Alert in polar ice

Print edition : June 27, 2014

A schematic of the IceCube architecture. Although the telescope is 2,820 m tall, the average hole is 2,452 m deep.

ICECUBE, located at the South Pole, is a cutting-edge second-generation neutrino telescope. First-generation neutrino telescopes, like AMANDA, built from the mid-1990s, could, in principle, observe such high-energy neutrinos, but none of them found any. This was not unexpected because they were not of the scale required to capture such low-probability events. AMANDA had essentially been built as proof of concept and the feasibility of the detection technique with large Cerenkov detectors for future bigger neutrino telescopes. IceCube is about 30 times bigger and thus substantially more sensitive than AMANDA. It is the biggest particle detector worldwide.

Built between 2004 and 2010, IceCube encloses a cubic kilometre of clear ice, beginning 1.5 km beneath the surface at the South Pole and extending downward by another kilometre. There are crucial advantages to having the South Pole as a site for deploying such an instrument that outweigh the remoteness of the site. Polar ice is about 3 km deep and the level of background light is extremely low. With that much frozen weight above it, the ice at this location gets compressed, driving out any air bubbles and making it perfectly clear and transparent. That makes it an ideal medium to detect the faint light signals emitted by charged particles produced by high-energy neutrinos.

The IceCube observatory consists of 5,160 basketball-sized detectors called digital optical modules (DOMs), which were conceived and largely designed at Berkeley Lab, California, United States. The DOMs are suspended along 86 strings that are embedded in a km of clear ice between 1,450 m to 2,450 m beneath the Antarctic surface (see figure). Each string comprises 60 DOMs and their power supply and the cables for the signal read-out. Each string was lowered into a vertical hole using a unique pressurised hot water drill to quickly bore through the ice when installing the array.

The telescope has to be this big because neutrino collisions with matter are exceedingly rare: out of trillions of neutrinos constantly passing through the ice, IceCube will observe just a few hundred a day. Seeing them at all is only possible because when neutrinos collide with the nuclei of oxygen atoms in the ice, they turn into energetic charged particles called muons, moving in the same direction. Because these muons are moving faster than light travels in ice, they radiate a shock wave of blue Cerenkov radiation that can be detected by IceCube’s optical sensors, which are photomultiplier tubes (PMTs).

The PMTs are so sensitive that they respond even to a single photon. The PMTs and circuit boards are housed together in transparent glass pressure vessels. Inside the DOMs, the photon signals are amplified and converted into electrical pulses and then translated into digital signals. Therefore, each module has its own minicomputer and a precision clock to measure the arrival time of the photons to an accuracy of five nanoseconds (five billionths of a second). From the depth, the digitised light signals are sent over kilometre-long cables to the central data acquisition system at surface station.

“Each of IceCube’s DOMs was designed to be a minicomputer server that you can log onto and download data from, or upload software to,” said Robert Stokstad of Berkeley Lab, who led the development of the DOMs and was one of the original proponents of IceCube.

“The DOMs are no more accessible than a space satellite in high orbit,” said Spencer Klein of Berkeley Lab, “but they’re a lot more reliable and extremely robust. They’re also performing far above specifications... their timing resolution is about two nanoseconds.”

To achieve this astonishing resolution, the DOMs use integrated circuits to sample the PMT signals 300 million times a second and convert each sample to a digital value. In a laboratory on the surface, signals from DOMs on many different strings are combined into a single data stream, which is analysed to determine the direction and energy of the neutrino events that left their tracks.

To separate neutrino signals from far more copious background events, the most important difference is whether the signal comes from overhead or below. Muons moving upward through IceCube must come from neutrinos that have passed through the earth. Downward-going muons, which are produced when cosmic rays hit the ice, are a million times more numerous. A smaller background comes from neutrinos produced when cosmic rays hit the earth’s atmosphere. To avoid this background, experimenters look for a cluster of neutrino events coming from a single direction in space, or for an excess of very energetic neutrinos; most atmospheric neutrinos are of lower energy.

R. Ramachandran