Spectra of He Isotopes and the 3He/4He Ratio

Since its launch, the Alpha Magnetic Spectrometer-02 (AMS-02) has delivered outstanding quality measurements of the spectra of cosmic-ray (CR) species, e±, , and nuclei (H–Si, S, Fe), which have resulted in a number of breakthroughs. Besides elemental spectra, AMS-02 also measures the spectra of light isotopes albeit within a smaller rigidity range. In this paper, we use the precise measurements of He isotopes and the 3He/4He ratio by AMS-02, together with Voyager 1 data, and investigate their origin. We show that there is an excess in 3He at higher energies compared to standard calculations assuming its fully secondary origin, which may indicate a source of primary 3He, or signify a change in the propagation parameters over the Galactic volume probed by the 3He/4He ratio. We provide updated local interstellar spectra (LIS) of 3He and 4He in the rigidity range from 2 to 40 GV. Our calculations employ the self-consistent GalProp–HelMod framework that has proved to be a reliable tool in deriving the LIS of CR e−, , and nuclei Z ≤ 28.