Kamioka ObservatoryICRR, The University of Tokyo

The Hyper-Kamiokande detector is huge, so if we run the cables to read the signals from the photosensors outside the tank, the length of the cables would exceed 150 meters. The faint signal from the photosensors is significantly attenuated while traveling through the 150m long cable and it will be difficult to make accurate measurements because it is easily affected by noises. Therefore, in Hyper-Kamiokande, we decided to install the electronics modules housed in a water pressure-tolerant case next to the photosensors to shorten the cables from the photosensors. Each electronics module will digitize the signals from 24 photosensors and transmit the digital data outside the tank via optical fibers.

 

 

The electronics module consists of the high-voltage power supply for photosensors, the digitizer boards, which convert  photosensors’ analog signal to digital form, and the data processing board which processes and transfers the digitalized data. These components are safely installed in a water-tight stainless-steel case. These modules will be connected through special cables for underwater use to photosensors in the water and readout computers outside of the water tank. The development of each component has been shared among eight countries: Italy, Switzerland, Spain, France, Poland, England, Korea, and Japan.

 

 

The Swiss Group is responsible for the water pressure-tolerant container and the power supplies, which have already been designed and are being prepared for production or have just started. Japan is in charge of special underwater cables and connectors, and the development of these cables and connectors is almost finished. The Italian group is responsible for the digitizer boards, while the electronic boards for data processing are developed and provided by Spain and Poland, respectively. The UK group provides the electronics for the outer detector in collaboration with the Italian group. We will use almost 1000 underwater modules and all of them must be synchronized. The French group is responsible for this synchronization system with the Italian group.  The French and the Korean group provide the system to evaluate and calibrate the system. We have performed various tests and confirmed that the developed prototypes met the requirements to achieve the physics goals of Hyper-Kamiokande.

In this test, we replaced a part of the Super-Kamiokande electronics modules with a prototype underwater electronics module for Hyper-Kamiokande and verified its performance by measuring the signal of the photosensors of the Super-Kamiokande, which is actually the same ring imaging water Cherenkov detector as Hyper-Kamiokande. We performed this test to confirm whether the required performance can be achieved as close to the actual situation, by observing cosmic ray muons and atmospheric neutrinos, as well as reference signals from light sources in Super-Kamiokande, whose repetition rates are up to approximately 1 million per second per channel, which is equivalent to the rates of neutrinos from nearby supernova explosions. We also  checked the stability of the data readout and the monitoring of each device.

We will make final adjustments based on the test results, perform the final tests this summer, and proceed to mass production. In the second half of FY2025, all components will be delivered  to CERN, the European Organization for Nuclear Research in Switzerland, and the underwater electronic modules will be assembled there. Then, they will be installed together with the photosensors in the Hyper-Kamiokande tank.