Left: energy loss (ΔE) in the L0A detector versus residual energy (E′) deposited in L1A for energetic H and He nuclei collected by the EPI-Hi instrument during the early orbits of PSP. These data include particles incident through the A-end (generally sunward viewing end) of the LET1 telescope. The blue points indicate particles identified as H or He onboard; the red points indicate background events identified as not being consistent with any element. The region with a reduced density of points immediately below the H track is due to sampling of different regions of the plot with different efficiencies. Black lines show Geant4 calculations of the expected H, ³He, and ⁴He track locations. Right: He mass histogram from a two-day time period identified by Wiedenbeck et al. (2020) as having high values of ³He/⁴He (a few % or greater, as compared to ~0.04% in the solar wind). These He events were selected to have total energy greater than 4.8 MeV (E∕M > 1.2 MeV nuc⁻¹ for ⁴He). Red line shows Gaussian fit to the central part of the ⁴He peak, which yields a mass resolution of 0.20 amu.