P. B. Price

The AMANDA neutrino telescope: principle of operation and first results

AMANDA is a high-energy neutrino telescope presently under construction at the geographical South Pole. In the Antarctic summer 1995/96, an array of 80 optical modules (OMs) arranged on 4 strings (AMANDA-B4) was deployed at depths between 1.5 and 2 km. In this paper we describe the design and performance of the AMANDA-B4 prototype, based on data collected between February and November 1996. Monte Carlo simulations of the detector response to down-going atmospheric muon tracks show that the global behavior of the detector is understood. We describe the data analysis method and present first results on atmospheric muon reconstruction and separation of neutrino candidates. The AMANDA array was upgraded with 216 OMs on 6 new strings in 1996/97 (AMANDA-B10), and 122 additional OMs on 3 strings in 1997/98.

Optical properties of the South pole ice at depths between 0.8 and 1 kilometer.

The optical properties of the ice at the geographical South Pole have been investigated at depths between 0.8 and 1 kilometer. The absorption and scattering lengths of visible light (∼515 nanometers) have been measured in situ with the use of the laser calibration setup of the Antarctic Muon and Neutrino Detector Array (AMANDA) neutrino detector. The ice is intrinsically extremely transparent. The measured absorption length is 59 � 3 meters, comparable with the quality of the ultrapure water used in the Irvine-Michigan-Brookhaven and Kamiokande proton-decay and neutrino experiments and more than twice as long as the best value reported for laboratory ice. Because of a residual density of air bubbles at these depths, the trajectories of photons in the medium are randomized. If the bubbles are assumed to be smooth and spherical, the average distance between collisions at a depth of 1 kilometer is about 25 centimeters. The measured inverse scattering length on bubbles decreases linearly with increasing depth in the volume of ice investigated.

Kinetics of conversion of air bubbles to air hydrate crystals in Antarctic ice

The depth dependence of bubble concentration at pressures above the transition to the air hydrate phase and the optical scattering length due to bubbles in deep ice at the South Pole are modeled with diffusion-growth data from the laboratory, taking into account the dependence of age and temperature on depth in the ice. The model fits the available data on bubbles in cores from Vostok and Byrd and on scattering length in deep ice at the South Pole. It explains why bubbles and air hydrate crystals coexist in deep ice over a range of depths as great as 800 meters and predicts that at depths below ∼1400 meters the AMANDA neutrino observatory at the South Pole will operate unimpaired by light scattering from bubbles.

Age vs depth of glacial ice at South Pole

Knowledge of age as a function of depth in glacial ice is important for both glaciology and paleoclimatology. For sites near a ridge or dome, an ice flow model together with information on accumulation rate provides a first approximation. If the accumulation rate is high enough, annual layering of isotopes and dust measured in a solid core can provide a precise age vs depth relationship. For South Pole, the flow geometry is not simple and no deep core exists. Nevertheless, by remotely sensing peaks in scattering and absorption of light from pulsed sources buried at depths down to 2200 m, we have been able to determine age vs depth for ages up to 65,000 years. Analysis of radar isochrons by Siegert and Hodgkins provides a rough extension of the age vs depth model to ∼165,000 years near bedrock.

UV and optical light transmission properties in deep ice at the South Pole

Both absorption and scattering of light at wavelengths 410 to 610 nanometers were measured in the South Pole ice at depths 0.8 to 1 kilometer with the laser calibration system of the Antarctic Muon And Neutrino Detector Array (AMANDA). At the shortest wavelengths the absorption lengths exceeded 200 meters - an order of magnitude longer than has been reported for laboratory ice. The absorption shows a strong wavelength dependence while the scattering length is found to be independent of the wavelength, consistent with the hypothesis of a residual density of air bubbles in the ice. The observed linear decrease of the inverse scattering length with depth is compatible with an earlier measurement by the AMANDA collaboration (at ∼515 nanometers).

Optical properties of deep ice at the South Pole: scattering.

Recently, absorption and scattering at depths 800-1000 m in South Pole ice have been studied with transit-time distributions of pulses from a variable-frequency laser sent between emitters and receivers embedded in the ice. At 800-1000 m, scattering is independent of wavelength and the scattering centers are air bubbles of size ? wavelength. At 1500-2000 m it is predicted that all bubbles will have transformed into air-hydrate clathrate crystals and that scattering occurs primarily at dust grains, at liquid acids concentrated along three-crystal boundaries, and at salt grains. Scattering on decorated dislocations, at ice-ice boundaries, and at hydrate-ice boundaries will be of minor importance. Scattering from liquid acids in veins at three-crystal boundaries goes as ~lambda(-1) to ~lambda(-2) and should show essentially no depth dependence. Scattering from dust grains goes as ~lambda(-2) and should show peaks at depths of ~1050, ~1750, and ~2200 m in South Pole ice. If marine salt grains remain undissolved, they will scatter like insoluble dust grains. Refraction at ice-ice boundaries and at hydrate-ice boundaries is manifested by a multitude of small-angle scatters, independent of wavelength. The largest contribution to Rayleigh-like scattering is likely due to dislocations decorated discontinuously with impurities. Freshly grown laboratory ice exhibits a large Rayleigh-like scattering that we attribute to the much higher density of decorated dislocations than in glacial ice.

Gas-Rich Meteorites: Possible Evidence for Origin on a Regolith

The asymmetry of irradiation features of grains in the Kapoeta and Fayetteville meteorites suggests irradiation on a regolith before meteorite formation. Chondrules and broken grains require approximately 104 years of irradiation time between formation or fracturing and compaction into the meteorite. Shock erasure of tracks from irradiated Kapoeta feldspars requires a severe shock event during or after meteorite formation.

Comparison of optical, radio, and acoustical detectors for ultrahigh-energy neutrinos

For electromagnetic cascades induced by electron-neutrinos in South Pole ice, the effective volume per detector element (phototube, radio antenna, or acoustic transducer) as a function of cascade energy is estimated, taking absorption and scattering into account. A comparison of the three techniques shows that the optical technique is most effective for energies below ~0.5 PeV, nthat the radio technique shows promise of being the most effective for higher energies, and that the acoustic method is not competitive. Due to the great transparency of ice, the event rate of AGN ne-induced cascades is an order of magnitude greater than in water. For hard source spectra, the rate of Glashow resonance events may be much greater than the rate for non-resonant energies. The radio technique will be particularly useful in the study of Glashow events and in studies of sources with very hard energy spectra.

Role of group and phase velocity in high-energy neutrino observatories

Abstract Kuzmichev recently showed that use of phase velocity rather than group velocity for Cherenkov light signals and pulses from calibration lasers in high-energy neutrino telescopes leads to errors in track reconstruction and distance measurement. We amplify on his remarks and show that errors for four cases of interest to AMANDA, IceCube, and radio Cherenkov detector are negligibly small.

Remote sensing of dust in deep ice at the South Pole

A three-dimensional array of phototubes in deep ice at the South Pole called the Antarctic Muon and Neutrino Detector Array (AMANDA) is recording Cherenkov light pulses that serve as tracers of high-energy neutrinos from throughout the Universe. The performance of this neutrino observatory will ultimately be constrained by the optical properties of the ice at near-ultraviolet and visible wavelengths. At depths greater than {approximately}1.4 km, air bubbles are absent and light travels great distances, limited only by absorption and scattering by dust in the ice. In this paper, the Mie theory is used to predict the magnitude and wavelength dependence of the scattering and absorption coefficients and mean cosine of the scattering angle for deep South Pole ice. The results depend on the composition, size distribution, and depth profile of insoluble mineral grains, sea salt grains, liquid acid droplets, and soot particles. With most probable values for mineral grains, sea salt, acid, and soot, we fit optical data in the wavelength interval of 300{endash}500 nm for depths of 1.6{endash}1.83 km, taken with pulsed laser beams as light sources and with AMANDA phototubes as receivers. Our work provides quantitative evidence that aerosols deposited in snow and compacted into the ice accountmorexa0» for the optical properties at wavelengths {approximately}300{endash}500 nm. We finally predict optical properties of the South Pole ice at 2.5 km, a depth future AMANDA strings may reach. We expect that at 2.5 km the effective scattering, which is predominantly due to acid droplets, decreases by a factor of {approximately}1.5 relative to that at 1.7 km and that absorption, which is predominantly due to the mineral and soot, decreases by a factor of {approximately}3{endash}5 relative to that at 1.7 km. {copyright} 1998 American Geophysical Union«xa0less