# Precision Measurement of the Boron to Carbon Flux Ratio in Cosmic Rays from 1.9 GV to 2.6 TV with the Alpha Magnetic Spectrometer on the International Space Station

Abstract

Knowledge of the rigidity dependence of the boron to carbon flux ratio (B/C) is important in understanding the propagation of cosmic rays. The precise measurement of the B/C ratio from 1.9 GV to 2.6 TV, based on 2.3 million boron and 8.3 million carbon nuclei collected by AMS during the first 5 years of operation, is presented. The detailed variation with rigidity of the B/C spectral index is reported for the first time. The B/C ratio does not show any significant structures in contrast to many cosmic ray models that require such structures at high rigidities. Remarkably, above 65 GV, the B/C ratio is well described by a single power law $R^\Delta$ with index $\Delta = -0.333 \pm 0.014(fit) \pm 0.005(syst)$, in good agreement with the Kolmogorov theory of turbulence which predicts $\Delta = -1/3$ asymptotically.

Supplemental Material
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### Table-SM-I

The boron to carbon flux ratio (B/C) as a function of rigidity including errors due to statistics (stat), contributions to the systematic error from the backgrounds subtraction (back), the trigger and the acceptance calculation (acc), the unfolding procedure and the rigidity resolution function (unf), the absolute rigidity scale (scale), and the total systematic error (syst).

### Table-SM-II

The boron to carbon flux ratio (B/C) as a function of kinetic energy including errors due to statistics (stat), contributions to the systematic error from the backgrounds subtraction (back), the trigger and the acceptance calculation (acc), the unfolding procedure and the rigidity resolution function (unf), the absolute rigidity scale (scale), the error associated to rigidity to kinetic energy conversion due to uncertainties in the boron isotopic composition (conv), and the total systematic error (syst).