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

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