Galactic Cosmic Ray Nuclei Abundances from 10Ne to 56Ba with SuperTIGER

SuperTIGER (Super Trans-Iron Galactic Element Recorder) is a long-duration-balloon instrument that completed its first Antarctic flight during the 2012-2013 austral summer, spending 55 days at an average float altitude of 125,000 feet, and a second flight in the 2019-2020 austral summer completing 32 days of flight at an average float altitude of 120,000 feet. The first SuperTIGER flight measured the relative abundances of Galactic cosmic-ray (GCR) nuclei with high statistical precision and well resolved individual element peaks from ₁₀Ne to ₄₀Zr, with further measurements out to 56Ba. Although statistics are low for these elements heavier than ₄₀Zr, we present preliminary relative abundance measurements of charges Z = 41–56 with individual element resolution. (Walsh et. al, Adv. in Space Research (under review))

Recent multi messenger observations have established NS mergers as one of the sites of r-process nucleosynthesis. After this landmark discovery, the question has become, where is the primary site of r-process nucleosynthesis? Is it NS-NS mergers, supernovae and other core-collapse events, a mixture, or some other process that contributes significantly to the heavy r-process budget of the universe? Our GCR measurements up to ₄₀Zr do support a source acceleration model where supernovae in OB associations preferentially accelerate refractory elements that are more readily embedded in interstellar dust grains than volatiles. In addition, injection into the GCR for both refractory and volatile elements appear to follow a charge dependence consistent with their grain sputtering cross sections. Our preliminary measurements of the Z=41–56 range suggest the existence of an alternative GCR source or acceleration model for Z>40 elements. We report progress in refining this interesting result and note that future heavy cosmic-ray abundance measurements in the Milky Way may be able to further constrain sites of r-process nucleosynthesis.