Transparency measurements of the South Pole ice: Implications for AMANDA (Antarctic Muon and Neutrino Detector Array)

The first stage of AMANDA, the Antarctic Muon and Neutrino Detector Array was deployed at 1 km depth in the South Pole ice during the Austral Summer of 1993-1994. Four strings of optical sensors with 20 sensors per string separated by a distance of 10 m constitute this stage of the detector.; The performance of the array as a neutrino detector is greatly governed by the optical properties of the detection medium. Using a pulsed laser and diffusive spheres, measurement and modeling of transit times of photons of several wavelengths in the ice indicated that the ice was phenomenally transparent, the absorption length increasing with decreasing wavelength to a value of {dollar}sim{dollar}250 m at 410 nm. The presence of residual bubbles at this depth was also revealed by this analysis with the mean free path between bubbles ranging from {dollar}sim{dollar}8 cm to {dollar}sim{dollar}20 cm, increasing as a function of depth in ice, but independent of the wavelength.; Muons traversing the ice emit Cerenkov radiation and there is an abundance of these photons in the range of wavelengths ({dollar}sim{dollar}300 nm) in which the AMANDA optical modules are sensitive. These photons scatter off bubbles (similar to the laser photons) and hence the arrival time distributions of these photons should be indicative of the optical properties of ice at such low wavelengths. Extensive simulations of the AMANDA detector, with absorption and scattering in ice as tunable parameters, in comparison with muon data confirmed the long absorption lengths. The best agreement between simulations and data was obtained with a peak absorption length in ice of {dollar}sim{dollar}310 m.; To take advantage of the excellent clarity of ice, it is necessary to move to depths at which the bubbles completely disappear. Hence, detailed modeling of the depth profile of the South Pole ice has been done in comparison with glaciological data from ice cores extracted from other sites in the Antarctic subcontinent. It has been concluded that the next stage of the detector to be deployed during the Austral Summer of 1995-1996 has to be at depths greater than {dollar}sim{dollar}1500 m.