  HELIX is a new NASA-sponsored instrument aimed at measuring the spectra and
composition of light cosmic-ray isotopes from hydrogen to neon nuclei, in
particular the clock isotopes 10Be (radioactive, with 1.4 Myr lifetime) and 9Be
(stable). The latter are unique markers of the production and Galactic
propagation of secondary cosmic-ray nuclei, and are needed to resolve such
important mysteries as the proportion of secondary positrons in the excess of
antimatter observed by the AMS-02 experiment. By using a combination of a 1 T
superconducting magnet spectrometer (with drift-chamber tracker) with a
high-resolution time-of-flight detector system and ring-imaging Cherenkov
detector, mass-resolved isotope measurements of light cosmic-ray nuclei will be
possible up to 3 GeV/n in a first stratospheric balloon flight from Kiruna,
Sweden to northern Canada, anticipated to take place in early summer 2024. An
eventual longer Antarctic balloon flight of HELIX will yield measurements up to
10 GeV/n, sampling production from a larger volume of the Galaxy extending into
the halo. We review the instrument design, testing, status and scientific
prospects.
