised a precise gamma-ray observation project, Gamma-Ray Astro-Imager with Nuclear Emulsion (GRAINE), using balloon-borne emulsion gamma-ray telescopes to realize high angular resolution, polarization-sensitive, and large-aperture observations in the 0.01-100 GeV energy region. On April 26, 2018, the 3rd balloon experiment of the GRAINE project was conducted in Alice Springs, Australia, to detect celestial gamma-ray sources and to demonstrate the overall performance of the middle-sized emulsion telescope (aperture area of 0.4 m 2). The balloon floated at the altitude of 36-38 km for 15 h, and the telescope observed the target object, Vela pulsar, for 6 h. Following the recovery and the photofinishing, the data acquisition by the emulsion scanning system were completed, and then the flight data analysis has been performed using reconstructed gamma-ray events. In this presentation, the in-flight performance focusing on the converter part of the emulsion telescope employed in the GRAINE 2018 balloon experiment is reported.
We report a precise measurement of the sub-GeV atmospheric gamma-ray spectrum at balloon altitude on GRAINE 2018 experiment, and comparisons with the predictions calculated by the latest HKKM, which is widely known as a model for atmospheric neutrino flux calculation. Understanding the interactions between cosmic rays and atmospheric nuclei is important for accurate atmospheric neutrino flux calculations. Observation data of sub-GeV atmospheric gamma rays at balloon altitudes are useful for verifying such hadronic interaction models and pion productions in the low energy region. In April 2018, we conducted a balloon experiment (GRAINE 2018) in Australia with the aim of detecting and imaging the celestial gamma-ray sources with the nuclear emulsion telescope. Following flight data analysis, we derived an atmospheric gamma-ray spectrum in 0.1-1 GeV region at altitudes of ∼36 km (residual depth ∼ 4 g / cm 2 ). The flux around the 1 GeV region is in good agreement with the HKKM prediction and smoothly connects to the multi-GeV observations of past balloon experiments. On the other hand, the flux around 0.1 GeV shows a discrepancy with the prediction. In this presentation, the balloon experiment, flight data analysis, and observation results are described.
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