News Release

Waseda University researchers measure boron flux in high-energy cosmic rays with the CALorimetric Electron Telescope (CALET)

The data, collected over 6.4 years at the International Space Station, could help us understand cosmic ray propagation in the supernova remnant and our galaxy

Peer-Reviewed Publication

Waseda University

Measuring Boron Flux in High-Energy Cosmic Rays

image: Researchers have measured boron flux and the boron-to-carbon ratio in high-energy cosmic rays based on the data collected with the Calorimetric Electron Telescope over more than six years on the International Space Station view more 

Credit: Yosui Akaike of Waseda University, Japan

Cosmic rays (CR) constitute high-energy particles that mainly originate outside our solar system. These primary CR interact with interstellar matter to produce secondary CR. The secondary nature of their origin is reflected in the higher abundance of light elements, such as boron (B), in secondary CR relative to the solar system. Likewise, the primary CR can be quantified in terms of the amount of carbon (C) nuclei. Consequently, measurements of the secondary-to-primary abundance ratios (as B/C) are possible. This can help realize better galactic CR propagation models by constraining their parameters that are proportional to the average material path length (λ) traversed by CR in the galaxy at high energies.

Existing studies indicate that λ follows a power-law variation. It is inversely related to E raised to the δth power, where E is the CR energy per nucleon and δ is called the diffusion spectral index. The galactic CR propagation can be investigated by precisely determining the energy dependence of λ.

In this regard, a team of international researchers, led by Professor Emeritus Shoji Torii from Waseda University, Japan, has now extended the measurements of secondary CR in the tera electronvolts/nucleon (TeV/n) region with high statistics and reduced systematic uncertainties. Their work, published in Physical Review Letters on December 16, 2022, involved contributions from Dr. Paolo Maestro from the University of Siena, Italy, and Dr. Yosui Akaike from Waseda University.

Akaike briefly discusses the fundamental contribution of their study. “Our research presents new direct measurements of the energy spectrum of B and the B/C flux ratio in the energy range 8.4 GeV/n to 3.8 TeV/n, based on the data collected by the CALorimetric Electron Telescope (CALET) from October 13, 2015, to February 28, 2022 aboard the International Space Station. The C energy spectrum has also been updated. The measurements indicate excellent charge identification, accurate tracking, and good energy sampling of the CR particles up to the TeV region,” he says.

By analyzing the CALET data, the researchers observed that B shows a different energy spectrum from C in terms of its spectral index value: -3.047 for low energies. The index seems to harden (by 0.25) more than it does for C (by 0.19), albeit with low statistical significance, at a transition energy of around 200 GeV/n. The B/C ratio can be fitted with a single power law function with a spectral index of -0.366, even at high energies.

Further, due to the slight difference in the hardening between B and C, the researchers tried to fit a “leaky-box model” of CR propagation in the galaxy to the B/C ratio. In this approach, the CR were modeled as “leaking” from the galaxy. Their results pointed to the possibility of a non-zero residual value of λ. Physically, this implies that the primary CR cross a column density of matter within the acceleration region. They then produce new secondary B nuclei near the CR source, hardening the ratio.

“The presented results could significantly contribute to our understanding of cosmic ray propagation mechanism in supernova remnants and our galaxy. More importantly, however, pure science research can stimulate an intellectual curiosity about our universe and help us better comprehend what life might look like at places like the Moon and Mars,” concludes Akaike.




Authors: O. Adriani,1,2 Y. Akaike,3,4 K. Asano,5 Y. Asaoka,5 E. Berti,1,2 G. Bigongiari,6,7 W. R. Binns,8 M. Bongi,1,2 P. Brogi,6,7 A. Bruno,9 J. H. Buckley,8 N. Cannady,10,11,12 G. Castellini,13 C. Checchia,6,7 M. L. Cherry,14 G. Collazuol,15,16 G. A. de Nolfo,9 K. Ebisawa,17 A. W. Ficklin,14 H. Fuke,17 S. Gonzi,1,2 T. G. Guzik,14 T. Hams,10 K. Hibino,18 M. Ichimura,19 K. Ioka,20 W. Ishizaki,5 M. H. Israel,8 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,8 J. F. Krizmanic,11 P. Maestro ,6,7 P. S. Marrocchesi,6,7 A. M. Messineo,27,7 J. W. Mitchell,11 S. Miyake,28 A. A. Moiseev,29,11,12 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,1,13,2 P. Papini,2 B. F. Rauch,8 S. B. Ricciarini,13,2 K. Sakai,10,11,12 T. Sakamoto,34 M. Sasaki,29,11,12 Y. Shimizu,18 A. Shiomi,35 P. Spillantini,1 F. Stolzi,6,7 S. Sugita,34 A. Sulaj,6,7 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. Zober8


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

2INFN Sezione di Florence, 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

6Department of Physical Sciences, Earth and Environment, University of Siena, via Roma 56, 53100 Siena, Italy

7INFN Sezione di Pisa, Polo Fibonacci, Largo B. Pontecorvo, 3–56127 Pisa, Italy

8Department of Physics and McDonnell Center for the Space Sciences, Washington University, One Brookings Drive, St. Louis, Missouri 63130-4899, USA

9Heliospheric Physics Laboratory, NASA/GSFC, Greenbelt, Maryland 20771, USA

10Center for Space Sciences and Technology, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, Maryland 21250, USA

11Astroparticle Physics Laboratory, NASA/GSFC, Greenbelt, Maryland 20771, USA

12Center for Research and Exploration in Space Sciences and Technology, NASA/GSFC, Greenbelt, Maryland 20771, USA

13Institute of Applied Physics (IFAC), National Research Council (CNR), Via Madonna del Piano, 10, 50019 Sesto, Fiorentino, Italy

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 Oiwakecho, Sakyo, Kyoto 606-8502, Japan

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

22Waseda Research Institute for 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, 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, 3-3-138, Sugimoto, Sumiyoshi, Osaka 558-8585, Japan

37Nambu Yoichiro Institute for Theoretical and Experimental Physics, Osaka Metropolitan University, 3-3-138, 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 Associate Professor Yosui Akaike

Yosui Akaike is an Associate Professor at the Waseda Research Institute for Science and Engineering at Waseda University, Japan. He has been actively involved in astronomical research for more than 15 years. During this period, he has published over 60 research papers with 200-plus co-authors. His work has been cited nearly 800 times in more than 400 documents. His research interests include cosmos, dark matter, antiprotons, cosmic ray showers, and particle telescopes. He has even worked with the CALorimetric Electron Telescope (CALET) aboard the International Space Station.


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 

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