Magnetic shielding for large photoelectron multipliers for the OSIRIS facility of the JUNO detector

Abstract: We present the technical design and characteristics of the magnetic shield developed for 20" PMTs of the low-background OSIRIS facility. A ribbon of amorphous alloy with the extreme magnetic permeability was used in its design providing the excellent efficiency in screening the Earth's magnetic field with a relatively small amount of the material. The mass of materials is crucial for the construction of low-background facilities from the point of view of radioactive backgrounds. Using amorphous materials is cost-efficient compared to other methods for screening the Earth's magnetic field.