Volume 501 - 39th International Cosmic Ray Conference (ICRC2025) - Gamma-Ray Astrophysics
Investigation of the intrinsic detector background for SMILE-3 balloon experiment
H. Iiyama*, T. Nakamori, D. Yaegashi, T. Ikeda, T. Oka, A. Takada, M. Abe, H. Tsukamoto, T. Sato, S. Deguchi, T. Sawano, M. Sakata, Y. Munakata, K. Okamoto, Y. Mizumura, S. Kurosawa, M. Mori, J. Kushida, K. Hamaguchi, K. Miuchi, T. Tanimori and R. Yoshioka
*: corresponding author
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Pre-published on: September 23, 2025
Published on: —
Abstract
MeV gamma-ray observations are a probe for uncovering various physical phenomena, such as the search for dark matter and primordial black holes, and the study of the nucleosynthesis in the Universe.

To achieve high sensitivity in this band, we are developing an electron-tracking Compton camera (ETCC), which combines a gaseous time projection chamber (TPC) and pixelated GSO(Ce) scintillator arrays (PSAs). The ETCC tracks recoil electrons in the TPC and reconstructs Compton events to uniquely determine gamma-ray arrival directions.
We are now planning the Sub-MeV/MeV gamma-ray Imaging Loaded-on-balloon Experiment 3 (SMILE-3), which will use an upgraded ETCC from the previous balloon experiment, SMILE-2+, with improved dynamic range and effective area. The first one-day flight of SMILE-3 is scheduled for spring 2027 in Australia.

The ETCC has background events originating from radioactive isotopes contained in the GSO(Ce) scintillators. Therefore, we measured the intrinsic background of all GSO(Ce) scintillators to be used in SMILE-3 and investigated their properties to evaluate the feasibility of selecting low-background scintillators.
DOI: https://doi.org/10.22323/1.501.0690
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