Toward new frontiers : Encounter and synergy of state-of-the-art astronomical detectors and exotic quantum beams

Program C01-7	Imaging MeV gamma rays from TGF with high-resolution semiconductor imagers
Principal Investigator	NAKAZAWA, Kazuhiro (Nagoya University)

The MeV gamma rays emitted from lightning activity are the only direct evidence of large-scale electrostatic acceleration, without no relation to magnetic field, of electrons in nature (e.g., Dwyer 2012), a high-energy phenomenon that was discovered in the late 1980s and began to be studied in detail after around 2000.

Thunderstorm gamma rays can be divided into two types: “thunderstorm gamma ray glow” which is emitted from the thundercloud itself for several minutes, and “lightning gamma-ray flash” (or Terrestrial Gamma-ray Flash, a name originating from the fact the it was first observed by satellites) which is a sudden (about 100 μs) emission synchronized with lightning discharges. In the winter, the Hokuriku region is active in lightning activity, and the low cloud height enables the gamma rays to reach the ground by avoiding atmospheric absorption, making the region suitable for ground-based observations. It is attracting attention from around the world. We have been conducting the GROWTH collaboration and have achieved world-leading results. In particular, we have confirmed that the lightning gamma-ray flash has a large dose of around 1.4 μGy in a short time of around 100 μs. Since this phenomenon contains a large amount of gamma-rays of >10 MeV, a photonuclear reaction occurs in the atmosphere, producing a large amount of fast neutrons and proton-rich nuclei. The main goal of this research is to understand where the electrons that produce these powerful gamma rays are accelerated and what is the maximum dose produced.

The ground-based observation of lightning gamma-ray flashes is an extreme high-luminosity observation, with as many as 10,000 photons per cm² arriving in only 100 μs. To capture this, we are adopting a semiconductor imager using the recently developed TimePix3 ASIC. The imager has a temporal resolution of 1.56 ns, a position resolution of 53 μm, and a detector size of 14×14 mm². Its thickness of 0.5–2 mm) is thicker than the position resolution, and it therefore can track the high-energy electrons. Although the electron-positron pairs produced by 10 MeV gamma-rays penetrate the detector, the direction of gamma-ray incidence can be determined by observing their tracks.

The goal of this two-year research project is to verify the observation performance for 10 MeV gamma-rays, to demonstrate the directional detection capability of the detector, and to actually set up the detector in the observation hut of the Nagoya University team in Kanazawa City for observation. If the observation is successful, we will compare the direction of the 10 MeV gamma-rays with the positional evaluation of the radio wave observation of the lightning discharge to identify the location of the electron acceleration.


Fig. 1. (Left) Schematic diagram of electron acceleration in thundercloud and its observations. (Center) Nagoya-U hut in Kanazawa City. (Right) Tracking observation of 10 MeV gamma rays interacting in the Si (or CdTe) TimePix3 imager devices

Members

Reference Materials

• Y. Wada, T. Enoto, K. Nakazawa, Y. Furuta, T. Yuasa, Y. Nakamura, …, H. Tsuchiya, “Downward terrestrial gamma-ray flash observed in a winter thunderstorm,” Phys. Rev. Lett. 123, 061103 (2019).
• T. Enoto, Y. Wada, Y. Furuta, K. Nakazawa, T. Yuasa, K. Okuda, …, H. Tsuchiya, “Photonuclear reactions triggered by lightning discharge,” Nature 551, 481–484 (2017). DOI: 10.1038/nature24630 .
• K. Nakazawa, G. Sato, M. Kokubun, T. Enoto et al., “The hard X-ray imager (HXI) onboard Hitomi (ASTRO-H),” J. Astron. Telesc. Instrum. Syst. 4(2), 021410 (2018).
• K. Nakazawa, K. Oonuki, T. Tanaka, Y. Kobayashi et al., “Improvement of the CdTe diode detectors using a guard-ring electrode,” IEEE Trans. Nucl. Sci. 51(4), 1881–1885 (2004).