HELIX - the High Energy Light Isotope eXperiment

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The HELIX (High-Energy Light Isotope eXperiment) is designed to measure the charged cosmic rays that continually bombard the Earth. Cosmic rays were discovered just over a century ago and, although much has been learned about them over the years, many mysteries remain. It is known, for example, that they mostly comprise the nuclei of hydrogen and helium, but there are lesser amounts of heavier nuclei as well. Cosmic rays are blocked by the Earth's atmosphere so most information about them comes from particle detectors deployed in space or on balloons at altitudes in excess of 35 km.Since cosmic rays are electrically charged, their trajectories are curved by magnetic fields in the galaxy and they do not 'point back' to their place of origin so the sources of cosmic rays have not been unambiguously identified. Despite the uncertainties, one can use models of cosmic-ray production and propagation to learn about the local galactic neighbourhood. The purpose of HELIX is to provide key measurements that will help decide among competing models of our galactic environment and help us understand the nature of nearby interstellar space.The motivation for such measurements is the recent discovery of a rise with energy of the ratio of positrons to electrons in the cosmic-ray flux. This effect has been measured by several independent experiments and is thought to be due either to particle production in nearby sources or to the annihilation or decay of dark-matter particles. The dark-matter explanation is tantalizing since these elusive particles have been the subject of intense searches for more than three decades at accelerators, in low-backgound underground laboratories, and with gamma-ray astronomy telescopes. To decide between these two explanations one needs a better understanding of the nearby part of our galaxy. HELIX will probe the local environment by measuring the relative flux of two isotopes of the beryllium nucleus, Be9 and Be10. Both are produced by collisions of heavier nuclei with the interstellar medium but Be10 is radioactive and decays with a half-life of 1.4 million years; it is a 'clock isotope'. In a procedure analogous to carbon dating, one can use the flux ratio of the two isotopes to calculate how long, and through what, they have been travelling. To make the measurement, HELIX will use a magnetic spectrometer comprising a superconducting magnet and a suite of components found in a typical particle-physics experiment. The payload will be carried to the edge of space by a large helium balloon launched from the coast of Antarctica in 2019. It will remain aloft for two weeks, circumnavigating the continent in 24-hour daylight. The combination of high-resolution afforded by the superconducting magnet and superior statistical power resulting from the long flight will enable measurements to be made in a previously unexplored energy range, a region where theory needs the most guidance.