The P-ONE Prototype line: a proof of concept

To launch P-ONE, the next milestone is to deploy the very first instrumented line in order to verify the working principle of the newly developed optical instruments, the deployment technique, and to serve as the blueprint of the neutrino telescope’s mooring lines. The development of this first line, which will involve 20 optical instruments distributed along an approximately 1000 m long mooring, started early 2021 and is in full progress.

The prototype line concept

P-ONE is currently envisioned as a segmented structure, consisting of 7 clusters with 10 mooring lines each. Each line will include 20 optical modules of two different types: a calibration module and an optical receiver module.

The P-ONE prototype line (see left) aims to be the blueprint for the P-ONE moorings and follows three objectives: First, the development and construction of the optical modules together with the mechanical mounting structure.

Second, the refinement of the deployment strategy and the necessary mechanical deployment support structures. This will allow the collaboration to reach an efficient deployment of several mooring lines during each deployment cruise, guaranteeing a rapid construction of each individual P-ONE cluster.

Third, the initial development of around 16-17 optical and 3-4 calibration modules on an instrumented line with a planned length of 750 to 1000 m. The optical instruments rely heavily on an unimpaired field of view of the photosensors and the integrating spheres of the calibration module, respectively.

Time synchronization between the individual modules, moorings, and clusters of P-ONE will be based on fiber-optical cables. The collaboration is currently exploring options for the most suitable data transfer protocol.

Entanglement and damage of individual components pose a significant threat when deploying 1000 m mooring lines. The P-ONE collaboration is currently looking into the possibility to combine the mechanical backbone with the hybrid electro-optical cable. This will simplify the structure of the line, reducing the risk of failure and facilitate preparations and handling during deployment.

The instrument mountings must serve as robust, safe, and long-lasting connection to the mechanical backbone of the mooring, while still keeping the field of view unimpeded. Regardless of the final approach, the mounting will be designed closely alongside the backbone cable and the instrument core.

The P-ONE optical module

The P-ONE optical module will have a multi photo-multiplier tube (mPMT) configuration in order to achieve the best possible performance within the high light-background environment of the deep Ocean (O(10) kHz per PMT). Photons from slow biological processes are not correlated on a nanosecond timescale, which allows the strong suppression of this background by requiring nanosecond-scale coincidences of photon hits in two or more PMTs. This coincidence is expected for neutrino-induced flashes of Cherenkov light, enabling a strong discrimination of signal from background events at the trigger level.

At the current development stage, candidate PMTs for the optical modules are being evaluated at calibration setups at the University of Alberta and the Technical University Munich. Performing the PMT evaluation at two locations allows independent verification of key PMT parameters of the candidate PMTs. The prototyping of electronics, reflectors, and the internal mounting structure is in full progress.

The P-ONE calibration module

The optical properties and the geometry of the P-ONE sensor array need to be constantly mon-
itored during operation in order to obtain the best possible data and angular resolution. The special-
ized calibration modules that complement the standard P-ONE modules within the array will serve this purpose. The calibration modules emit nanosecond isotropic flashes of light, which are detected by the sur-
rounding optical modules, facilitating the measurement of light attenuation via the flash intensity. This concept has been proven with the STRAW experiment.

As a key novelty for neutrino telescopes worldwide, the P-ONE collaboration will develop a system that allows one to base the entire geometry calibration on this module, exploiting the excellent timing properties of the system. The light output of the flashers will be monitored in-situ by built-in optical sensors. This general concept and the required electronics have already been explored in great detail in the design of the POCAM for the IceCube Upgrade and STRAW. The proven POCAM design will form the basis for successful adaptation and further development of the P-ONE calibration module.