Thin flexible sheets of high-permeability FINEMET foils encased in thinplastic layers have been used to shield various types of 20-cm-diameterphotomultiplier tubes from ambient magnetic fields. In the presence of theEarth's magnetic field this type of shielding is shown to increase thecollection efficiency of photoelectrons and can improve the uniformity ofresponse of these photomultiplier tubes.
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