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STRAW: pathfinder for a neutrino telescope in the Pacific

The Cascadia Basin in the northern Pacific provides ideal prerequisites for a possible large scale neutrino telescope in the deep sea: At its sea floor at a depth of 2600 meter below sea level, the University of Victoria's Ocean Networks Canada (ONC) operates a variety of instruments connected by undersea cables that provide ample power and communication capability to support interdisciplinary science.

With the vision of a new large scale neutrino facility at this site, a research team led by Prof. Elisa Resconi at the Technical University of Munich (TUM), has designed the STRAW pathfinder mission (STRAW STRings for Absorption length in Water). The goal of STRAW is a systematic investigation of the optical transparency and the ambient background light of the site. Specifically, knowlege of the absorption and scattering length of the deep-sea water in the range between 350 nm and 600 nm, the most sensitve wavelength band for the detection of Cherenkov light, forms the basis for the design of a new neutrino telesope.

The concept of STRAW follows the principles of neutrino telescopes and consists of two vertical mooring lines instrumented with light emitter tools and light sensor modules mounted at different heights above sea floor (see left). The light emitter is based on the design of the Precision Optical Calibration Module (POCAM) that was initially designed to act as light emitting calibration device for the IceCube neutrino telescope. In STRAW, the POCAMs are used for emitting intense, adjustable, isotropic, nanosecond light flashes that are observed by the surrounding sDOM units (STRAW Digital Optical Modules).

The assembly of STRAW took place at TUM in Garching from January to March 2018. On 25 June 2018, the two STRAW mooring lines were anchored by ONC in the Pacific at 47º 46' N, 127º 46' W. The sensores have been collecting data since then. First results of the measurements will be published in a forthcoming paper.