Simulation study for an in-situ calibration system for the measurement of the snow accumulation and the index-of-refraction profile for radio neutrino detectors

TL;DR

This paper proposes an in-situ calibration system for radio neutrino detectors in ice, enabling precise monitoring of snow accumulation and index-of-refraction profiles to improve neutrino property reconstruction.

Contribution

The study introduces a novel calibration method using two radio emitters to continuously measure firn properties, significantly enhancing ice property accuracy for neutrino detection.

Findings

Snow accumulation can be measured with 3mm resolution.

Index-of-refraction profile parameters can be determined with high precision.

Calibration improves neutrino property reconstruction accuracy.

Abstract

Sensitivity to ultra-high-energy neutrinos ($E>10^{17}$eV) can be obtained cost-efficiently by exploiting the Askaryan effect in ice, where a particle cascade induced by the neutrino interaction produces coherent radio emission that can be picked up by antennas. As the near-surface ice properties change rapidly within the upper $\mathcal{O}$(100m), a good understanding of the ice properties is required to reconstruct the neutrino properties. In particular, continuous monitoring of the snow accumulation (which changes the depth of the antennas) and the index-of-refraction $n(z)$ profile are crucial for an accurate determination of the neutrino's direction and energy. We present an in-situ calibration system that extends the radio detector station with two radio emitters to continuously monitor the firn properties within the upper 40m by measuring the time differences between direct and reflected (off the surface) signals (D'n'R). We determine the optimal positions of two transmitters at all three sites of current and future in-ice radio detectors: Greenland, Moore's Bay, and the South Pole. For the South Pole we find that the snow accumulation $\Delta h$ can be measured with a resolution of 3mm and the parameters of an exponential $n(z)$ profile $\alpha$ and $z_0$ with 0.04% and 0.14% precision respectively, which constitutes an improvement of more than a factor of 10 as compared to the inference of the $n(z)$ profile from density measurements. Additionally, as this technique is based on the measurement of the signal propagation times we are not bound to the conversion of density to index-of-refraction. We quantify the impact of these ice uncertainties on the reconstruction of the neutrino vertex, direction, and energy and find that the calibration device measures the ice properties to sufficient precision to have negligible influence.