# Properties of Cosmic Helium Isotopes Measured by the Alpha Magnetic Spectrometer

Abstract

Precision measurements by the Alpha Magnetic Spectrometer (AMS) on the International Space Station of $^3\mathrm{He}$ and $^4\mathrm{He}$ fluxes are presented. The measurements are based on 100 million $^4\mathrm{He}$ nuclei in the rigidity range from 2.1 to 21 GV, and 18 million $^3\mathrm{He}$ from 1.9 to 15 GV collected from May 2011 to November 2017. We observed that the $^3\mathrm{He}$ and $^4\mathrm{He}$ fluxes exhibit nearly identical variations with time. The relative magnitude of the variations decreases with increasing rigidity. The rigidity dependence of the $^3\mathrm{He}/^4\mathrm{He}$ flux ratio is measured for the first time. Below 4 GV, the $^3\mathrm{He}/^4\mathrm{He}$ flux ratio was found to have a significant long-term time dependence. Above 4 GV, the $^3\mathrm{He}/^4\mathrm{He}$ flux ratio was found to be time independent and its rigidity dependence is well described by a single power law ${\propto R^{\Delta}}$ with ${\Delta = -0.294 \pm 0.004}$. Unexpectedly, this value is in agreement with the B/O and B/C spectral indices at high energies.

Supplemental Material
The ${^{3}\textrm{He}}$ and ${^{4}\textrm{He}}$ fluxes in units of $[\textrm{m}^{2}\,{\cdot}\,\textrm{sr}\,{\cdot}\,\textrm{s}\,{\cdot}\,\textrm{GV}]^{-1}$, and their ratio, as functions of rigidity at the top of AMS, for different time periods, including errors due to statistical (stat.) and systematic (sys.) errors.
The time-averaged ${^{3}\textrm{He}}$ and ${^{4}\textrm{He}}$ fluxes in units of $[\textrm{m}^{2}\,{\cdot}\,\textrm{sr}\,{\cdot}\,\textrm{s}\,{\cdot}\,\textrm{GV}]^{-1}$, and their ratio, as functions of rigidity at the top of AMS, including statistical (stat.) and systematic (sys.) errors.
The time-averaged ${^{3}\textrm{He}}$ and ${^{4}\textrm{He}}$ fluxes in units of $[\textrm{m}^{2}\,{\cdot}\,\textrm{sr}\,{\cdot}\,\textrm{s}\,{\cdot}\,\textrm{GV}]^{-1}$, and their ratio, as functions of kinetic energy per nucleon at the top of AMS, including statistical (stat.) and systematic (sys.) errors.