News Release

Helium nuclei research advances our understanding of cosmic ray origin and propagation

The latest observations from Low Earth Orbit with the International Space Station provide further evidence of spectral hardening and softening of cosmic ray particles

Peer-Reviewed Publication

Waseda University

Measurement of the Cosmic Ray-Helium Spectrum with CALorimetric Electron Telescope

image: Cosmic ray helium particles were found to follow a Double Broken Power Law, with spectral hardening from 1.3 TeV and softening from 30 TeV view more 

Credit: Waseda University

Much of our understanding of the Universe and its mysterious phenomena is based on theoretical interpretations. In order to deepen the understanding of distant objects and energetic phenomena, astronomers are looking at cosmic rays, which are high-energy charged particles composed of protons, electrons, atomic nuclei, and other subatomic particles. Such studies have revealed that cosmic rays contain all the elements known to us in the periodic table, suggesting that these elements originate from stars and high-energy events such as supernovae. Additionally, due to their charged nature, the path of cosmic rays through space is influenced by the magnetic fields of interstellar phenomena and objects.

Detailed observations of cosmic rays can, thus, not only shed light on the origins of these particles but also decode the existence of high-energy objects and phenomena such as supernova remnants, pulsars, and even dark matter. In an effort to better observe high-energy radiations, Japan, Italy, and USA collaboratively established the CALorimetric Electron Telescope (CALET) on the International Space Station in 2015.

In 2018, observations of the cosmic ray proton spectrum from 50 GeV to 10 TeV revealed that the particle flux of protons at high energies was significantly higher than expected. These results deviated from the conventional cosmic ray acceleration and propagation models that assume a “single power-law distribution,” wherein the number of particles decrease with increasing energy.

Consequently, in a study published in 2022, the CALET team, including researchers from Waseda University, found cosmic ray protons in the energy range of 50 GeV to 60 TeV to follow a “Double Broken Power Law.” This law assumes that the number of high-energy particles initially increase until 10 TeV (known as spectral hardening) and then decrease with an increase in energy (known as spectral softening).

Extending these observations further, the team has now found similar trends of spectral hardening and softening in the cosmic ray helium spectrum captured over a broad range of energy, from 40 GeV to 250 TeV.

The study, published in the journal Physical Review Letters on 27 April, 2023, was led by Associate Professor Kazuyoshi Kobayashi from Waseda University, Japan, along with contributions from Professor Emeritus Shoji Torii, Principal Investigator of the CALET project, also affiliated with Waseda University, and Research Assistant Paolo Brogi from the University of Siena in Italy.

CALET has successfully observed energy spectral structure of cosmic ray helium, especially spectral hardening starting from around 1.3 TeV, and the tendency of softening starting from around 30 TeV,” says Kobayashi.

These observations are based on data collected by CALET aboard the International Space Station (ISS) between 2015 to 2022. Representing the largest energy range to date for cosmic helium nuclei particles, these observations provide additional evidence for deviation of the particle flux from the single power-law model. The researchers noticed that deviation from the expected power-law distribution was more than eight standard deviations away from the mean, indicating a very low probability of this deviation occurring by chance.

Notably, the initial spectral hardening observed in this data suggests that there may be unique sources or mechanisms that are responsible for accelerating and propagating the helium nuclei to high energies. The discovery of these spectral features is also supported by recent observations from the Dark Matter Particle Explorer, and questions our current understanding of the origin and nature of cosmic rays.

These results would significantly contribute to the understanding of cosmic ray acceleration in the supernova remnant and propagation mechanism,” says Torii.

These findings undoubtedly enhance our understanding of the Universe. Even as we prepare for manned missions to the Moon and Mars, the energy distribution of cosmic ray particles can also provide further insight into the radiation environment in space and its effects on astronauts.






Authors: O. Adriani,1,2 Y. Akaike,3,4 K. Asano,5 Y. Asaoka,5 E. Berti,2,6 G. Bigongiari,7,8 W.R. Binns,9 M. Bongi,1,2 P. Brogi,7,8, A. Bruno,10 J.H. Buckley,9 N. Cannady,11,12,13 G. Castellini,6 C. Checchia,7,8 M.L. Cherry,14G. Collazuol,15,16 G.A. de Nolfo,10 K. Ebisawa,17 A. W. Ficklin,14 H. Fuke,17 S. Gonzi,1,2,6 T.G. Guzik,14 T. Hams,11 K. Hibino,18 M. Ichimura,19 K. Ioka,20 W. Ishizaki,5 M.H. Israel,9 K. Kasahara,21 J. Kataoka,22 R. Kataoka,23 Y. Katayose,24 C. Kato,25 N. Kawanaka,20 Y. Kawakubo,14 K. Kobayashi,3,4,† K. Kohri,26 H.S. Krawczynski,9 J.F. Krizmanic,12 P. Maestro,7, 8 P.S. Marrocchesi,7, 8 A.M. Messineo,8, 27 J.W. Mitchell,12 S. Miyake,28 A.A. Moiseev,12,13,29 M. Mori,30 N. Mori,2 H.M. Motz,31 K. Munakata,25 S. Nakahira,17 J. Nishimura,17 S. Okuno,18 J.F. Ormes,32 S. Ozawa,33 L. Pacini,2,6 P. Papini,2 B.F. Rauch,9 S.B. Ricciarini,2,6 K. Sakai,11,12,13 T. Sakamoto,34 M. Sasaki,12,13,29 Y. Shimizu,18 A. Shiomi,35 P. Spillantini,1 F. Stolzi,7, 8 S. Sugita,34 A. Sulaj,7, 8 M. Takita,5 T. Tamura,18 T. Terasawa,5 S. Torii,3 Y. Tsunesada,36, 37 Y. Uchihori,38 E. Vannuccini,2 J.P. Wefel,14 K. Yamaoka,39 S. Yanagita,40 A. Yoshida,34 K. Yoshida,21 and W. V. Zober9



1Department of Physics, University of Florence, Via Sansone, 1 - 50019, Sesto Fiorentino, Italy

2INFN Sezione di Firenze, Via Sansone, 1 - 50019, Sesto Fiorentino, Italy

3Waseda Research Institute for Science and Engineering, Waseda University, 17 Kikuicho, Shinjuku, Tokyo 162-0044, Japan

4JEM Utilization Center, Human Spaceflight Technology Directorate,

Japan Aerospace Exploration Agency, 2-1-1 Sengen, Tsukuba, Ibaraki 305-8505, Japan

5Institute for Cosmic Ray Research, The University of Tokyo,

5-1-5 Kashiwa-no-Ha, Kashiwa, Chiba 277-8582, Japan

6Institute of Applied Physics (IFAC), National Research Council (CNR),

Via Madonna del Piano, 10, 50019, Sesto Fiorentino, Italy

7Department of Physical Sciences, Earth and Environment, University of Siena, via Roma 56, 53100 Siena, Italy

8INFN Sezione di Pisa, Polo Fibonacci, Largo B. Pontecorvo, 3 - 56127 Pisa, Italy

9Department of Physics and McDonnell Center for the Space Sciences,

Washington University, One Brookings Drive, St. Louis, Missouri 63130-4899, USA

10Heliospheric Physics Laboratory, NASA/GSFC, Greenbelt, Maryland 20771, USA

11Center for Space Sciences and Technology, University of Maryland,

Baltimore County, 1000 Hilltop Circle, Baltimore, Maryland 21250, USA

12Astroparticle Physics Laboratory, NASA/GSFC, Greenbelt, Maryland 20771, USA

13Center for Research and Exploration in Space Sciences and Technology, NASA/GSFC, Greenbelt, Maryland 20771, USA

14Department of Physics and Astronomy, Louisiana State University,

202 Nicholson Hall, Baton Rouge, Louisiana 70803, USA

15Department of Physics and Astronomy, University of Padova, Via Marzolo, 8, 35131 Padova, Italy

16INFN Sezione di Padova, Via Marzolo, 8, 35131 Padova, Italy

17Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo, Sagamihara, Kanagawa 252-5210, Japan

18Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa, Yokohama, Kanagawa 221-8686, Japan

19Faculty of Science and Technology, Graduate School of Science and Technology, Hirosaki University, 3, Bunkyo, Hirosaki, Aomori 036-8561, Japan

20Yukawa Institute for Theoretical Physics, Kyoto University,

Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto, 606-8502, Japan

21Department of Electronic Information Systems, Shibaura Institute of Technology, 307 Fukasaku, Minuma, Saitama 337-8570, Japan

22School of Advanced Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan

23National Institute of Polar Research, 10-3, Midori-cho, Tachikawa, Tokyo 190-8518, Japan

24Faculty of Engineering, Division of Intelligent Systems Engineering,

Yokohama National University, 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501, Japan

25Faculty of Science, Shinshu University, 3-1-1 Asahi, Matsumoto, Nagano 390-8621, Japan

26Institute of Particle and Nuclear Studies, High Energy Accelerator

Research Organization, 1-1 Oho, Tsukuba, Ibaraki, 305-0801, Japan

27University of Pisa, Polo Fibonacci, Largo B. Pontecorvo, 3 - 56127 Pisa, Italy

28Department of Electrical and Electronic Systems Engineering,

National Institute of Technology (KOSEN), Ibaraki College,

866 Nakane, Hitachinaka, Ibaraki 312-8508, Japan

29Department of Astronomy, University of Maryland, College Park, Maryland 20742, USA

30Department of Physical Sciences, College of Science and Engineering, Ritsumeikan University, Shiga 525-8577, Japan

31Faculty of Science and Engineering, Global Center for Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan

32Department of Physics and Astronomy, University of Denver, Physics Building, Room 211, 2112 East Wesley Avenue, Denver, Colorado 80208-6900, USA

33Quantum ICT Advanced Development Center, National Institute of Information and Communications Technology, 4-2-1 Nukui-Kitamachi, Koganei, Tokyo 184-8795, Japan

34College of Science and Engineering, Department of Physics and Mathematics, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo, Sagamihara, Kanagawa 252-5258, Japan

35College of Industrial Technology, Nihon University, 1-2-1 Izumi, Narashino, Chiba 275-8575, Japan

36Graduate School of Science, Osaka Metropolitan University, Sugimoto, Sumiyoshi, Osaka 558-8585, Japan

37Nambu Yoichiro Institute for Theoretical and Experimental Physics,

Osaka Metropolitan University, Sugimoto, Sumiyoshi, Osaka 558-8585, Japan

38National Institutes for Quantum and Radiation Science and Technology, 4-9-1 Anagawa, Inage, Chiba 263-8555, Japan

39Nagoya University, Furo, Chikusa, Nagoya 464-8601, Japan

40College of Science, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki 310-8512, Japan


About Waseda University

Located in the heart of Tokyo, Waseda University is a leading private research university that has long been dedicated to academic excellence, innovative research, and civic engagement at both the local and global levels since 1882. The University has produced many changemakers in its history, including nine prime ministers and many leaders in business, science and technology, literature, sports, and film. Waseda has strong collaborations with overseas research institutions and is committed to advancing cutting-edge research and developing leaders who can contribute to the resolution of complex, global social issues. The University has set a target of achieving a zero-carbon campus by 2032, in line with the Sustainable Development Goals (SDGs) adopted by the United Nations in 2015. 

To learn more about Waseda University, visit  


About Associate Professor Kazuyoshi Kobayashi from Waseda University

Kazuyoshi Kobayashi is an Associate Professor at the Waseda Research Institute for Science and Engineering, Waseda University, Japan. Dr. Kobayashi has significant expertise in the domain of physics and astronomy, and has published more than 105 papers spanning a wide range of topics, including proton decay, neutrino oscillations, and dark matter. His papers are highly cited, with more than 20,000 citations to his credit. In addition, he has also contributed to conference presentations and articles.

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