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Nuclear emulsion plates for studying elementary particle physics as well as cosmic ray physics are very powerful tracking tools with sub-micron spatial resolutions of charged particle trajectories. Even if gamma rays have to be detected, electron-positron pair tracks can provide precise information to reconstruct their direction and energy with high accuracy. Recent developments of emulsion analysis technology can digitally handle almost all tracks recorded in emulsion plates by using the Hyper Track Selector of the OPERA group at NAGOYA University. On the other hand, the potential of time resolutions have been equipped by emulsion multilayer shifter technology in the GRAINE (Gamma Ray Astro-Imager with Nuclear Emulsion) experiments, the aims of which are to detect cosmic gamma rays such as the Vela pulsar stellar object by precise emulsion tracking analysis and to study cosmic ray particle interactions and chemical compositions. In this paper, we focus on the subject of cosmic ray nuclei detection in the GRAINE balloon flight experiments launched at Alice Springs, Australia in May 2015.

The compact EAS Extensive Air Shower (EAS) array (200 m2) and the Linsley method have been used with the aim to observe the primary energy spectrum. The spectral indexes αproton and αiron are obtained as − 2.46 ± 0.17 and − 3.08 ± 0.24, respectively, in the primary energy region of 1016eV − 1018.5eV in the single observation of the compact EAS array. Additionally, αproton and αiron are obtained as − 3.12 ± 0.13 and − 3.52 ± 0.17, respectively, in the same primary energy region in the observation by restricting the zenith angle of EAS events.