There are ice sculptures and ice hotels, and now there is a particle detector made of ice. But far from being a novelty, IceCube is the world's largest neutrino observatory and scientists expect that, among other mysteries, it will yield data that may illuminate Dark Matter.
The IceCube detector, located at the U.S. National Science Foundation's Amundsen-Scott South Pole Station, was completed on December 18th with the installation of a final string of optical sensors into a 1 cubic kilometre section of ice that is both the detector's medium and vessel.
The final deployed sensor string of the observatory is part of the low-energy neutrino detection system, called DeepCore, designed by a team led by University of Alberta physicist Darren Grant.
Grant established Canada's first institutional presence in the IceCube collaboration, which includes over 30 universities and research centres from around the world, including its lead institution, the University of Wisconsin.
"The ice at the South Pole is an ideal detector medium for neutrinos," explains Grant, an assistant professor who joined the U of A Department of Physics in 2010.
"At the largest depths the ice is incredibly pure. Light produced when a neutrino interacts in the deep ice can travel large distances (over 100 metres) without being scattered or absorbed. This means you can build a very large optical detector for the universe's highest energy neutrinos."
IceCube construction
The $279 million (U.S.) experiment included the design and construction of a special hot water drill that bore 86 holes into the ice to depths of 1,450 and 2,450 metres. A string of 60 sensors was lowered into each hole to make up the main IceCube and DeepCore detectors. Four additional sensors sit on top of the ice above each string, forming the IceTop array. The IceTop array, combined with the IceCube and DeepCore detectors, constitute the IceCube Observatory, whose sensors record neutrino interactions. Grant says IceCube is designed to operate for 15 to 20 years, after which it will be decommissioned.
"The heart of the detector, the optical modules and their electrical/signal cables, are now a permanent part of the Antarctic glacier," he says. "At a depth of 1.5 to 2.5 kilometres, there is no method to extract them again, and they are considered part of the ice. All other pieces of the detector, from the drill which deployed it, to the electronics and computers housed in the IceCube surface laboratory, will ultimately be removed from the continent when their role there is complete."
Cool experience
Having joined the experiment as a Penn State researcher in 2007, the Harrow, Ont. native has participated in all phases of the experiment, including designing, building, testing and installing the DeepCore modules. The latter task gave him the opportunity to work in the Antarctic.
"I was lucky enough to spend nearly three weeks living at the South Pole during the early summer for the 2009/10 deployment season," Grant recalls. "Probably the highlight that sticks in my mind most occurred in the very first hours after landing at South Pole Station and unpacking. I made my way to the IceCube office space, sat down, looked out a window at the most incredible barren wasteland landscape, and saw a barbershop pole surrounded by flags about 20 metres away. I must have been in shock because the conversation with my colleague went:
Me: "What is that pole?"
Karen: "The South Pole."
Me: "You mean, 'the South Pole?' "
Karen: "Yes, 'the.' "
Me: "That is so cool."
The memory still makes Grant smile. "The thing that always surprised me was to wake up each morning there and pinch myself to realize I was really at the very bottom world. Only a few hundred people get to travel there each year and you are immediately welcomed as part of a large family when you arrive. The community is very close because you essentially depend on each other for survival there. It's simply an incredible experience."
Scientific potential
Grant will continue to work on the experiment in its analysis phase with the support of a three-year, $180,000 award from the National Science and Engineering Council of Canada (NSERC). Major funding for all other aspects of IceCube came from U.S., Belgian, German and Swedish research agencies.
While the completion of the detector is a milestone for the experiment, IceCube actually started taking data as it was being built.
"So far, we have used the data from the partially constructed detector to place world-leading limits in searches for Dark Matter and astrophysical neutrino sources," Grant says.
"But the real discovery potential of this observatory is in the full cubic-kilometre detector, which is now complete. I am most excited that at the University of Alberta we will be leading a number of the Dark Matter searches with the full detector and be the first in the world to begin stringent testing of some of the popular theoretical models for this most important physics problem.
"To use a phrase, for which I probably won't be forgiven, we really are now at the tip of the iceberg for the scientific potential this detector makes possible."