Properties of Cosmic Lithium Isotopes Measured by the Alpha Magnetic Spectrometer

We present the first measurement of cosmic-ray fluxes of $^6\textrm{Li}$ and $^7\textrm{Li}$ isotopes in the rigidity range from 1.9 to 25 GV. The measurements are based on $9.7 \times 10^5$ $^6\textrm{Li}$ and $1.04 \times 10^6$ $^7\textrm{Li}$ nuclei collected by the Alpha Magnetic Spectrometer on the International Space Station from May 2011 to October 2023. We observe that over the entire rigidity range the $^6\textrm{Li}$ and $^7\textrm{Li}$ fluxes exhibit nearly identical time variations and, above $\sim$4  GV, the time variations of $^6\textrm{Li}$, $^7\textrm{Li}$, He, Be, B, C, N, and O fluxes are identical. Above $\sim$7  GV, we find an identical rigidity dependence of the $^6\textrm{Li}$ and $^7\textrm{Li}$ fluxes. This shows that they are both produced by collisions of heavier cosmic-ray nuclei with the interstellar medium and, in particular, excludes the existence of a sizable primary component in the $^7\textrm{Li}$ flux.