Moonshadow: IceCube raps to a Cat Stevens tune
I’m being followed by a moon shadow, moon shadow-moon shadow, sang Cat Stevens. At the South Pole, the IceCube neutrino detector is singing much the same song.
IceCube is a neutrino telescope that is monitoring a cubic kilometer of ice at the South Pole for neutrino interactions. When a neutrino hits the ice, it results in a production of a charged particle called the muon. The muon streaks through the ice faster than the speed of light in ice (but not faster than the speed of ice light in vacuum – which is the absolute speed limit). This speeding muon creates a cone of blue light called Cherenkov light. By detecting this Cherenkov light, IceCube (and indeed many other neutrino detectors) can figure out the direction of the original neutrino.
But muons in the ice are coming not just from neutrino interactions. When cosmic rays hit the Earth’s atmosphere, they also generate muons, and these muons can also create Cherenkov light in the ice. It’s these cosmic ray-muons that are the source of a moon shadow. The moon blocks some of the cosmic rays reaching the Earth. When IceCube studies the entire sky for cosmic ray-muons, it sees a deficit of such muons coming from the direction of the moon. This is the moon’s shadow!
The moon’s shadow was first proposed in 1957.
IceCube is made of 80 strings of digital optical modules (DOMs), with each string contain 60 DOMs. Each DOM is capable of detecting the Cherenkov light emitted by a speeding muon.
In this paper, IceCube scientists report the results of 40 strings observing cosmic ray muons, operating between April 2008 and April 2009. A total of 8 lunar months’ worth of data was analysed by IceCube – and it clearly saw the shadow of the moon.
Why is the moon’s shadow important? It helps the IceCube scientists figure out that their telescope is pointing correctly. How does a bunch of strings of DOMs embedded in the ice function like a telescope that can point at a region of the sky? That’s for another blog post.
Meanwhile, here is a video I made in the IceCube lab at the South Pole in January 2008, of a display of muons streaking through the ice. First, a paragraph from The Edge of Physics describing the display:
The display is a graphic on a computer screen showing, in near real-time, the muons that are streaming through the ice. The ice is an inky black background, against which the IceCube strings are a line of dots, each dot a DOM frozen in the ice. The sky is a checkered pattern of blue lines. Every few seconds a muon comes streaking down from the sky, sometimes at an angle to the strings, sometimes straight down. The DOMs register the muon as it passes by, and they light up in one of five colors: red, orange, yellow, green, or blue. The closer a muon is to a DOM, the redder it looks; the farthest muons are blue. The greater the energy of the muon, the more the DOM flares up. I knew it was just a visual representation of what was happening kilometers beneath the surface, but it was hypnotic. Low-intensity muons caused the DOMs to light up a little, making them look like pearls. High-intensity muons caused nearby DOMs to become so big and bulbous that they overlapped and stuck together, like lumps of gummy candy. The display rotated slowly to provide a 3-D view, a psychedelic soiree of subatomic particles.