  The IceCube neutrino observatory is composed of more than five thousand light
sensors, Digital Optical Modules (DOMs), installed on the surface and at depths
between 1450 and 2450 m in clear ice at the South Pole. Each DOM incorporates a
10-inch diameter photomultiplier tube (PMT) intended to detect light emitted
when high energy neutrinos interact with atoms in the ice. Depending on the
energy of the neutrino and the distance from secondary particle tracks, PMTs
can be hit by up to several thousand photons within a few hundred nanoseconds.
The number of photons per PMT and their time distribution is used to reject
background events and to determine the energy and direction of each neutrino.
The detector energy scale was established from previous lab measurements of DOM
optical sensitivity, then refined based on observed light yield from stopping
muons and calibration of ice properties. A laboratory setup has now been
developed to more precisely measure the DOM optical sensitivity as a function
of angle and wavelength. DOMs are calibrated in water using a broad beam of
light whose intensity is measured with a NIST calibrated photodiode. This study
will refine the current knowledge of the IceCube response and lay a foundation
for future precision upgrades to the detector.
