# Precision Measurement of the Helium Flux in Primary Cosmic Rays of Rigidities 1.9 GV to 3 TV with the Alpha Magnetic Spectrometer on the International Space Station

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Phys. Rev. Lett. 115, 211101 (2015)
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Abstract

Knowledge of the precise rigidity dependence of the helium flux is important in understanding the origin, acceleration, and propagation of cosmic rays. A precise measurement of the helium flux in primary cosmic rays with rigidity (momentum/charge) from 1.9 GV to 3 TV based on 50 million events is presented and compared to the proton flux. The detailed variation with rigidity of the helium flux spectral index is presented for the first time. The spectral index progressively hardens at rigidities larger than 100 GV. The rigidity dependence of the helium flux spectral index is similar to that of the proton spectral index though the magnitudes are different. Remarkably, the spectral index of the proton to helium flux ratio increases with rigidity up to 45 GV and then becomes constant; the flux ratio above 45 GV is well described by a single power law.

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### Table-I

The helium flux as a function of rigidity $\tilde{R}$ at the top of AMS including errors due to statistics (stat); contributions to the systematic error from the trigger (trig); geomagnetic cutoff, acceptance, and background contamination (acc); the rigidity resolution function and unfolding (unf); and the absolute rigidity scale (scale); and the total systematic error (syst).

### Table-II

The proton to helium flux ratio p/He as a function of rigidity including errors due to statistics (stat); contributions to the systematic error from the trigger (trig); geomagnetic cutoff, acceptance, and background contamination (acc); the rigidity resolution function and unfolding (unf); the absolute rigidity scale (scale); and the total systematic error (syst).