Spectra of He isotopes and the $^3$He/$^4$He ratio

TL;DR

This paper analyzes AMS-02 measurements of helium isotopes and the $^3$He/$^4$He ratio, revealing an excess of $^3$He at high energies and providing updated interstellar spectra using a comprehensive propagation model.

Contribution

It introduces an analysis of helium isotope data with the GalProp-HelMod framework, highlighting potential primary sources of $^3$He and updating local interstellar spectra.

Findings

Detection of $^3$He excess at high energies.

Updated local interstellar spectra for $^3$He and $^4$He.

Implication of possible primary $^3$He sources or propagation changes.

Abstract

Since its launch, the Alpha Magnetic Spectrometer-02 (AMS-02) has delivered outstanding quality measurements of the spectra of cosmic-ray (CR) species, $e^{\pm}$, $\bar{p}$, 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 $^3$He/$^4$He ratio by AMS-02, together with Voyager 1 data, and investigate their origin. We show that there is an excess in $^3$He at higher energies compared to standard calculations assuming its fully secondary origin, which may indicate a source of primary $^3$He, or signify a change in the propagation parameters over the Galactic volume probed by the $^3$He/$^4$He ratio. We provide updated local interstellar spectra (LIS) of $^3$He and $^4$He in the rigidity range from 2-40 GV. Our calculations employ the self-consistent GalProp-HelMod framework that has proved to be a reliable tool in deriving the LIS of CR $e^{-}$, $\bar{p}$, and nuclei $Z\le28$.