The STRAW Pathfinder Experiments
The P-ONE Collaboration has developed and deployed two pathfinder missions at the chosen experimental site, STRAW and STRAW-b (STRings for Absorption in Water), in 2018 and 2020, respectively, that were decommissioned after a successful operation in the Summer of 2023. Their purpose was to explore the optical properties of the local water, measure the possible optical background, and test solutions planned for the regular measurement lines in the future.
Scientific Purpose & Deployment History
The first pathfinder mission, STRAW, was mainly constructed to measure the attenuation length of water for wavelengths between 350 and 600 nm. It was composed of two mooring lines (or strings), each containing a number of light-emitting LED flashers, the POCAMs (Precision Optical CAlibration Modules), and light-detecting sDOMs (STRAW Digital Optical Modules), each hosting two photomultipliers facing downwards and upwards, respectively. The modules were located at depths between 30 and 100 m above the sea floor and allowed for recording light intensity from POCAMs at various distances.
The subsequent pathfinder, STRAW-b, was located 40 m from STRAW and consisted of a 444 m long string-hosting module enclosed in 13-inch glass-housing spheres. The modules included two LIDARs, three modules to measure environmental conditions, one muon tracker (composed of scintillator planes and SiPMs), and two spectrometer modules. The two latter also contained low-light cameras to register bioluminescence events in the deep ocean.
Results
Attenuation Length
The first pathfinder measured the light attenuation length in water at the experimental site. The measurement was performed by fitting a parametric model of the whole STRAW setup using two years of exposure data. The attenuation length for the wavelength of 450 nm (closest to that of Cherenkov radiation) is 27.7+1.9/−1.3 m.
Salinity & Radioactivity
The radioactive isotope Potassium-40 (40K) in the sea salt results in an ambient-light background that the optical sensors of P-ONE will record. The β electrons from 40K decays, as well as Compton-scattered electrons from excited Argon nuclei (created in the electron-capture process by 40K), will generate Cherenkov light:
40K → 40Ca + e− + ¯νe
40K + e− → 40Ar + νe + γ
Its pattern will be different from the expected future signal in the detector and will consist of coincident illumination of the top and bottom PMT in sDOMs. This measurement was performed using only photons coincident between the top and bottom sensor of the given sDOM within a narrow time window. The obtained 40K decay rate was 78 ± 44 mHz. A comparison with simulations was performed to check the simulation parameters, and a good agreement was found. With this result, a comparison of salinity value with results previously obtained by Ocean Networks Canada (OCN) can be performed to prove that the simulation inputs were correct. A salinity value of 2.5 ± 1.4% was found, in agreement with an ONC value of 3.482 ± 0.001%.
Bioluminescence Background
Significant light emissions from bioluminescence are expected for 440–500 nm wavelengths. As this range coincides with the Cherenkov light, estimating its rate as a background for P-ONE measurements is crucial. It is especially essential to estimate if the planned design of the data acquisition system (DAQ) is sufficient to withstand the bioluminescence background rates.
The results obtained with two years of exposure indicate that the maximum detection rate of 10 MHz is rarely exceeded, which is essential for the design of the P-ONE DAQ system. In addition, the fraction of time above the threshold rate is computed to estimate the background-induced dead time of the photosensors. The temporal evolution of rate percentiles shows modulation with tidal cycles (12.5 h).
The emission spectra for many organisms are known and exhibit a characteristic wavelength thanks to the particular underlying biochemical reactions. The information from the STRAW-b pathfinder cameras, in the form of RGB channels, could be transformed into hue or color angle. These data can be unfolded using the water and glass-housing transmittance information to obtain the emission wavelengths. The figure shows the resulting spectral population and presents the entries from bioluminescence catalogs. Most remarkably, the Annelida, with a spectral peak around 585 nm, visible only near the bottom (Camera 1), are planktonic organisms that cannot move against water currents.