Cosmic-Ray Boron Flux Measured from <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mn>8.4</mml:mn><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>GeV</mml:mi><mml:mo>/</mml:mo><mml:mi>n</mml:mi></mml:mrow></mml:math> to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mn>3.8</mml:mn><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>TeV</mml:mi><mml:mo>/</mml:mo><mml:mi>n</mml:mi></mml:mrow></mml:math> with the Calorimetric Electron Telescope on the International Space Station
O. Adriani, Yosui Akaike, Katsuaki Asano, Y. Asaoka, E. Berti, G. Bigongiari, W. R. Binns, M. Bongi, P. Brogi, A. Bruno, J. H. Buckley, N. Cannady, G. Castellini, C. Checchia, M. L. Cherry, G. Collazuol, G. A. de Nolfo, Ken Ebisawa, Anthony W. Ficklin, H. Fuke, S. Gonzi, T. G. Guzik, T. Hams, K. Hibino, M. Ichimura, Kunihito Ioka, W. Ishizaki, M. H. Israel, K. Kasahara, J. Kataoka, Ryuho Kataoka, Y. Katayose, C. Kato, Norita Kawanaka, Y. Kawakubo, Kazuyoshi Kobayashi, Kazunori Kohri, H. Krawczynski, John Krizmanic, P. Maestro, P. S. Marrocchesi, A. Messineo, J. W. Mitchell, Shoko Miyake, А.А. Моисеев, M. Mori, N. Mōri, Holger Motz, K. Munakata, S. Nakahira, Jun Nishimura, S. Okuno, J. F. Ormes, S. Ozawa, L. Pacini, P. Papini, B. F. Rauch, S. Ricciarini, Kazuhiro Sakai, T. Sakamoto, M. Sasaki, Yuki Shimizu, A. Shiomi, П. Спиллантини, F. Stolzi, S. Sugita, A. Sulaj, M. Takita, T. Tamura, T. Terasawa, S. Torii, Y. Tsunesada, Y. Uchihori, E. Vannuccini, J. P. Wefel, K. Yamaoka, S. Yanagita, A. Yoshida, Kenji Yoshida, Wolfgang V Zober
Abstract
We present the measurement of the energy dependence of the boron flux in cosmic rays and its ratio to the carbon flux in an energy interval from $8.4\text{ }\text{ }\mathrm{GeV}/n$ to $3.8\text{ }\text{ }\mathrm{TeV}/n$ based on the data collected by the Calorimetric Electron Telescope (CALET) during $\ensuremath{\sim}6.4\text{ }\text{ }\mathrm{yr}$ of operation on the International Space Station. An update of the energy spectrum of carbon is also presented with an increase in statistics over our previous measurement. The observed boron flux shows a spectral hardening at the same transition energy ${E}_{0}\ensuremath{\sim}200\text{ }\text{ }\mathrm{GeV}/n$ of the C spectrum, though B and C fluxes have different energy dependences. The spectral index of the B spectrum is found to be $\ensuremath{\gamma}=\ensuremath{-}3.047\ifmmode\pm\else\textpm\fi{}0.024$ in the interval $25<E<200\text{ }\text{ }\mathrm{GeV}/n$. The B spectrum hardens by $\mathrm{\ensuremath{\Delta}}{\ensuremath{\gamma}}_{B}=0.25\ifmmode\pm\else\textpm\fi{}0.12$, while the best fit value for the spectral variation of C is $\mathrm{\ensuremath{\Delta}}{\ensuremath{\gamma}}_{C}=0.19\ifmmode\pm\else\textpm\fi{}0.03$. The $\mathrm{B}/\mathrm{C}$ flux ratio is compatible with a hardening of $0.09\ifmmode\pm\else\textpm\fi{}0.05$, though a single power-law energy dependence cannot be ruled out given the current statistical uncertainties. A break in the $\mathrm{B}/\mathrm{C}$ ratio energy dependence would support the recent AMS-02 observations that secondary cosmic rays exhibit a stronger hardening than primary ones. We also perform a fit to the $\mathrm{B}/\mathrm{C}$ ratio with a leaky-box model of the cosmic-ray propagation in the Galaxy in order to probe a possible residual value ${\ensuremath{\lambda}}_{0}$ of the mean escape path length $\ensuremath{\lambda}$ at high energy. We find that our $\mathrm{B}/\mathrm{C}$ data are compatible with a nonzero value of ${\ensuremath{\lambda}}_{0}$, which can be interpreted as the column density of matter that cosmic rays cross within the acceleration region.