2008 IceCube Update - Section 6
VI. THE ICE IN ICECUBE
The ice surrounding the DOMs is a critical part of IceCube. Both absorption and scattering are significant. Both are strongly affected by impurities in the ice. These impurities are a reflection of the impurities in the air when the ice was first laid down as snow. This happened over roughly the last 100,000 years. Because of variations in the long-term dust level in the atmosphere during this period, as well as the occasional volcanic eruption, the impurity concentrations are depth dependent.
Much effort has gone into measuring the optical properties of the ice, using artificial light sources and in-situ measurements. In AMANDA and IceCube, studies have been done using LEDs and lasers that emit at a variety of wavelengths. By measuring the arrival time distributions of photons at different distances from a light source, it is possible to measure both the attenuation length and scattering length of the light. These measurements, although useful, suffer from a limited resolution in depth .
Higher resolution depth-dependence measurements of the ice properties come from a 'dust logger' which is lowered down a water-filled hole immediately after drilling. The dust logger shines a thin beam of 404 nm light into the ice, and measures the reflected light . This provides a measure of the ice properties on a depth scale given by the width of the emitted beam – a few mm.
Figure 4 shows our understanding of ice absorption and scattering distances, as a function of depth and wavelength. At depths below about 1400 m, air bubbles are present in the ice. These bubbles greatly limit the optical scattering length in the ice. At deeper depths, the broad peaks in both the absorption and scattering lengths are due to dust in the ice. Not visible are the very narrow peaks due to thin layers of dust produced by volcanoes. The underlying scattering lengths are derivable from Mie scattering. The exponential rise in absorption at long wavelengths is believed to be due to molecular absorption.