Detection of the Cherenkov light diffused by Sea Water with the ULTRA Experiment

TL;DR

This paper presents the ULTRA experiment's measurements of Cherenkov light diffusion by sea water, aiding ultra-high-energy cosmic ray detection and improving shower reconstruction for space-based observatories.

Contribution

It provides the first measurements of sea water's diffusing properties for Cherenkov light, addressing a key gap in cosmic ray observation techniques.

Findings

Measured sea water diffusion coefficient for Cherenkov light

Determined the energy threshold for detection in sea water

Demonstrated the feasibility of using sea water reflections for cosmic ray studies

Abstract

The study of Ultra High Energy Cosmic Rays represents one of the most challenging topic in the Cosmic Rays and in the Astroparticle Physics fields. The interaction of primary particles with atmospheric nuclei produces a huge Extensive Air Shower together with isotropic emission of UV fluorescence light and highly directional Cherenkov photons, that are reflected/diffused isotropically by the impact on the Earth's surface or on high optical depth clouds. For space-based observations, detecting the reflected Cherenkov signal in a delayed coincidence with the fluorescence light improves the accuracy of the shower reconstruction in space and in particular the measurement of the shower maximum, giving a strong signature for discriminating hadrons and neutrinos, and helping to estimate the primary chemical composition. Since the Earth's surface is mostly covered by water, the ULTRA (UV Light Transmission and Reflection in the Atmosphere)experiment has been designed to provide the diffusing properties of sea water, overcoming the lack of information in this specific field. A small EAS array, made up of 5 particle detectors, and an UV optical device, have been coupled to detect in coincidence both electromagnetic and UV components. The detector was in operation from May to December, 2005, in a small private harbor in Capo Granitola (Italy); the results of these measurements in terms of diffusion coefficient and threshold energy are presented here.