  The IceCube neutrino observatory in operation at the South Pole, Antarctica,
comprises three distinct components: a large buried array for ultrahigh energy
neutrino detection, a surface air shower array, and a new buried component
called DeepCore. DeepCore was designed to lower the IceCube neutrino energy
threshold by over an order of magnitude, to energies as low as about 10 GeV.
DeepCore is situated primarily 2100 m below the surface of the icecap at the
South Pole, at the bottom center of the existing IceCube array, and began
taking physics data in May 2010. Its location takes advantage of the
exceptionally clear ice at those depths and allows it to use the surrounding
IceCube detector as a highly efficient active veto against the principal
background of downward-going muons produced in cosmic-ray air showers. DeepCore
has a module density roughly five times higher than that of the standard
IceCube array, and uses photomultiplier tubes with a new photocathode featuring
a quantum efficiency about 35% higher than standard IceCube PMTs. Taken
together, these features of DeepCore will increase IceCube's sensitivity to
neutrinos from WIMP dark matter annihilations, atmospheric neutrino
oscillations, galactic supernova neutrinos, and point sources of neutrinos in
the northern and southern skies. In this paper we describe the design and
initial performance of DeepCore.
