Development of a magnetic shield for 20-inch microchannel plate photomultiplier tubes

A newly developed 20-inch photomultiplier tube with good timing response and improved energy resolution uses a microchannel plate instead of traditional dynodes. Thin flexible sheets of high-permeability iron-based amorphous alloy foils encased in thin polyethylene terephthalate layers are used to shield the photomultiplier tube from the geomagnetic field. The work reported in this paper shows that the detection efficiency is increased by 87% on average with the help of this magnetic shield, and the number of secondary electrons is reduced by a factor of 2 for 20-inch MCP-PMT at LHAASO-WCDA.