The Experiment

HELIX is a new long-duration balloon payload designed to measure light cosmic ray isotopes from 0.2 GeV/n eventually up to 10 GeV/n. HELIX can measure isotopes from proton (Z=1) up to neon (Z=10), and the detector is optimized to provide precision measurements of the ratio of the beryllium isotopes. These measurements are of particular interest at the present time due to several recent high-profile ‘anomalies’ (addressed below) detected in the cosmic-ray flux.

Recent Updates from Direct Measurements

A new era of precision space-based measurements has brought some real surprises

$Figure of positron fraction versus energy from the Particle Data Group$

Rising positron fraction

The most highly publicized recent result has been the remarkable increase observed in the positron fraction reported by the PAMELA and AMS-02 collaborations. Early hints of this increase were previously reported by the HEAT collaboration. This behavior is in conflict with traditional models of cosmic-ray physics (black) and has been attributed to a wide variety of phenomena, including particle production in nearby astrophysical objects such as pulsars (red), to propagation physics, or to more exotic sources, including the annihilation or decay of dark matter particles (green and blue). Figure taken from the Particle Data Group.

Figure of proton spectrum taken from O. Adriani et al, PRL 2019 (see paragraph below for link)

Element spectral hardening

New measurements of the proton and helium spectra by AMS-02 and CALET deviate from an expected single power-law distribution at a rigidity of around 300 GV, with the spectral indices becoming harder for higher energies. This spectral hardening has been observed for the fluxes of other elements up to oxygen as well. This hardening cannot be explained by traditional models; complementary measurements are necessary for better understanding. Figure taken from O. Adriani et al, PRL 2019

It is critical to understand cosmic ray propagation to interpret these results. With HELIX we aim to measure the isotopic abundance ratios as they provide key information necessary for discriminating between leading propagation models.

Secondary to primary ratio

Best measured observable to study the propagation

Figure of boron-to-carbon ratio versus energy per nucleon taken from R. Cowsik et al, 2016 (see paragraph below for link)

B/C ratio

The data most commonly used to study the propagation of cosmic rays are measurements of the boron-to-carbon ratio, where boron is a secondary nucleus generated by the inelastic scattering of primary nuclei with the interstellar medium and carbon is a primary nucleus originating from the source. This ratio probes the total material path length traversed by the cosmic rays during their containment time in our Galaxy before they reach the Earth. It thus has a degeneracy between the material density and the containment time of the cosmic rays. Figure taken from R. Cowsik et al, 2016

Figure of beryllium 10 to beryllium 9 ratio versus energy per nucleon taken from I. Moskalenko - “AMS02 Days”

¹⁰Be/⁹Be ratio

A way to break the degeneracies is to measure isotopic abundance ratios. If one of the isotopes is unstable (a ‘clock’ isotope) one can get an estimate of the diffusion time. The ratio of ¹⁰Be to ⁹Be is especially good for this. Both isotopes are secondaries, produced by spallation reactions along the path of the heavier nuclei, but ¹⁰Be is unstable, undergoing beta decay with a half-life of 1.39 Myr. The observed flux ratio at Earth can be used to estimate the travel time since production. Indeed, at lower energies these isotopes have been measured (ACE/CRIS and ISOMAX) to estimate the mean confinement time for cosmic rays within the Galaxy to be about 15 Myr. At higher energies, a few GeV/n, the ¹⁰Be/⁹Be ratio can provide strong discrimination between entire classes of propagation model.

Our mission

We follow a two stage approach to cover wider range of energy

Stage 1

The stage 1 of the experiment aims to measure the ¹⁰Be/⁹Be ratio up to ~3 GeV/n with a very high mass resolution. It will be a 7-14 day exposure flight with 0.1 m²sr geometry factor.

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Stage 2

Stage 2 will include the evolution and refinement of the detector systems on future flights to increase statistics and reach energies up to at least 10 GeV/n. It would require a 28 day flight.