Long term monitoring of the optical background in the Capo Passero deep-sea site with the NEMO tower prototype

TL;DR

This paper reports on over a year of underwater optical background monitoring at the Capo Passero deep-sea site using the NEMO tower prototype, confirming the site's stability for neutrino telescope deployment.

Contribution

It presents the first long-term optical background measurements at the Capo Passero site with a deep-sea detector prototype, validating site stability for future neutrino observatories.

Findings

Stable and low baseline optical rates observed

Bioluminescence contributes minimally to light bursts

Site conditions are suitable for neutrino detection

Abstract

The NEMO Phase-2 tower is the first detector which was operated underwater for more than one year at the "record" depth of 3500 m. It was designed and built within the framework of the NEMO (NEutrino Mediterranean Observatory) project. The 380 m high tower was successfully installed in March 2013 80 km offshore Capo Passero (Italy). This is the first prototype operated on the site where the italian node of the KM3NeT neutrino telescope will be built. The installation and operation of the NEMO Phase-2 tower has proven the functionality of the infrastructure and the operability at 3500 m depth. A more than one year long monitoring of the deep water characteristics of the site has been also provided. In this paper the infrastructure and the tower structure and instrumentation are described. The results of long term optical background measurements are presented. The rates show stable and low baseline values, compatible with the contribution of 40K light emission, with a small percentage of light bursts due to bioluminescence. All these features confirm the stability and good optical properties of the site.