Lawrence Berkeley Labs - 19 April 2010
A radio antenna to capture neutrinos from galaxies far, far away
We’re constantly being peppered by showers of debris from cosmic rays colliding with atoms in the atmosphere. 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. Some originate from our own sun but most come from farther off, from the Milky Way or beyond.
“The most energetic cosmic rays are the rarest, and they pose the biggest mystery,” says Spencer Klein of Berkeley Lab’s Nuclear Science Division. 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.
“If they’re protons, they have about 40 million times the energy of the protons accelerated at the Large Hadron Collider,” Klein says. “With present technology we’d need to build an accelerator around the sun to produce protons that energetic. Not only do we not know how these cosmic accelerators work, we don’t even know where they are.”
Being 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.
Yet they can’t come from too far away. 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. This sounds like a long distance, but, on cosmic scales, it isn’t very far.”
In all that volume of “nearby” space, sources capable of producing such high-energy nuclei have not been clearly identified. 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.
An energetic neutrino striking the upper atmosphere creates a shower of particles in which electrons predominate. When the shower enters the ice, it sheds Cherenkov radiation in the form of radio waves, which reflect from the interface of ice and water and are detected by antennas buried in the snow.