Properties of Cosmic Deuterons Measured by the Alpha Magnetic Spectrometer

Precision measurements of the cosmic ray D flux are presented as function of rigidity from 1.9 to 21 GV, based on 21 million D nuclei. We observed that over the entire rigidity range D exhibits nearly identical time variations with 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 $\sim R^{\Delta}$ with $\Delta_{D/^4He}$ = -0.108 $\pm$ 0.005. This is in contrast with the $^3$He/$^4$He flux ratio for which we find $\Delta_{^3He/^4He}$= -0.289 $\pm$ 0.003. The significance of $\Delta_{D/^4He}$ > $\Delta_{^3He/^4He}$ exceeds 10$\sigma$. In addition, we found that above $\sim$ 13 GV the rigidity dependence of D and p fluxes is identical with a D/p flux ratio of 0.027 $\pm$ 0.001. These unexpected observations show that, contrary to expectations, cosmic deuterons have a sizeable primary component.