New paper: isotopic composition of cosmic helium

Our new paper is now out. The paper is titled “Properties of Cosmic Helium Isotopes Measured by the Alpha Magnetic Spectrometer” and it is published in Physical Review Letter 123, 181102 (November 2019), selected as Editor’s suggestion. In this article, we reports new data of the AMS experiment in the ISS on the isotopic composition of cosmic-ray helium (3He and 4He isotopes) as function of energy (or rigidity) and time. The study is based on 100 million 4He nuclei in the rigidity range from 2.1 to 21 GV and 18 million 3He from 1.9 to 15 GV collected from May 2011 to November 2017. We observed that the 3He and 4He fluxes exhibit nearly identical variations with time. The relative magnitude of the variations decreases with increasing rigidity. The rigidity dependence of the 3He/4He flux ratio is measured for the first time. Below 4 GV, the 3He/4He flux ratio was found to have a significant long-term time dependence. Above 4 GV, the 3He/4He flux ratio was found to be time independent, and its rigidity dependence is well described by a single power law ∝RΔ with Δ=−0.294±0.004. Quite remarkably, this value is in agreement with the B/O and B/C spectral indices at high energies.