Precision Measurement of Cosmic Ray Deuterons with Alpha Magnetic Spectrometer

Precision measurements by the Alpha Magnetic Spectrometer (AMS) on the International Space Station of the deuteron (𝐷) flux are presented. The measurements are based on 21 million 𝐷 nuclei in the rigidity range from 1.9 to 21 GV collected from May 2011 to April 2021. We observe that over the entire rigidity range the 𝐷 flux exhibits nearly identical time variations with the 𝑝, ${}^3$He , and ${}^4$He fluxes. Above 4.5 GV, the 𝐷/${}^4$He flux ratio is time independent and its rigidity dependence is well described by a single power law ∝𝑅$^\Delta$ with $Δ_{𝐷/{}^4\text{He}}$ =−0.108±0.005. This is in contrast with the ${}^3$He/${}^4$He flux ratio for which we find $Δ_{{}^3\text{He}/{}^4\text{He}}$ =−0.289±0.003. Above ∼13 GV we find a nearly identical rigidity dependence of the 𝐷 and 𝑝 fluxes with a 𝐷/𝑝 flux ratio of 0.027 ±0.001. These unexpected observations indicate that cosmic deuterons have a sizable primarylike component. With a method independent of cosmic ray propagation, we obtain the primary component of the 𝐷 flux equal to 9.4 ±0.5% of the ${}^4$He flux and the secondary component of the 𝐷 flux equal to 58 ±5% of the ${}^3$He flux.