Measurement of the radio-frequency ice dielectric permittivity and implications for neutrino reconstruction

Abstract

Using radio receiver data triggered by in-ice transmitters, at 0-1.6 km depths at South Pole, we present measurements of the radio-frequency ice dielectric permittivity. Three custom high-amplitude pulsers, with 1–5 ns rise times, were used: a) a piezo-electric pulser, based on the same design previously used for the NASA-sponsored HiCal project in Antarctica, a pulser developed by the KU Instrumentation Design Lab (“IDL”), based on the fast discharge of a DC→DC converter, and the High-Voltage SPark “HVSP” pulser, based on the breakdown of a transistor array. The pulser output is fed into a custom fat-dipole designed to match the geometry of the SPICE core, with one of the dipole chambers hermetically sealed to contain the signal generator and associated power. Signals from the transmitters were recorded on the five englacial multi-receiver ARA stations, as well as ARIANNA Station 51, with receiver depths between 0–200 m, and horizontal baselines of 0.5–5 km. This permits improved measurements of the radio-frequency response of cold, polar ice and also comparison with models of the RF properties of ice crystals, based on direct measurements of the crystal-fabric from SPICE core samples. We find: a) the real part of the permittivity, corresponding to the refractive index, has a depth dependence which can be (at our current level of precision) parameterized as n(z)=1.78-0.43 exp(0.0125z) , with z in meters and negative with depth, b) from five separate data samples, the field attenuation length, corresponding to the imaginary part of the permittivity is measured to be 1.4–1.8 km in the upper km of the ice sheet, c) the tensorial component of the permittivity, corresponding to birefringence, is measured at a level commensurate with first-principles modeling of the ice fabric, based on core samples extracted during SPICE drilling. Birefringence therefore holds the possibility that the range to an in-ice neutrino interaction can be estimated from the arrival time difference of two polarizations.