Klein and Stezelberger were setting up a prototype station for the proposed ARIANNA array of neutrino detectors (ARIANNA stands for the Antarctic Ross Ice Shelf Antenna Neutrino Array). Which is how Klein came to find himself tent-camping on the Ross Ice Shelf last December (the middle of summer in Antarctica), along with his colleague Thorsten Stezelberger of the Lab’s Engineering Division and camp manager Martha Story from the Berg Field Center, a support service at McMurdo Station, the main U.S. Locating neutrinos produced by UHE cosmic rays needs a detector covering a huge area. The flip side is that it’s quite a trick to catch neutrinos, especially those produced by rare events. “The only way they interact is through the weak interaction, so they aren’t deflected by magnetic fields in flight, and they easily slip through dense matter like stars that would stop the cosmic rays themselves.” “Neutrinos have important advantages as observational tools,” says Klein. One clue to the origin of the highest-energy cosmic rays is the neutrinos they produce when they interact with the very cosmic microwave photons that slow them down. In all that volume of “nearby” space, sources capable of producing such high-energy nuclei have not been clearly identified. This sounds like a long distance, but, on cosmic scales, it isn’t very far.” Klein explains that because cosmic rays lose energy by plowing into the photons of the cosmic microwave background as they travel, “the ones that we observe must come from the ‘local’ universe, within about 225 million light years of Earth.

Not only do we not know how these cosmic accelerators work, we don’t even know where they are.”īeing electrically charged, even the most energetic cosmic rays are forced to bend when they traverse interstellar magnetic fields, so it’s not possible to extrapolate where they came from by looking back along their paths when they arrive on Earth. “With present technology we’d need to build an accelerator around the sun to produce protons that energetic. “If they’re protons, they have about 40 million times the energy of the protons accelerated at the Large Hadron Collider,” Klein says. He compares the energy of an ultra-high-energy (UHE) cosmic ray to a well-hit tennis ball or a boxer’s punch – all packed into a single atomic nucleus. “The most energetic cosmic rays are the rarest, and they pose the biggest mystery,” says Spencer Klein of Berkeley Lab’s Nuclear Science Division. Some originate from our own sun but most come from farther off, from the Milky Way or beyond. Cosmic rays aren’t actually rays, of course, they’re particles ninety percent are protons, the nuclei of hydrogen atoms, and most of the rest are heavier nuclei like iron. We’re constantly being peppered by showers of debris from cosmic rays colliding with atoms in the atmosphere. Spencer Klein with the prototype ARIANNA station, Ross Ice Shelf

Neutrino radiation