The purpose of the magnetic shielding is to suppress the modulation of the phoswich photomultiplier tube gain as the external magnetic field vector varies in orbit. The magnetic shield consists of three sections, all of which are in mechanical contact with each other to form a single assembly. These sections surround the photomultiplier tube, the detector crystal housing and the collimator (see Figure 5.2). In this way each phoswich photomultiplier tube is embedded within a magnetic shield environment, and systematic effects due to the varying magnetic field vector will be reduced to less than 0.1%.