An Spáinn – 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 an international project to build a neutrino telescope, an evolution of the Kamiokande and Super-Kamiokande detectors. The HK detector will be constructed 620 m underground to observe neutrinos, proton decay and other physical phenomena. The experiment will be carried out in a huge cylindrical water tank, approximately 70m in diameter and 70m in depth, filled with a megatonne of “ultra-pure” water, extremely transparent, in the walls of which tens of thousands of light-sensitive photosensors (Photo-Multiplier Tubes, ‘PMTs’ from here on) will be installed, sensitive to the light known as Cherenkov light produced by particles, such as the neutrino, when interacting with the water. Spain is participating in this major research infrastructure and is doing so through the LSC Consortium, which will be responsible for coordinating and developing Spanish participation in the construction of the HK neutrino detector, including the financing of new projects and actions to be carried out in the HK, for the design, manufacture and installation of photomultiplier 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 back of the PMT; and the second, formed from sheets of polymethyl methacrylate with very high ultraviolet light transparency (PMMA UVT), which is located 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 with a sheet of PMMA UVT of 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 that reduces this 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 set for the dome is reached. Throughout the process, and particularly during the heating and forming process, it must be avoided at all costs 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 from 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 sheets of PMMA UVT 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 minimise the damage caused to the experiment by the loss of broken PMTs.