Spain – Plastic caps – Suministro de fabricación de cúpulas de polimetacrilato de metilo de muy alta transparencia a luz ultravioleta por termo conformado para la obtención de cubiertas anti-implosión para fotomultiplicadores de 20 pulgadas para el experimento Hyperkamiokande.

Hyper-Kamiokande (HK) is the international project to construct a neutrino telescope, an evolution of the Kamiokande and Super-Kamiokande detectors. The HK detector will be built 620 m underground, with the aim of observing neutrinos, proton decay and other physical phenomena. The experiment will be carried out in a huge cylindrical water tank, approximately 70 m in diameter and 70 m deep, filled with a megaton of “ultrapure” water, extremely transparent, in whose walls tens of thousands of light-sensitive photosensors (Photo-Multiplier Tubes, ‘PMTs’ hereafter) will be installed. These detect Cherenkov light produced by particles, such as the neutrino, when they interact with the water. Spain is participating in this large research infrastructure through the LSC Consortium, which will be responsible for coordinating and developing Spanish participation in the construction of the HK neutrino detector. This includes the financing of new projects and actions to be carried out in the HK, for the design, manufacture and installation of PMT protection capsules. To this end, an “anti-implosion” cover has been developed for each of the 20-inch PMTs of the HK experiment, composed of two parts joined together: the first, made of steel, which covers the rear part of the PMT; and the second, formed from sheets of polymethyl methacrylate of very high transparency to ultraviolet light (PMMA UVT), which is placed at the front of the PMT and must be transparent to the Cherenkov photons that it is intended to detect with it. The transparency to ultraviolet rays of the PMMA UVT dome is, by far, the most important parameter that determines the functionality of the protective cover for the 20” PMT of HK. It is for this reason that, in addition to starting from a PMMA UVT sheet with excellent transparency, which has been produced explicitly to meet the transparency needs of the HK project, a transformation process is required until the dome is made, which reduces said transparency to the minimum necessary. It is for this reason that the transformation process that has been validated to date is by thermoforming, in which the PMMA UVT sheet is heated to a temperature at which it can be deformed by an air overpressure, until the necessary geometry fixed for the dome is reached. Throughout the process, and particularly during the heating and forming process, it must be avoided in any way that dust or any other type of impurity can adhere to the surface of the formed dome, causing a potential reduction in the initial high UV light transparency of the sheet. In addition to its transparency, another crucial factor for the survival of more than 10 years of data collection for the HK experiment is the mechanical strength of the protective covers. The LSC has carried out validation tests with covers of different thicknesses formed from PMMA UVT sheets and validated that a thickness of 13 mm meets the minimum resistance requirements for the project. Thus, the PMMA UVT cover generated from a 13 mm thick sheet is sufficient to mitigate the propagation of the shock wave, and minimize the damage caused to the experiment by the loss of broken PMTs.