Characterization of large-area photomultipliers under low magnetic fields: Design and performance of the magnetic shielding for the Double Chooz neutrino experiment

A precise quantitative measurement of the effect of low magnetic fields in Hamamatsu R7081 photomultipliers has been performed. These large-area photomultipliers will be used in the Double Chooz neutrino experiment. A magnetic shielding has been developed for these photomultipliers. Its design and performance is also reported in this paper.