This spring NASA will launch what could become one of this decade’s most transformative missions in astrophysics. But you’ve almost certainly never heard of it—and it’s not even going to space. Dubbed the High-Energy Light Isotope eXperiment (HELIX), the mission seeks to solve a long-standing mystery about just how much antimatter there is in the universe and where it comes from—all from a lofty perch in Earth’s stratosphere, slung beneath a giant balloon set for long-duration flights above each of our planet’s desolate poles.
Led by Scott Wakely, an astrophysicist at the University of Chicago, HELIX is designed to study cosmic rays—subatomic particles that pelt our planet from the depths of interstellar and even intergalactic space. These particles include those of ordinary matter’s opposite-charge version, called antimatter. Scientists suspect the sources for the antimatter showering Earth from space could be almost anything, ranging from emissions by conventional astrophysical objects to the esoteric behavior of dark matter, the invisible stuff that seems to govern the large-scale behavior of galaxies. Figuring out which explanation is right may depend on a deceptively simple measurement: gauging how much time each of two specific particles spent hurtling through the galaxy. It’s like carbon-dating cosmic rays. “The models are all over the place. A measurement of this ratio is what everybody wants,” says Nahee Park, an astrophysicist at Queen’s University in Ontario and a member of the HELIX team.
Most cosmic rays are protons and light atomic nuclei that are thought to be accelerated by shock waves from supernova explosions within the galaxy. Others are produced when these nuclei collide with interstellar gas as they travel. But another particle—the antimatter counterpart of the electron, called the positron—presents a puzzle: observations since 2008 have repeatedly concluded that there are more positrons than can be explained by known phenomena. Astrophysicists have proposed models to explain where these particles came from and what interactions they encountered in the Milky Way. HELIX is designed to measure a parameter that could rule out some speculations about antimatter and cosmic-ray origins.
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Traditional models of the galaxy postulate a magnetized region, or “halo,” that extends beyond the Milky Way’s starry disk and influences cosmic-ray travel paths. But these models cannot easily account for observed antimatter levels. To bridge this gap, researchers posit an additional as-yet-unknown positron source lurking somewhere out there. One possibility is that dark matter is a sea of slowly moving heavy particles, and their annihilation or decay could be the source of the mysterious positron excess. Another is that the positrons could come from outbursts from undiscovered local pulsars—the rapidly rotating versions of stellar corpses called neutron stars—in our arm of the galaxy. An alternative explanation is that if particles spend more time within the halo, the observed antimatter flux can be caused by positron-producing collisions with interstellar gas, without the need for any additional astrophysical source. The crux of the debate lies in estimates of how long cosmic rays spend within the galaxy and predictions of which ones—and how many of them—find their way to Earth. These numbers, in turn, are proxies for the size of the galaxy’s halo, whose extent evades measurement using existing techniques. The halo’s size influences the detectable flux of positrons on Earth.
Cosmic-ray-propagation models start with the set of products created when atoms with heavier nuclei, such as carbon, crash into a proton or a helium nucleus. Such reactions can chip off part of the nucleus, resulting in lighter elements, such as beryllium. “When the starting gun goes off, you have some mix of [beryllium] isotopes,” says David Hanna, a physicist at McGill University and a member of the HELIX team, referring to beryllium varieties that have the same number of protons but different numbers of neutrons. The mixture that reaches detectors on Earth, however, depends on how long the isotopes spend in transit and what happens to them on their way. Because one of the beryllium isotopes that the scientists are looking for is radioactive, it serves as a “cosmic clock” that traces the time spent in the galaxy from production to detection: While beryllium 9 (9Be) is stable, beryllium 10 (10Be) decays to half its original amount over 1.4 million years. Measuring the ratio of 10Be to 9Be thus gives a timescale of how much time cosmic rays spend in the galaxy.
Every Particle Counts
The HELIX team is preparing to measure the beryllium isotope ratio from 120,000 feet in the atmosphere during its first-ever flight, which will occur north of the Arctic Circle. The researchers’ goal is to count each high-energy particle that reaches the detector. HELIX will use a strong magnet to deflect each particle’s path and, from the curvature of its trajectory through the magnetic field, calculate its momentum. Another detector will measure the particles’ speed, allowing the team to determine each one’s mass and identity. The detectors have been specially designed for lightweight particles (that is, those with an atomic number below 10; beryllium’s atomic number is 4, for instance) with energies up to 10 giga-electron-volts per nucleon (GeV/n)—the amount of energy that a grain of sand would have falling from a centimeter height. But for a tiny atomic particle hurtling through space, that’s a huge amount of energy. The flux of these lightweight, high-energy particles is where competing models diverge in their predictions. “The measurements get harder and harder as you go to higher energies,” Park says. It’s a numbers game: Because fewer high-energy particles reach Earth, determining their flux is harder. And because their trajectory bends less, determining their momentum is harder, too.
To obtain maximum bending power—and therefore improved momentum resolution—HELIX uses a superconducting magnet. But this isn’t without its drawbacks. Superconductivity requires cryogenically cold temperatures; in Park’s words, superconducting magnets “drink liquid helium.” That makes them nonstarters for long-duration space missions, where replenishing the liquid helium is very costly or impossible—but the approach works well for balloon flights of days or weeks, where resupply is easier. The trade-off is that space-based experiments such as NASA’s Alpha Magnetic Spectrometer (AMS-02) offer much longer observation times well above the bulk of Earth’s atmosphere, whereas HELIX and other balloon-borne stratospheric experiments have shorter observation windows, with their view somewhat muddied by our planet’s cosmic-ray-blocking air.
Seeking the best of both worlds, the original plan for AMS-02 had included a superconducting magnet, but it was replaced with a permanent one that requires no power—like a fridge-door magnet. This magnet allows for a longer duration but has a much weaker field. “We realized the weaker field provided a scientific opportunity for [beryllium] if we could find a superconducting magnet,” Wakely says. That realization led to the birth of HELIX.
HELIX uses the same superconducting magnet that the balloon-borne High-Energy Antimatter Telescope (HEAT) experiment carried in 2000. But the similarities end there. Every other element of the payload is brand-new and designed specifically for HELIX, allowing the experiment to distinguish between beryllium isotopes on a particle-by-particle basis. “We want to say, ‘This particle was [beryllium 9]; that one was [beryllium 10].’ That’s the thing that, as far as we know, nobody else can do right now,” Wakely says. Having that ratio of 10Be to 9Be could prove crucial for clarifying where cosmic antimatter comes from.
Carmelo Evoli, an astrophysicist at the Gran Sasso Science Institute in Italy, says that HELIX’s design “specificity sets it apart from large, multipurpose experiments like AMS-02.” AMS-02 measures the flux of particles across the energy spectrum, including the total amount of beryllium, with good precision. But it cannot distinguish between individual isotopes: their mass is too similar for the AMS-02 hardware to reliably discern them. Yet that experiment’s venerable age can be beneficial in other ways: “While HELIX is designed to have a better mass resolution, AMS-02 has already collected 12 years of data,” says Alberto Oliva, a senior physicist at AMS-02. This vast dataset should allow for differentiation between isotopes using statistical tools. But, Hanna says, “that’s nothing like seeing them separated. It’s like looking at two stars blurred together versus using a telescope that shows each one.”
Looking Up
In January HELIX passed its “hang test” at NASA’s Columbia Scientific Balloon Facility in Palestine, Tex., proving it could communicate with NASA data transmitters, antennas and other infrastructure that are crucial to the mission. It’s now ready for a weeks-long launch window from Kiruna, Sweden, that will open on May 15. The balloon will carry HELIX for roughly a week before it touches down somewhere in northern Canada. The team would be thrilled to bring home clean data that could differentiate the isotopes at the lower end of the targeted energy spectrum. But unforeseen effects, such as heavier-than-expected showers of other nonberyllium cosmic rays, could compromise the measurements. “Not quite getting the resolution you wanted would be painful,” Wakely says.
If everything goes as planned, the next step will be a two-week flight over Antarctica to collect enough data to measure particles with an energy of 3 GeV/n. “You need to be up high for as long as possible to get enough of these rare particles,” Park says. Eventually, with upgraded instruments, a 28-day flight could make measurements of beryllium isotopes at up to 10 GeV/n. Hopefully this will suffice to establish the critical transit time through the galaxy for these particles and a clue to their origin.
We’re lucky to have beryllium. Its lifetime is perfect for exploring the local galaxy: if it were 10 times longer, it would be good for exploring a larger region; if it were 10 times shorter, it would disappear too fast to reach us. “The HELIX mission emerges as a critical player” for helping to illuminate the mysteries of antimatter and all cosmic-ray behavior in the Milky Way, Evoli says. For the first time, an experiment is strategically poised to resolve the discrepancies between divergent predictions, offering unprecedented insights into the fundamental processes that govern cosmic-ray transport across the galaxy’s vast expanse.
But for now “we’re just hoping for a nice flight,” Wakely says.
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