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Precision Spectral Measurements of Chromium and Titanium from 10 to 250  GeV/n and Sub-Iron to Iron Ratio with the Calorimetric Electron Telescope on the International Space Station

O. Adriani1,2, Y. Akaike3,4,*, K. Asano5, Y. Asaoka5, E. Berti2,6, P. Betti2,6, G. Bigongiari7,8, W. R. Binns9, M. Bongi1,2 et al. (CALET Collaboration)

M. Bongi1,2, P. Brogi7,8, A. Bruno10, N. Cannady11, G. Castellini6, C. Checchia7,8,†, M. L. Cherry12, G. Collazuol13,14, G. A. de Nolfo10, K. Ebisawa15, A. W. Ficklin12, H. Fuke15, S. Gonzi1,2,6, T. G. Guzik12, T. Hams16, K. Hibino17, M. Ichimura18, M. H. Israel9, K. Kasahara19, J. Kataoka20, R. Kataoka21, Y. Katayose22, C. Kato23, N. Kawanaka24,25, Y. Kawakubo26, K. Kobayashi3,4, K. Kohri25,27, H. S. Krawczynski9, J. F. Krizmanic11, P. Maestro7,8, P. S. Marrocchesi7,8, M. Mattiazzi13,14, A. M. Messineo8,28, J. W. Mitchell11, S. Miyake29, A. A. Moiseev11,30,31, M. Mori32, N. Mori2, H. M. Motz33, K. Munakata23, S. Nakahira15, J. Nishimura15, M. Negro12, S. Okuno17, J. F. Ormes34, S. Ozawa35, L. Pacini2,6, P. Papini2, B. F. Rauch9, S. B. Ricciarini2,6, K. Sakai36, T. Sakamoto26, M. Sasaki11,30,31, Y. Shimizu17, A. Shiomi37, P. Spillantini1, F. Stolzi7,8,‡, S. Sugita26, A. Sulaj7,8, M. Takita5, T. Tamura17, T. Terasawa5, S. Torii3, Y. Tsunesada38,39, Y. Uchihori40, E. Vannuccini2, J. P. Wefel12, K. Yamaoka41, S. Yanagita42, A. Yoshida26, K. Yoshida19, and W. V. Zober9 (CALET Collaboration)

  • 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 Bruno 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
  • 11Astroparticle Physics Laboratory, NASA/GSFC, Greenbelt, Maryland 20771, USA
  • 12Department of Physics and Astronomy, Louisiana State University, 202 Nicholson Hall, Baton Rouge, Louisiana 70803, USA
  • 13Department of Physics and Astronomy, University of Padova, Via Marzolo, 8, 35131 Padova, Italy
  • 14INFN Sezione di Padova, Via Marzolo, 8, 35131 Padova, Italy
  • 15Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo, Sagamihara, Kanagawa 252-5210, Japan
  • 16Center for Space Sciences and Technology, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, Maryland 21250, USA
  • 17Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa, Yokohama, Kanagawa 221-8686, Japan
  • 18Faculty of Science and Technology, Graduate School of Science and Technology, Hirosaki University, 3, Bunkyo, Hirosaki, Aomori 036-8561, Japan
  • 19Department of Electronic Information Systems, Shibaura Institute of Technology, 307 Fukasaku, Minuma, Saitama 337-8570, Japan
  • 20School of Advanced Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan
  • 21National Institute of Polar Research, 10-3, Midori-cho, Tachikawa, Tokyo 190-8518, Japan
  • 22Faculty of Engineering, Division of Intelligent Systems Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501, Japan
  • 23Faculty of Science, Shinshu University, 3-1-1 Asahi, Matsumoto, Nagano 390-8621, Japan
  • 24Department of Physics, Graduate School of Science, Tokyo Metropolitan University, 1-1 Minanii-Osawa, Hachioji, Tokyo 192-0397, Japan
  • 25National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
  • 26Department of Physical Sciences, College of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo, Sagamihara, Kanagawa 252-5258, Japan
  • 27Institute of Particle and Nuclear Studies, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan
  • 28University of Pisa, Polo Fibonacci, Largo Bruno Pontecorvo, 3–56127 Pisa, Italy
  • 29Department of Electrical and Computer Engineering, National Institute of Technology (KOSEN), Gifu College, 2236-2 Kamimakuwa, Motosu-city, Gifu 501-0495, Japan
  • 30Center for Research and Exploration in Space Sciences and Technology, NASA/GSFC, Greenbelt, Maryland 20771, USA
  • 31Department of Astronomy, University of Maryland, College Park, Maryland 20742, USA
  • 32Department of Physical Sciences, College of Science and Engineering, Ritsumeikan University, Shiga 525-8577, Japan
  • 33Faculty of Science and Engineering, Global Center for Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan
  • 34Department of Physics and Astronomy, University of Denver, Physics Building, Room 211, 2112 East Wesley Avenue, Denver, Colorado 80208-6900, USA
  • 35Quantum ICT Advanced Development Center, National Institute of Information and Communications Technology, 4-2-1 Nukui-Kitamachi, Koganei, Tokyo 184-8795, Japan
  • 36Kavli Institute for Cosmological Physics, The University of Chicago, 5640 South Ellis Avenue, Chicago, Illinois 60637, USA
  • 37College of Industrial Technology, Nihon University, 1-2-1 Izumi, Narashino, Chiba 275-8575, Japan
  • 38Graduate School of Science, Osaka Metropolitan University, Sugimoto, Sumiyoshi, Osaka 558-8585, Japan
  • 39Nambu Yoichiro Institute for Theoretical and Experimental Physics, Osaka Metropolitan University, Sugimoto, Sumiyoshi, Osaka 558-8585, Japan
  • 40National Institutes for Quantum and Radiation Science and Technology, 4-9-1 Anagawa, Inage, Chiba 263-8555, Japan
  • 41Nagoya University, Furo, Chikusa, Nagoya 464-8601, Japan
  • 42College of Science, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki 310-8512, Japan

  • *Contact author: yakaike@aoni.waseda.jp
  • †Contact author: caterina.checchia2@unisi.it
  • ‡Contact author: francesco.stolzi@unisi.it
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Phys. Rev. Lett. 135, 021002 – Published 8 July, 2025

DOI: https://doi.org/10.1103/py17-74rk

Abstract

The Calorimetric Electron Telescope (CALET), in operation on the International Space Station since 2015, collected a large sample of cosmic-ray (CR) iron and sub-iron events over a wide energy interval. In this Letter, we report an update of our previous measurement of the iron flux and we present—for the first time—a high statistics measurement of the spectra of two sub-iron elements Cr and Ti in the energy interval from 10 to 250  GeV/n. The analyses are based on 8 years of data. Differently from older generations of cosmic-ray instruments which, in most cases, could not resolve individual sub-iron elements, CALET can identify each nuclear species from proton to nickel (and beyond) with a measurement of their electric charge. Thanks to the improvement in statistics and a more refined assessment of systematic uncertainties, the iron spectral shape is better resolved, at high energy, than in our previous paper, and we report its flux ratio to chromium and titanium. The measured fluxes of Cr and Ti show energy dependences compatible with a single power law with spectral indices −2.74±0.06 and −2.88±0.06, respectively.

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Phys. Rev. Lett. 135, 021002– Published 8 July, 2025
  • Received 5 February 2025
  • Revised 22 May 2025
  • Accepted 18 June 2025
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